Initial commit: Go 1.23 release state

src/testing/allocs.go

@@ -0,0 +1,45 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"runtime"
+)
+
+// AllocsPerRun returns the average number of allocations during calls to f.
+// Although the return value has type float64, it will always be an integral value.
+//
+// To compute the number of allocations, the function will first be run once as
+// a warm-up. The average number of allocations over the specified number of
+// runs will then be measured and returned.
+//
+// AllocsPerRun sets GOMAXPROCS to 1 during its measurement and will restore
+// it before returning.
+func AllocsPerRun(runs int, f func()) (avg float64) {
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+
+	// Warm up the function
+	f()
+
+	// Measure the starting statistics
+	var memstats runtime.MemStats
+	runtime.ReadMemStats(&memstats)
+	mallocs := 0 - memstats.Mallocs
+
+	// Run the function the specified number of times
+	for i := 0; i < runs; i++ {
+		f()
+	}
+
+	// Read the final statistics
+	runtime.ReadMemStats(&memstats)
+	mallocs += memstats.Mallocs
+
+	// Average the mallocs over the runs (not counting the warm-up).
+	// We are forced to return a float64 because the API is silly, but do
+	// the division as integers so we can ask if AllocsPerRun()==1
+	// instead of AllocsPerRun()<2.
+	return float64(mallocs / uint64(runs))
+}

src/testing/allocs_test.go

@@ -0,0 +1,29 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import "testing"
+
+var global any
+
+var allocsPerRunTests = []struct {
+	name   string
+	fn     func()
+	allocs float64
+}{
+	{"alloc *byte", func() { global = new(*byte) }, 1},
+	{"alloc complex128", func() { global = new(complex128) }, 1},
+	{"alloc float64", func() { global = new(float64) }, 1},
+	{"alloc int32", func() { global = new(int32) }, 1},
+	{"alloc byte", func() { global = new(byte) }, 1},
+}
+
+func TestAllocsPerRun(t *testing.T) {
+	for _, tt := range allocsPerRunTests {
+		if allocs := testing.AllocsPerRun(100, tt.fn); allocs != tt.allocs {
+			t.Errorf("AllocsPerRun(100, %s) = %v, want %v", tt.name, allocs, tt.allocs)
+		}
+	}
+}

src/testing/benchmark.go

@@ -0,0 +1,840 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"internal/sysinfo"
+	"io"
+	"math"
+	"os"
+	"runtime"
+	"slices"
+	"strconv"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+	"unicode"
+)
+
+func initBenchmarkFlags() {
+	matchBenchmarks = flag.String("test.bench", "", "run only benchmarks matching `regexp`")
+	benchmarkMemory = flag.Bool("test.benchmem", false, "print memory allocations for benchmarks")
+	flag.Var(&benchTime, "test.benchtime", "run each benchmark for duration `d` or N times if `d` is of the form Nx")
+}
+
+var (
+	matchBenchmarks *string
+	benchmarkMemory *bool
+
+	benchTime = durationOrCountFlag{d: 1 * time.Second} // changed during test of testing package
+)
+
+type durationOrCountFlag struct {
+	d         time.Duration
+	n         int
+	allowZero bool
+}
+
+func (f *durationOrCountFlag) String() string {
+	if f.n > 0 {
+		return fmt.Sprintf("%dx", f.n)
+	}
+	return f.d.String()
+}
+
+func (f *durationOrCountFlag) Set(s string) error {
+	if strings.HasSuffix(s, "x") {
+		n, err := strconv.ParseInt(s[:len(s)-1], 10, 0)
+		if err != nil || n < 0 || (!f.allowZero && n == 0) {
+			return fmt.Errorf("invalid count")
+		}
+		*f = durationOrCountFlag{n: int(n)}
+		return nil
+	}
+	d, err := time.ParseDuration(s)
+	if err != nil || d < 0 || (!f.allowZero && d == 0) {
+		return fmt.Errorf("invalid duration")
+	}
+	*f = durationOrCountFlag{d: d}
+	return nil
+}
+
+// Global lock to ensure only one benchmark runs at a time.
+var benchmarkLock sync.Mutex
+
+// Used for every benchmark for measuring memory.
+var memStats runtime.MemStats
+
+// InternalBenchmark is an internal type but exported because it is cross-package;
+// it is part of the implementation of the "go test" command.
+type InternalBenchmark struct {
+	Name string
+	F    func(b *B)
+}
+
+// B is a type passed to [Benchmark] functions to manage benchmark
+// timing and to specify the number of iterations to run.
+//
+// A benchmark ends when its Benchmark function returns or calls any of the methods
+// FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods must be called
+// only from the goroutine running the Benchmark function.
+// The other reporting methods, such as the variations of Log and Error,
+// may be called simultaneously from multiple goroutines.
+//
+// Like in tests, benchmark logs are accumulated during execution
+// and dumped to standard output when done. Unlike in tests, benchmark logs
+// are always printed, so as not to hide output whose existence may be
+// affecting benchmark results.
+type B struct {
+	common
+	importPath       string // import path of the package containing the benchmark
+	context          *benchContext
+	N                int
+	previousN        int           // number of iterations in the previous run
+	previousDuration time.Duration // total duration of the previous run
+	benchFunc        func(b *B)
+	benchTime        durationOrCountFlag
+	bytes            int64
+	missingBytes     bool // one of the subbenchmarks does not have bytes set.
+	timerOn          bool
+	showAllocResult  bool
+	result           BenchmarkResult
+	parallelism      int // RunParallel creates parallelism*GOMAXPROCS goroutines
+	// The initial states of memStats.Mallocs and memStats.TotalAlloc.
+	startAllocs uint64
+	startBytes  uint64
+	// The net total of this test after being run.
+	netAllocs uint64
+	netBytes  uint64
+	// Extra metrics collected by ReportMetric.
+	extra map[string]float64
+}
+
+// StartTimer starts timing a test. This function is called automatically
+// before a benchmark starts, but it can also be used to resume timing after
+// a call to [B.StopTimer].
+func (b *B) StartTimer() {
+	if !b.timerOn {
+		runtime.ReadMemStats(&memStats)
+		b.startAllocs = memStats.Mallocs
+		b.startBytes = memStats.TotalAlloc
+		b.start = highPrecisionTimeNow()
+		b.timerOn = true
+	}
+}
+
+// StopTimer stops timing a test. This can be used to pause the timer
+// while performing complex initialization that you don't
+// want to measure.
+func (b *B) StopTimer() {
+	if b.timerOn {
+		b.duration += highPrecisionTimeSince(b.start)
+		runtime.ReadMemStats(&memStats)
+		b.netAllocs += memStats.Mallocs - b.startAllocs
+		b.netBytes += memStats.TotalAlloc - b.startBytes
+		b.timerOn = false
+	}
+}
+
+// ResetTimer zeroes the elapsed benchmark time and memory allocation counters
+// and deletes user-reported metrics.
+// It does not affect whether the timer is running.
+func (b *B) ResetTimer() {
+	if b.extra == nil {
+		// Allocate the extra map before reading memory stats.
+		// Pre-size it to make more allocation unlikely.
+		b.extra = make(map[string]float64, 16)
+	} else {
+		clear(b.extra)
+	}
+	if b.timerOn {
+		runtime.ReadMemStats(&memStats)
+		b.startAllocs = memStats.Mallocs
+		b.startBytes = memStats.TotalAlloc
+		b.start = highPrecisionTimeNow()
+	}
+	b.duration = 0
+	b.netAllocs = 0
+	b.netBytes = 0
+}
+
+// SetBytes records the number of bytes processed in a single operation.
+// If this is called, the benchmark will report ns/op and MB/s.
+func (b *B) SetBytes(n int64) { b.bytes = n }
+
+// ReportAllocs enables malloc statistics for this benchmark.
+// It is equivalent to setting -test.benchmem, but it only affects the
+// benchmark function that calls ReportAllocs.
+func (b *B) ReportAllocs() {
+	b.showAllocResult = true
+}
+
+// runN runs a single benchmark for the specified number of iterations.
+func (b *B) runN(n int) {
+	benchmarkLock.Lock()
+	defer benchmarkLock.Unlock()
+	defer func() {
+		b.runCleanup(normalPanic)
+		b.checkRaces()
+	}()
+	// Try to get a comparable environment for each run
+	// by clearing garbage from previous runs.
+	runtime.GC()
+	b.resetRaces()
+	b.N = n
+	b.parallelism = 1
+	b.ResetTimer()
+	b.StartTimer()
+	b.benchFunc(b)
+	b.StopTimer()
+	b.previousN = n
+	b.previousDuration = b.duration
+}
+
+// run1 runs the first iteration of benchFunc. It reports whether more
+// iterations of this benchmarks should be run.
+func (b *B) run1() bool {
+	if ctx := b.context; ctx != nil {
+		// Extend maxLen, if needed.
+		if n := len(b.name) + ctx.extLen + 1; n > ctx.maxLen {
+			ctx.maxLen = n + 8 // Add additional slack to avoid too many jumps in size.
+		}
+	}
+	go func() {
+		// Signal that we're done whether we return normally
+		// or by FailNow's runtime.Goexit.
+		defer func() {
+			b.signal <- true
+		}()
+
+		b.runN(1)
+	}()
+	<-b.signal
+	if b.failed {
+		fmt.Fprintf(b.w, "%s--- FAIL: %s\n%s", b.chatty.prefix(), b.name, b.output)
+		return false
+	}
+	// Only print the output if we know we are not going to proceed.
+	// Otherwise it is printed in processBench.
+	b.mu.RLock()
+	finished := b.finished
+	b.mu.RUnlock()
+	if b.hasSub.Load() || finished {
+		tag := "BENCH"
+		if b.skipped {
+			tag = "SKIP"
+		}
+		if b.chatty != nil && (len(b.output) > 0 || finished) {
+			b.trimOutput()
+			fmt.Fprintf(b.w, "%s--- %s: %s\n%s", b.chatty.prefix(), tag, b.name, b.output)
+		}
+		return false
+	}
+	return true
+}
+
+var labelsOnce sync.Once
+
+// run executes the benchmark in a separate goroutine, including all of its
+// subbenchmarks. b must not have subbenchmarks.
+func (b *B) run() {
+	labelsOnce.Do(func() {
+		fmt.Fprintf(b.w, "goos: %s\n", runtime.GOOS)
+		fmt.Fprintf(b.w, "goarch: %s\n", runtime.GOARCH)
+		if b.importPath != "" {
+			fmt.Fprintf(b.w, "pkg: %s\n", b.importPath)
+		}
+		if cpu := sysinfo.CPUName(); cpu != "" {
+			fmt.Fprintf(b.w, "cpu: %s\n", cpu)
+		}
+	})
+	if b.context != nil {
+		// Running go test --test.bench
+		b.context.processBench(b) // Must call doBench.
+	} else {
+		// Running func Benchmark.
+		b.doBench()
+	}
+}
+
+func (b *B) doBench() BenchmarkResult {
+	go b.launch()
+	<-b.signal
+	return b.result
+}
+
+// launch launches the benchmark function. It gradually increases the number
+// of benchmark iterations until the benchmark runs for the requested benchtime.
+// launch is run by the doBench function as a separate goroutine.
+// run1 must have been called on b.
+func (b *B) launch() {
+	// Signal that we're done whether we return normally
+	// or by FailNow's runtime.Goexit.
+	defer func() {
+		b.signal <- true
+	}()
+
+	// Run the benchmark for at least the specified amount of time.
+	if b.benchTime.n > 0 {
+		// We already ran a single iteration in run1.
+		// If -benchtime=1x was requested, use that result.
+		// See https://golang.org/issue/32051.
+		if b.benchTime.n > 1 {
+			b.runN(b.benchTime.n)
+		}
+	} else {
+		d := b.benchTime.d
+		for n := int64(1); !b.failed && b.duration < d && n < 1e9; {
+			last := n
+			// Predict required iterations.
+			goalns := d.Nanoseconds()
+			prevIters := int64(b.N)
+			prevns := b.duration.Nanoseconds()
+			if prevns <= 0 {
+				// Round up, to avoid div by zero.
+				prevns = 1
+			}
+			// Order of operations matters.
+			// For very fast benchmarks, prevIters ~= prevns.
+			// If you divide first, you get 0 or 1,
+			// which can hide an order of magnitude in execution time.
+			// So multiply first, then divide.
+			n = goalns * prevIters / prevns
+			// Run more iterations than we think we'll need (1.2x).
+			n += n / 5
+			// Don't grow too fast in case we had timing errors previously.
+			n = min(n, 100*last)
+			// Be sure to run at least one more than last time.
+			n = max(n, last+1)
+			// Don't run more than 1e9 times. (This also keeps n in int range on 32 bit platforms.)
+			n = min(n, 1e9)
+			b.runN(int(n))
+		}
+	}
+	b.result = BenchmarkResult{b.N, b.duration, b.bytes, b.netAllocs, b.netBytes, b.extra}
+}
+
+// Elapsed returns the measured elapsed time of the benchmark.
+// The duration reported by Elapsed matches the one measured by
+// [B.StartTimer], [B.StopTimer], and [B.ResetTimer].
+func (b *B) Elapsed() time.Duration {
+	d := b.duration
+	if b.timerOn {
+		d += highPrecisionTimeSince(b.start)
+	}
+	return d
+}
+
+// ReportMetric adds "n unit" to the reported benchmark results.
+// If the metric is per-iteration, the caller should divide by b.N,
+// and by convention units should end in "/op".
+// ReportMetric overrides any previously reported value for the same unit.
+// ReportMetric panics if unit is the empty string or if unit contains
+// any whitespace.
+// If unit is a unit normally reported by the benchmark framework itself
+// (such as "allocs/op"), ReportMetric will override that metric.
+// Setting "ns/op" to 0 will suppress that built-in metric.
+func (b *B) ReportMetric(n float64, unit string) {
+	if unit == "" {
+		panic("metric unit must not be empty")
+	}
+	if strings.IndexFunc(unit, unicode.IsSpace) >= 0 {
+		panic("metric unit must not contain whitespace")
+	}
+	b.extra[unit] = n
+}
+
+// BenchmarkResult contains the results of a benchmark run.
+type BenchmarkResult struct {
+	N         int           // The number of iterations.
+	T         time.Duration // The total time taken.
+	Bytes     int64         // Bytes processed in one iteration.
+	MemAllocs uint64        // The total number of memory allocations.
+	MemBytes  uint64        // The total number of bytes allocated.
+
+	// Extra records additional metrics reported by ReportMetric.
+	Extra map[string]float64
+}
+
+// NsPerOp returns the "ns/op" metric.
+func (r BenchmarkResult) NsPerOp() int64 {
+	if v, ok := r.Extra["ns/op"]; ok {
+		return int64(v)
+	}
+	if r.N <= 0 {
+		return 0
+	}
+	return r.T.Nanoseconds() / int64(r.N)
+}
+
+// mbPerSec returns the "MB/s" metric.
+func (r BenchmarkResult) mbPerSec() float64 {
+	if v, ok := r.Extra["MB/s"]; ok {
+		return v
+	}
+	if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
+		return 0
+	}
+	return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
+}
+
+// AllocsPerOp returns the "allocs/op" metric,
+// which is calculated as r.MemAllocs / r.N.
+func (r BenchmarkResult) AllocsPerOp() int64 {
+	if v, ok := r.Extra["allocs/op"]; ok {
+		return int64(v)
+	}
+	if r.N <= 0 {
+		return 0
+	}
+	return int64(r.MemAllocs) / int64(r.N)
+}
+
+// AllocedBytesPerOp returns the "B/op" metric,
+// which is calculated as r.MemBytes / r.N.
+func (r BenchmarkResult) AllocedBytesPerOp() int64 {
+	if v, ok := r.Extra["B/op"]; ok {
+		return int64(v)
+	}
+	if r.N <= 0 {
+		return 0
+	}
+	return int64(r.MemBytes) / int64(r.N)
+}
+
+// String returns a summary of the benchmark results.
+// It follows the benchmark result line format from
+// https://golang.org/design/14313-benchmark-format, not including the
+// benchmark name.
+// Extra metrics override built-in metrics of the same name.
+// String does not include allocs/op or B/op, since those are reported
+// by [BenchmarkResult.MemString].
+func (r BenchmarkResult) String() string {
+	buf := new(strings.Builder)
+	fmt.Fprintf(buf, "%8d", r.N)
+
+	// Get ns/op as a float.
+	ns, ok := r.Extra["ns/op"]
+	if !ok {
+		ns = float64(r.T.Nanoseconds()) / float64(r.N)
+	}
+	if ns != 0 {
+		buf.WriteByte('\t')
+		prettyPrint(buf, ns, "ns/op")
+	}
+
+	if mbs := r.mbPerSec(); mbs != 0 {
+		fmt.Fprintf(buf, "\t%7.2f MB/s", mbs)
+	}
+
+	// Print extra metrics that aren't represented in the standard
+	// metrics.
+	var extraKeys []string
+	for k := range r.Extra {
+		switch k {
+		case "ns/op", "MB/s", "B/op", "allocs/op":
+			// Built-in metrics reported elsewhere.
+			continue
+		}
+		extraKeys = append(extraKeys, k)
+	}
+	slices.Sort(extraKeys)
+	for _, k := range extraKeys {
+		buf.WriteByte('\t')
+		prettyPrint(buf, r.Extra[k], k)
+	}
+	return buf.String()
+}
+
+func prettyPrint(w io.Writer, x float64, unit string) {
+	// Print all numbers with 10 places before the decimal point
+	// and small numbers with four sig figs. Field widths are
+	// chosen to fit the whole part in 10 places while aligning
+	// the decimal point of all fractional formats.
+	var format string
+	switch y := math.Abs(x); {
+	case y == 0 || y >= 999.95:
+		format = "%10.0f %s"
+	case y >= 99.995:
+		format = "%12.1f %s"
+	case y >= 9.9995:
+		format = "%13.2f %s"
+	case y >= 0.99995:
+		format = "%14.3f %s"
+	case y >= 0.099995:
+		format = "%15.4f %s"
+	case y >= 0.0099995:
+		format = "%16.5f %s"
+	case y >= 0.00099995:
+		format = "%17.6f %s"
+	default:
+		format = "%18.7f %s"
+	}
+	fmt.Fprintf(w, format, x, unit)
+}
+
+// MemString returns r.AllocedBytesPerOp and r.AllocsPerOp in the same format as 'go test'.
+func (r BenchmarkResult) MemString() string {
+	return fmt.Sprintf("%8d B/op\t%8d allocs/op",
+		r.AllocedBytesPerOp(), r.AllocsPerOp())
+}
+
+// benchmarkName returns full name of benchmark including procs suffix.
+func benchmarkName(name string, n int) string {
+	if n != 1 {
+		return fmt.Sprintf("%s-%d", name, n)
+	}
+	return name
+}
+
+type benchContext struct {
+	match *matcher
+
+	maxLen int // The largest recorded benchmark name.
+	extLen int // Maximum extension length.
+}
+
+// RunBenchmarks is an internal function but exported because it is cross-package;
+// it is part of the implementation of the "go test" command.
+func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
+	runBenchmarks("", matchString, benchmarks)
+}
+
+func runBenchmarks(importPath string, matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) bool {
+	// If no flag was specified, don't run benchmarks.
+	if len(*matchBenchmarks) == 0 {
+		return true
+	}
+	// Collect matching benchmarks and determine longest name.
+	maxprocs := 1
+	for _, procs := range cpuList {
+		if procs > maxprocs {
+			maxprocs = procs
+		}
+	}
+	ctx := &benchContext{
+		match:  newMatcher(matchString, *matchBenchmarks, "-test.bench", *skip),
+		extLen: len(benchmarkName("", maxprocs)),
+	}
+	var bs []InternalBenchmark
+	for _, Benchmark := range benchmarks {
+		if _, matched, _ := ctx.match.fullName(nil, Benchmark.Name); matched {
+			bs = append(bs, Benchmark)
+			benchName := benchmarkName(Benchmark.Name, maxprocs)
+			if l := len(benchName) + ctx.extLen + 1; l > ctx.maxLen {
+				ctx.maxLen = l
+			}
+		}
+	}
+	main := &B{
+		common: common{
+			name:  "Main",
+			w:     os.Stdout,
+			bench: true,
+		},
+		importPath: importPath,
+		benchFunc: func(b *B) {
+			for _, Benchmark := range bs {
+				b.Run(Benchmark.Name, Benchmark.F)
+			}
+		},
+		benchTime: benchTime,
+		context:   ctx,
+	}
+	if Verbose() {
+		main.chatty = newChattyPrinter(main.w)
+	}
+	main.runN(1)
+	return !main.failed
+}
+
+// processBench runs bench b for the configured CPU counts and prints the results.
+func (ctx *benchContext) processBench(b *B) {
+	for i, procs := range cpuList {
+		for j := uint(0); j < *count; j++ {
+			runtime.GOMAXPROCS(procs)
+			benchName := benchmarkName(b.name, procs)
+
+			// If it's chatty, we've already printed this information.
+			if b.chatty == nil {
+				fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
+			}
+			// Recompute the running time for all but the first iteration.
+			if i > 0 || j > 0 {
+				b = &B{
+					common: common{
+						signal: make(chan bool),
+						name:   b.name,
+						w:      b.w,
+						chatty: b.chatty,
+						bench:  true,
+					},
+					benchFunc: b.benchFunc,
+					benchTime: b.benchTime,
+				}
+				b.run1()
+			}
+			r := b.doBench()
+			if b.failed {
+				// The output could be very long here, but probably isn't.
+				// We print it all, regardless, because we don't want to trim the reason
+				// the benchmark failed.
+				fmt.Fprintf(b.w, "%s--- FAIL: %s\n%s", b.chatty.prefix(), benchName, b.output)
+				continue
+			}
+			results := r.String()
+			if b.chatty != nil {
+				fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
+			}
+			if *benchmarkMemory || b.showAllocResult {
+				results += "\t" + r.MemString()
+			}
+			fmt.Fprintln(b.w, results)
+			// Unlike with tests, we ignore the -chatty flag and always print output for
+			// benchmarks since the output generation time will skew the results.
+			if len(b.output) > 0 {
+				b.trimOutput()
+				fmt.Fprintf(b.w, "%s--- BENCH: %s\n%s", b.chatty.prefix(), benchName, b.output)
+			}
+			if p := runtime.GOMAXPROCS(-1); p != procs {
+				fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
+			}
+			if b.chatty != nil && b.chatty.json {
+				b.chatty.Updatef("", "=== NAME  %s\n", "")
+			}
+		}
+	}
+}
+
+// If hideStdoutForTesting is true, Run does not print the benchName.
+// This avoids a spurious print during 'go test' on package testing itself,
+// which invokes b.Run in its own tests (see sub_test.go).
+var hideStdoutForTesting = false
+
+// Run benchmarks f as a subbenchmark with the given name. It reports
+// whether there were any failures.
+//
+// A subbenchmark is like any other benchmark. A benchmark that calls Run at
+// least once will not be measured itself and will be called once with N=1.
+func (b *B) Run(name string, f func(b *B)) bool {
+	// Since b has subbenchmarks, we will no longer run it as a benchmark itself.
+	// Release the lock and acquire it on exit to ensure locks stay paired.
+	b.hasSub.Store(true)
+	benchmarkLock.Unlock()
+	defer benchmarkLock.Lock()
+
+	benchName, ok, partial := b.name, true, false
+	if b.context != nil {
+		benchName, ok, partial = b.context.match.fullName(&b.common, name)
+	}
+	if !ok {
+		return true
+	}
+	var pc [maxStackLen]uintptr
+	n := runtime.Callers(2, pc[:])
+	sub := &B{
+		common: common{
+			signal:  make(chan bool),
+			name:    benchName,
+			parent:  &b.common,
+			level:   b.level + 1,
+			creator: pc[:n],
+			w:       b.w,
+			chatty:  b.chatty,
+			bench:   true,
+		},
+		importPath: b.importPath,
+		benchFunc:  f,
+		benchTime:  b.benchTime,
+		context:    b.context,
+	}
+	if partial {
+		// Partial name match, like -bench=X/Y matching BenchmarkX.
+		// Only process sub-benchmarks, if any.
+		sub.hasSub.Store(true)
+	}
+
+	if b.chatty != nil {
+		labelsOnce.Do(func() {
+			fmt.Printf("goos: %s\n", runtime.GOOS)
+			fmt.Printf("goarch: %s\n", runtime.GOARCH)
+			if b.importPath != "" {
+				fmt.Printf("pkg: %s\n", b.importPath)
+			}
+			if cpu := sysinfo.CPUName(); cpu != "" {
+				fmt.Printf("cpu: %s\n", cpu)
+			}
+		})
+
+		if !hideStdoutForTesting {
+			if b.chatty.json {
+				b.chatty.Updatef(benchName, "=== RUN   %s\n", benchName)
+			}
+			fmt.Println(benchName)
+		}
+	}
+
+	if sub.run1() {
+		sub.run()
+	}
+	b.add(sub.result)
+	return !sub.failed
+}
+
+// add simulates running benchmarks in sequence in a single iteration. It is
+// used to give some meaningful results in case func Benchmark is used in
+// combination with Run.
+func (b *B) add(other BenchmarkResult) {
+	r := &b.result
+	// The aggregated BenchmarkResults resemble running all subbenchmarks as
+	// in sequence in a single benchmark.
+	r.N = 1
+	r.T += time.Duration(other.NsPerOp())
+	if other.Bytes == 0 {
+		// Summing Bytes is meaningless in aggregate if not all subbenchmarks
+		// set it.
+		b.missingBytes = true
+		r.Bytes = 0
+	}
+	if !b.missingBytes {
+		r.Bytes += other.Bytes
+	}
+	r.MemAllocs += uint64(other.AllocsPerOp())
+	r.MemBytes += uint64(other.AllocedBytesPerOp())
+}
+
+// trimOutput shortens the output from a benchmark, which can be very long.
+func (b *B) trimOutput() {
+	// The output is likely to appear multiple times because the benchmark
+	// is run multiple times, but at least it will be seen. This is not a big deal
+	// because benchmarks rarely print, but just in case, we trim it if it's too long.
+	const maxNewlines = 10
+	for nlCount, j := 0, 0; j < len(b.output); j++ {
+		if b.output[j] == '\n' {
+			nlCount++
+			if nlCount >= maxNewlines {
+				b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
+				break
+			}
+		}
+	}
+}
+
+// A PB is used by RunParallel for running parallel benchmarks.
+type PB struct {
+	globalN *atomic.Uint64 // shared between all worker goroutines iteration counter
+	grain   uint64         // acquire that many iterations from globalN at once
+	cache   uint64         // local cache of acquired iterations
+	bN      uint64         // total number of iterations to execute (b.N)
+}
+
+// Next reports whether there are more iterations to execute.
+func (pb *PB) Next() bool {
+	if pb.cache == 0 {
+		n := pb.globalN.Add(pb.grain)
+		if n <= pb.bN {
+			pb.cache = pb.grain
+		} else if n < pb.bN+pb.grain {
+			pb.cache = pb.bN + pb.grain - n
+		} else {
+			return false
+		}
+	}
+	pb.cache--
+	return true
+}
+
+// RunParallel runs a benchmark in parallel.
+// It creates multiple goroutines and distributes b.N iterations among them.
+// The number of goroutines defaults to GOMAXPROCS. To increase parallelism for
+// non-CPU-bound benchmarks, call [B.SetParallelism] before RunParallel.
+// RunParallel is usually used with the go test -cpu flag.
+//
+// The body function will be run in each goroutine. It should set up any
+// goroutine-local state and then iterate until pb.Next returns false.
+// It should not use the [B.StartTimer], [B.StopTimer], or [B.ResetTimer] functions,
+// because they have global effect. It should also not call [B.Run].
+//
+// RunParallel reports ns/op values as wall time for the benchmark as a whole,
+// not the sum of wall time or CPU time over each parallel goroutine.
+func (b *B) RunParallel(body func(*PB)) {
+	if b.N == 0 {
+		return // Nothing to do when probing.
+	}
+	// Calculate grain size as number of iterations that take ~100µs.
+	// 100µs is enough to amortize the overhead and provide sufficient
+	// dynamic load balancing.
+	grain := uint64(0)
+	if b.previousN > 0 && b.previousDuration > 0 {
+		grain = 1e5 * uint64(b.previousN) / uint64(b.previousDuration)
+	}
+	if grain < 1 {
+		grain = 1
+	}
+	// We expect the inner loop and function call to take at least 10ns,
+	// so do not do more than 100µs/10ns=1e4 iterations.
+	if grain > 1e4 {
+		grain = 1e4
+	}
+
+	var n atomic.Uint64
+	numProcs := b.parallelism * runtime.GOMAXPROCS(0)
+	var wg sync.WaitGroup
+	wg.Add(numProcs)
+	for p := 0; p < numProcs; p++ {
+		go func() {
+			defer wg.Done()
+			pb := &PB{
+				globalN: &n,
+				grain:   grain,
+				bN:      uint64(b.N),
+			}
+			body(pb)
+		}()
+	}
+	wg.Wait()
+	if n.Load() <= uint64(b.N) && !b.Failed() {
+		b.Fatal("RunParallel: body exited without pb.Next() == false")
+	}
+}
+
+// SetParallelism sets the number of goroutines used by [B.RunParallel] to p*GOMAXPROCS.
+// There is usually no need to call SetParallelism for CPU-bound benchmarks.
+// If p is less than 1, this call will have no effect.
+func (b *B) SetParallelism(p int) {
+	if p >= 1 {
+		b.parallelism = p
+	}
+}
+
+// Benchmark benchmarks a single function. It is useful for creating
+// custom benchmarks that do not use the "go test" command.
+//
+// If f depends on testing flags, then [Init] must be used to register
+// those flags before calling Benchmark and before calling [flag.Parse].
+//
+// If f calls Run, the result will be an estimate of running all its
+// subbenchmarks that don't call Run in sequence in a single benchmark.
+func Benchmark(f func(b *B)) BenchmarkResult {
+	b := &B{
+		common: common{
+			signal: make(chan bool),
+			w:      discard{},
+		},
+		benchFunc: f,
+		benchTime: benchTime,
+	}
+	if b.run1() {
+		b.run()
+	}
+	return b.result
+}
+
+type discard struct{}
+
+func (discard) Write(b []byte) (n int, err error) { return len(b), nil }

src/testing/benchmark_test.go

@@ -0,0 +1,214 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"bytes"
+	"cmp"
+	"runtime"
+	"slices"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"text/template"
+	"time"
+)
+
+var prettyPrintTests = []struct {
+	v        float64
+	expected string
+}{
+	{0, "         0 x"},
+	{1234.1, "      1234 x"},
+	{-1234.1, "     -1234 x"},
+	{999.950001, "      1000 x"},
+	{999.949999, "       999.9 x"},
+	{99.9950001, "       100.0 x"},
+	{99.9949999, "        99.99 x"},
+	{-99.9949999, "       -99.99 x"},
+	{0.000999950001, "         0.001000 x"},
+	{0.000999949999, "         0.0009999 x"}, // smallest case
+	{0.0000999949999, "         0.0001000 x"},
+}
+
+func TestPrettyPrint(t *testing.T) {
+	for _, tt := range prettyPrintTests {
+		buf := new(strings.Builder)
+		testing.PrettyPrint(buf, tt.v, "x")
+		if tt.expected != buf.String() {
+			t.Errorf("prettyPrint(%v): expected %q, actual %q", tt.v, tt.expected, buf.String())
+		}
+	}
+}
+
+func TestResultString(t *testing.T) {
+	// Test fractional ns/op handling
+	r := testing.BenchmarkResult{
+		N: 100,
+		T: 240 * time.Nanosecond,
+	}
+	if r.NsPerOp() != 2 {
+		t.Errorf("NsPerOp: expected 2, actual %v", r.NsPerOp())
+	}
+	if want, got := "     100\t         2.400 ns/op", r.String(); want != got {
+		t.Errorf("String: expected %q, actual %q", want, got)
+	}
+
+	// Test sub-1 ns/op (issue #31005)
+	r.T = 40 * time.Nanosecond
+	if want, got := "     100\t         0.4000 ns/op", r.String(); want != got {
+		t.Errorf("String: expected %q, actual %q", want, got)
+	}
+
+	// Test 0 ns/op
+	r.T = 0
+	if want, got := "     100", r.String(); want != got {
+		t.Errorf("String: expected %q, actual %q", want, got)
+	}
+}
+
+func TestRunParallel(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	testing.Benchmark(func(b *testing.B) {
+		procs := uint32(0)
+		iters := uint64(0)
+		b.SetParallelism(3)
+		b.RunParallel(func(pb *testing.PB) {
+			atomic.AddUint32(&procs, 1)
+			for pb.Next() {
+				atomic.AddUint64(&iters, 1)
+			}
+		})
+		if want := uint32(3 * runtime.GOMAXPROCS(0)); procs != want {
+			t.Errorf("got %v procs, want %v", procs, want)
+		}
+		if iters != uint64(b.N) {
+			t.Errorf("got %v iters, want %v", iters, b.N)
+		}
+	})
+}
+
+func TestRunParallelFail(t *testing.T) {
+	testing.Benchmark(func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			// The function must be able to log/abort
+			// w/o crashing/deadlocking the whole benchmark.
+			b.Log("log")
+			b.Error("error")
+		})
+	})
+}
+
+func TestRunParallelFatal(t *testing.T) {
+	testing.Benchmark(func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				if b.N > 1 {
+					b.Fatal("error")
+				}
+			}
+		})
+	})
+}
+
+func TestRunParallelSkipNow(t *testing.T) {
+	testing.Benchmark(func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				if b.N > 1 {
+					b.SkipNow()
+				}
+			}
+		})
+	})
+}
+
+func ExampleB_RunParallel() {
+	// Parallel benchmark for text/template.Template.Execute on a single object.
+	testing.Benchmark(func(b *testing.B) {
+		templ := template.Must(template.New("test").Parse("Hello, {{.}}!"))
+		// RunParallel will create GOMAXPROCS goroutines
+		// and distribute work among them.
+		b.RunParallel(func(pb *testing.PB) {
+			// Each goroutine has its own bytes.Buffer.
+			var buf bytes.Buffer
+			for pb.Next() {
+				// The loop body is executed b.N times total across all goroutines.
+				buf.Reset()
+				templ.Execute(&buf, "World")
+			}
+		})
+	})
+}
+
+func TestReportMetric(t *testing.T) {
+	res := testing.Benchmark(func(b *testing.B) {
+		b.ReportMetric(12345, "ns/op")
+		b.ReportMetric(0.2, "frobs/op")
+	})
+	// Test built-in overriding.
+	if res.NsPerOp() != 12345 {
+		t.Errorf("NsPerOp: expected %v, actual %v", 12345, res.NsPerOp())
+	}
+	// Test stringing.
+	res.N = 1 // Make the output stable
+	want := "       1\t     12345 ns/op\t         0.2000 frobs/op"
+	if want != res.String() {
+		t.Errorf("expected %q, actual %q", want, res.String())
+	}
+}
+
+func ExampleB_ReportMetric() {
+	// This reports a custom benchmark metric relevant to a
+	// specific algorithm (in this case, sorting).
+	testing.Benchmark(func(b *testing.B) {
+		var compares int64
+		for i := 0; i < b.N; i++ {
+			s := []int{5, 4, 3, 2, 1}
+			slices.SortFunc(s, func(a, b int) int {
+				compares++
+				return cmp.Compare(a, b)
+			})
+		}
+		// This metric is per-operation, so divide by b.N and
+		// report it as a "/op" unit.
+		b.ReportMetric(float64(compares)/float64(b.N), "compares/op")
+		// This metric is per-time, so divide by b.Elapsed and
+		// report it as a "/ns" unit.
+		b.ReportMetric(float64(compares)/float64(b.Elapsed().Nanoseconds()), "compares/ns")
+	})
+}
+
+func ExampleB_ReportMetric_parallel() {
+	// This reports a custom benchmark metric relevant to a
+	// specific algorithm (in this case, sorting) in parallel.
+	testing.Benchmark(func(b *testing.B) {
+		var compares atomic.Int64
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				s := []int{5, 4, 3, 2, 1}
+				slices.SortFunc(s, func(a, b int) int {
+					// Because RunParallel runs the function many
+					// times in parallel, we must increment the
+					// counter atomically to avoid racing writes.
+					compares.Add(1)
+					return cmp.Compare(a, b)
+				})
+			}
+		})
+
+		// NOTE: Report each metric once, after all of the parallel
+		// calls have completed.
+
+		// This metric is per-operation, so divide by b.N and
+		// report it as a "/op" unit.
+		b.ReportMetric(float64(compares.Load())/float64(b.N), "compares/op")
+		// This metric is per-time, so divide by b.Elapsed and
+		// report it as a "/ns" unit.
+		b.ReportMetric(float64(compares.Load())/float64(b.Elapsed().Nanoseconds()), "compares/ns")
+	})
+}

src/testing/cover.go

@@ -0,0 +1,124 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Support for test coverage.
+
+package testing
+
+import (
+	"fmt"
+	"internal/goexperiment"
+	"os"
+	"sync/atomic"
+)
+
+// CoverBlock records the coverage data for a single basic block.
+// The fields are 1-indexed, as in an editor: The opening line of
+// the file is number 1, for example. Columns are measured
+// in bytes.
+// NOTE: This struct is internal to the testing infrastructure and may change.
+// It is not covered (yet) by the Go 1 compatibility guidelines.
+type CoverBlock struct {
+	Line0 uint32 // Line number for block start.
+	Col0  uint16 // Column number for block start.
+	Line1 uint32 // Line number for block end.
+	Col1  uint16 // Column number for block end.
+	Stmts uint16 // Number of statements included in this block.
+}
+
+var cover Cover
+
+// Cover records information about test coverage checking.
+// NOTE: This struct is internal to the testing infrastructure and may change.
+// It is not covered (yet) by the Go 1 compatibility guidelines.
+type Cover struct {
+	Mode            string
+	Counters        map[string][]uint32
+	Blocks          map[string][]CoverBlock
+	CoveredPackages string
+}
+
+// Coverage reports the current code coverage as a fraction in the range [0, 1].
+// If coverage is not enabled, Coverage returns 0.
+//
+// When running a large set of sequential test cases, checking Coverage after each one
+// can be useful for identifying which test cases exercise new code paths.
+// It is not a replacement for the reports generated by 'go test -cover' and
+// 'go tool cover'.
+func Coverage() float64 {
+	if goexperiment.CoverageRedesign {
+		return coverage2()
+	}
+	var n, d int64
+	for _, counters := range cover.Counters {
+		for i := range counters {
+			if atomic.LoadUint32(&counters[i]) > 0 {
+				n++
+			}
+			d++
+		}
+	}
+	if d == 0 {
+		return 0
+	}
+	return float64(n) / float64(d)
+}
+
+// RegisterCover records the coverage data accumulators for the tests.
+// NOTE: This function is internal to the testing infrastructure and may change.
+// It is not covered (yet) by the Go 1 compatibility guidelines.
+func RegisterCover(c Cover) {
+	cover = c
+}
+
+// mustBeNil checks the error and, if present, reports it and exits.
+func mustBeNil(err error) {
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "testing: %s\n", err)
+		os.Exit(2)
+	}
+}
+
+// coverReport reports the coverage percentage and writes a coverage profile if requested.
+func coverReport() {
+	if goexperiment.CoverageRedesign {
+		coverReport2()
+		return
+	}
+	var f *os.File
+	var err error
+	if *coverProfile != "" {
+		f, err = os.Create(toOutputDir(*coverProfile))
+		mustBeNil(err)
+		fmt.Fprintf(f, "mode: %s\n", cover.Mode)
+		defer func() { mustBeNil(f.Close()) }()
+	}
+
+	var active, total int64
+	var count uint32
+	for name, counts := range cover.Counters {
+		blocks := cover.Blocks[name]
+		for i := range counts {
+			stmts := int64(blocks[i].Stmts)
+			total += stmts
+			count = atomic.LoadUint32(&counts[i]) // For -mode=atomic.
+			if count > 0 {
+				active += stmts
+			}
+			if f != nil {
+				_, err := fmt.Fprintf(f, "%s:%d.%d,%d.%d %d %d\n", name,
+					blocks[i].Line0, blocks[i].Col0,
+					blocks[i].Line1, blocks[i].Col1,
+					stmts,
+					count)
+				mustBeNil(err)
+			}
+		}
+	}
+	if total == 0 {
+		fmt.Println("coverage: [no statements]")
+		return
+	}
+	fmt.Printf("coverage: %.1f%% of statements%s\n", 100*float64(active)/float64(total), cover.CoveredPackages)
+}

src/testing/example.go

@@ -0,0 +1,97 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"fmt"
+	"slices"
+	"strings"
+	"time"
+)
+
+type InternalExample struct {
+	Name      string
+	F         func()
+	Output    string
+	Unordered bool
+}
+
+// RunExamples is an internal function but exported because it is cross-package;
+// it is part of the implementation of the "go test" command.
+func RunExamples(matchString func(pat, str string) (bool, error), examples []InternalExample) (ok bool) {
+	_, ok = runExamples(matchString, examples)
+	return ok
+}
+
+func runExamples(matchString func(pat, str string) (bool, error), examples []InternalExample) (ran, ok bool) {
+	ok = true
+
+	m := newMatcher(matchString, *match, "-test.run", *skip)
+
+	var eg InternalExample
+	for _, eg = range examples {
+		_, matched, _ := m.fullName(nil, eg.Name)
+		if !matched {
+			continue
+		}
+		ran = true
+		if !runExample(eg) {
+			ok = false
+		}
+	}
+
+	return ran, ok
+}
+
+func sortLines(output string) string {
+	lines := strings.Split(output, "\n")
+	slices.Sort(lines)
+	return strings.Join(lines, "\n")
+}
+
+// processRunResult computes a summary and status of the result of running an example test.
+// stdout is the captured output from stdout of the test.
+// recovered is the result of invoking recover after running the test, in case it panicked.
+//
+// If stdout doesn't match the expected output or if recovered is non-nil, it'll print the cause of failure to stdout.
+// If the test is chatty/verbose, it'll print a success message to stdout.
+// If recovered is non-nil, it'll panic with that value.
+// If the test panicked with nil, or invoked runtime.Goexit, it'll be
+// made to fail and panic with errNilPanicOrGoexit
+func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Duration, finished bool, recovered any) (passed bool) {
+	passed = true
+	dstr := fmtDuration(timeSpent)
+	var fail string
+	got := strings.TrimSpace(stdout)
+	want := strings.TrimSpace(eg.Output)
+	if eg.Unordered {
+		if sortLines(got) != sortLines(want) && recovered == nil {
+			fail = fmt.Sprintf("got:\n%s\nwant (unordered):\n%s\n", stdout, eg.Output)
+		}
+	} else {
+		if got != want && recovered == nil {
+			fail = fmt.Sprintf("got:\n%s\nwant:\n%s\n", got, want)
+		}
+	}
+	if fail != "" || !finished || recovered != nil {
+		fmt.Printf("%s--- FAIL: %s (%s)\n%s", chatty.prefix(), eg.Name, dstr, fail)
+		passed = false
+	} else if chatty.on {
+		fmt.Printf("%s--- PASS: %s (%s)\n", chatty.prefix(), eg.Name, dstr)
+	}
+
+	if chatty.on && chatty.json {
+		fmt.Printf("%s=== NAME   %s\n", chatty.prefix(), "")
+	}
+
+	if recovered != nil {
+		// Propagate the previously recovered result, by panicking.
+		panic(recovered)
+	} else if !finished {
+		panic(errNilPanicOrGoexit)
+	}
+
+	return
+}

src/testing/export_test.go

@@ -0,0 +1,11 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+var PrettyPrint = prettyPrint
+
+type HighPrecisionTime = highPrecisionTime
+
+var HighPrecisionTimeNow = highPrecisionTimeNow

src/testing/flag_test.go

@@ -0,0 +1,89 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"flag"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+var testFlagArg = flag.String("test_flag_arg", "", "TestFlag: passing -v option")
+
+const flagTestEnv = "GO_WANT_FLAG_HELPER_PROCESS"
+
+func TestFlag(t *testing.T) {
+	if os.Getenv(flagTestEnv) == "1" {
+		testFlagHelper(t)
+		return
+	}
+
+	testenv.MustHaveExec(t)
+
+	for _, flag := range []string{"", "-test.v", "-test.v=test2json"} {
+		flag := flag
+		t.Run(flag, func(t *testing.T) {
+			t.Parallel()
+			exe, err := os.Executable()
+			if err != nil {
+				exe = os.Args[0]
+			}
+			cmd := exec.Command(exe, "-test.run=^TestFlag$", "-test_flag_arg="+flag)
+			if flag != "" {
+				cmd.Args = append(cmd.Args, flag)
+			}
+			cmd.Env = append(cmd.Environ(), flagTestEnv+"=1")
+			b, err := cmd.CombinedOutput()
+			if len(b) > 0 {
+				// When we set -test.v=test2json, we need to escape the ^V control
+				// character used for JSON framing so that the JSON parser doesn't
+				// misinterpret the subprocess output as output from the parent test.
+				t.Logf("%q", b)
+			}
+			if err != nil {
+				t.Error(err)
+			}
+		})
+	}
+}
+
+// testFlagHelper is called by the TestFlagHelper subprocess.
+func testFlagHelper(t *testing.T) {
+	f := flag.Lookup("test.v")
+	if f == nil {
+		t.Fatal(`flag.Lookup("test.v") failed`)
+	}
+
+	bf, ok := f.Value.(interface{ IsBoolFlag() bool })
+	if !ok {
+		t.Errorf("test.v flag (type %T) does not have IsBoolFlag method", f)
+	} else if !bf.IsBoolFlag() {
+		t.Error("test.v IsBoolFlag() returned false")
+	}
+
+	gf, ok := f.Value.(flag.Getter)
+	if !ok {
+		t.Fatalf("test.v flag (type %T) does not have Get method", f)
+	}
+	v := gf.Get()
+
+	var want any
+	switch *testFlagArg {
+	case "":
+		want = false
+	case "-test.v":
+		want = true
+	case "-test.v=test2json":
+		want = "test2json"
+	default:
+		t.Fatalf("unexpected test_flag_arg %q", *testFlagArg)
+	}
+
+	if v != want {
+		t.Errorf("test.v is %v want %v", v, want)
+	}
+}

src/testing/fstest/mapfs.go

@@ -0,0 +1,244 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"io"
+	"io/fs"
+	"path"
+	"slices"
+	"strings"
+	"time"
+)
+
+// A MapFS is a simple in-memory file system for use in tests,
+// represented as a map from path names (arguments to Open)
+// to information about the files or directories they represent.
+//
+// The map need not include parent directories for files contained
+// in the map; those will be synthesized if needed.
+// But a directory can still be included by setting the [MapFile.Mode]'s [fs.ModeDir] bit;
+// this may be necessary for detailed control over the directory's [fs.FileInfo]
+// or to create an empty directory.
+//
+// File system operations read directly from the map,
+// so that the file system can be changed by editing the map as needed.
+// An implication is that file system operations must not run concurrently
+// with changes to the map, which would be a race.
+// Another implication is that opening or reading a directory requires
+// iterating over the entire map, so a MapFS should typically be used with not more
+// than a few hundred entries or directory reads.
+type MapFS map[string]*MapFile
+
+// A MapFile describes a single file in a [MapFS].
+type MapFile struct {
+	Data    []byte      // file content
+	Mode    fs.FileMode // fs.FileInfo.Mode
+	ModTime time.Time   // fs.FileInfo.ModTime
+	Sys     any         // fs.FileInfo.Sys
+}
+
+var _ fs.FS = MapFS(nil)
+var _ fs.File = (*openMapFile)(nil)
+
+// Open opens the named file.
+func (fsys MapFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	file := fsys[name]
+	if file != nil && file.Mode&fs.ModeDir == 0 {
+		// Ordinary file
+		return &openMapFile{name, mapFileInfo{path.Base(name), file}, 0}, nil
+	}
+
+	// Directory, possibly synthesized.
+	// Note that file can be nil here: the map need not contain explicit parent directories for all its files.
+	// But file can also be non-nil, in case the user wants to set metadata for the directory explicitly.
+	// Either way, we need to construct the list of children of this directory.
+	var list []mapFileInfo
+	var elem string
+	var need = make(map[string]bool)
+	if name == "." {
+		elem = "."
+		for fname, f := range fsys {
+			i := strings.Index(fname, "/")
+			if i < 0 {
+				if fname != "." {
+					list = append(list, mapFileInfo{fname, f})
+				}
+			} else {
+				need[fname[:i]] = true
+			}
+		}
+	} else {
+		elem = name[strings.LastIndex(name, "/")+1:]
+		prefix := name + "/"
+		for fname, f := range fsys {
+			if strings.HasPrefix(fname, prefix) {
+				felem := fname[len(prefix):]
+				i := strings.Index(felem, "/")
+				if i < 0 {
+					list = append(list, mapFileInfo{felem, f})
+				} else {
+					need[fname[len(prefix):len(prefix)+i]] = true
+				}
+			}
+		}
+		// If the directory name is not in the map,
+		// and there are no children of the name in the map,
+		// then the directory is treated as not existing.
+		if file == nil && list == nil && len(need) == 0 {
+			return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+		}
+	}
+	for _, fi := range list {
+		delete(need, fi.name)
+	}
+	for name := range need {
+		list = append(list, mapFileInfo{name, &MapFile{Mode: fs.ModeDir | 0555}})
+	}
+	slices.SortFunc(list, func(a, b mapFileInfo) int {
+		return strings.Compare(a.name, b.name)
+	})
+
+	if file == nil {
+		file = &MapFile{Mode: fs.ModeDir | 0555}
+	}
+	return &mapDir{name, mapFileInfo{elem, file}, list, 0}, nil
+}
+
+// fsOnly is a wrapper that hides all but the fs.FS methods,
+// to avoid an infinite recursion when implementing special
+// methods in terms of helpers that would use them.
+// (In general, implementing these methods using the package fs helpers
+// is redundant and unnecessary, but having the methods may make
+// MapFS exercise more code paths when used in tests.)
+type fsOnly struct{ fs.FS }
+
+func (fsys MapFS) ReadFile(name string) ([]byte, error) {
+	return fs.ReadFile(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Stat(name string) (fs.FileInfo, error) {
+	return fs.Stat(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	return fs.ReadDir(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Glob(pattern string) ([]string, error) {
+	return fs.Glob(fsOnly{fsys}, pattern)
+}
+
+type noSub struct {
+	MapFS
+}
+
+func (noSub) Sub() {} // not the fs.SubFS signature
+
+func (fsys MapFS) Sub(dir string) (fs.FS, error) {
+	return fs.Sub(noSub{fsys}, dir)
+}
+
+// A mapFileInfo implements fs.FileInfo and fs.DirEntry for a given map file.
+type mapFileInfo struct {
+	name string
+	f    *MapFile
+}
+
+func (i *mapFileInfo) Name() string               { return path.Base(i.name) }
+func (i *mapFileInfo) Size() int64                { return int64(len(i.f.Data)) }
+func (i *mapFileInfo) Mode() fs.FileMode          { return i.f.Mode }
+func (i *mapFileInfo) Type() fs.FileMode          { return i.f.Mode.Type() }
+func (i *mapFileInfo) ModTime() time.Time         { return i.f.ModTime }
+func (i *mapFileInfo) IsDir() bool                { return i.f.Mode&fs.ModeDir != 0 }
+func (i *mapFileInfo) Sys() any                   { return i.f.Sys }
+func (i *mapFileInfo) Info() (fs.FileInfo, error) { return i, nil }
+
+func (i *mapFileInfo) String() string {
+	return fs.FormatFileInfo(i)
+}
+
+// An openMapFile is a regular (non-directory) fs.File open for reading.
+type openMapFile struct {
+	path string
+	mapFileInfo
+	offset int64
+}
+
+func (f *openMapFile) Stat() (fs.FileInfo, error) { return &f.mapFileInfo, nil }
+
+func (f *openMapFile) Close() error { return nil }
+
+func (f *openMapFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.f.Data)) {
+		return 0, io.EOF
+	}
+	if f.offset < 0 {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *openMapFile) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case 0:
+		// offset += 0
+	case 1:
+		offset += f.offset
+	case 2:
+		offset += int64(len(f.f.Data))
+	}
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}
+	}
+	f.offset = offset
+	return offset, nil
+}
+
+func (f *openMapFile) ReadAt(b []byte, offset int64) (int, error) {
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[offset:])
+	if n < len(b) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// A mapDir is a directory fs.File (so also an fs.ReadDirFile) open for reading.
+type mapDir struct {
+	path string
+	mapFileInfo
+	entry  []mapFileInfo
+	offset int
+}
+
+func (d *mapDir) Stat() (fs.FileInfo, error) { return &d.mapFileInfo, nil }
+func (d *mapDir) Close() error               { return nil }
+func (d *mapDir) Read(b []byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.path, Err: fs.ErrInvalid}
+}
+
+func (d *mapDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.entry) - d.offset
+	if n == 0 && count > 0 {
+		return nil, io.EOF
+	}
+	if count > 0 && n > count {
+		n = count
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = &d.entry[d.offset+i]
+	}
+	d.offset += n
+	return list, nil
+}

src/testing/fstest/mapfs_test.go

@@ -0,0 +1,59 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"fmt"
+	"io/fs"
+	"strings"
+	"testing"
+)
+
+func TestMapFS(t *testing.T) {
+	m := MapFS{
+		"hello":             {Data: []byte("hello, world\n")},
+		"fortune/k/ken.txt": {Data: []byte("If a program is too slow, it must have a loop.\n")},
+	}
+	if err := TestFS(m, "hello", "fortune", "fortune/k", "fortune/k/ken.txt"); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMapFSChmodDot(t *testing.T) {
+	m := MapFS{
+		"a/b.txt": &MapFile{Mode: 0666},
+		".":       &MapFile{Mode: 0777 | fs.ModeDir},
+	}
+	buf := new(strings.Builder)
+	fs.WalkDir(m, ".", func(path string, d fs.DirEntry, err error) error {
+		fi, err := d.Info()
+		if err != nil {
+			return err
+		}
+		fmt.Fprintf(buf, "%s: %v\n", path, fi.Mode())
+		return nil
+	})
+	want := `
+.: drwxrwxrwx
+a: dr-xr-xr-x
+a/b.txt: -rw-rw-rw-
+`[1:]
+	got := buf.String()
+	if want != got {
+		t.Errorf("MapFS modes want:\n%s\ngot:\n%s\n", want, got)
+	}
+}
+
+func TestMapFSFileInfoName(t *testing.T) {
+	m := MapFS{
+		"path/to/b.txt": &MapFile{},
+	}
+	info, _ := m.Stat("path/to/b.txt")
+	want := "b.txt"
+	got := info.Name()
+	if want != got {
+		t.Errorf("MapFS FileInfo.Name want:\n%s\ngot:\n%s\n", want, got)
+	}
+}

src/testing/fstest/testfs.go

@@ -0,0 +1,621 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fstest implements support for testing implementations and users of file systems.
+package fstest
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"path"
+	"reflect"
+	"slices"
+	"strings"
+	"testing/iotest"
+)
+
+// TestFS tests a file system implementation.
+// It walks the entire tree of files in fsys,
+// opening and checking that each file behaves correctly.
+// It also checks that the file system contains at least the expected files.
+// As a special case, if no expected files are listed, fsys must be empty.
+// Otherwise, fsys must contain at least the listed files; it can also contain others.
+// The contents of fsys must not change concurrently with TestFS.
+//
+// If TestFS finds any misbehaviors, it returns either the first error or a
+// list of errors. Use [errors.Is] or [errors.As] to inspect.
+//
+// Typical usage inside a test is:
+//
+//	if err := fstest.TestFS(myFS, "file/that/should/be/present"); err != nil {
+//		t.Fatal(err)
+//	}
+func TestFS(fsys fs.FS, expected ...string) error {
+	if err := testFS(fsys, expected...); err != nil {
+		return err
+	}
+	for _, name := range expected {
+		if i := strings.Index(name, "/"); i >= 0 {
+			dir, dirSlash := name[:i], name[:i+1]
+			var subExpected []string
+			for _, name := range expected {
+				if strings.HasPrefix(name, dirSlash) {
+					subExpected = append(subExpected, name[len(dirSlash):])
+				}
+			}
+			sub, err := fs.Sub(fsys, dir)
+			if err != nil {
+				return err
+			}
+			if err := testFS(sub, subExpected...); err != nil {
+				return fmt.Errorf("testing fs.Sub(fsys, %s): %w", dir, err)
+			}
+			break // one sub-test is enough
+		}
+	}
+	return nil
+}
+
+func testFS(fsys fs.FS, expected ...string) error {
+	t := fsTester{fsys: fsys}
+	t.checkDir(".")
+	t.checkOpen(".")
+	found := make(map[string]bool)
+	for _, dir := range t.dirs {
+		found[dir] = true
+	}
+	for _, file := range t.files {
+		found[file] = true
+	}
+	delete(found, ".")
+	if len(expected) == 0 && len(found) > 0 {
+		var list []string
+		for k := range found {
+			if k != "." {
+				list = append(list, k)
+			}
+		}
+		slices.Sort(list)
+		if len(list) > 15 {
+			list = append(list[:10], "...")
+		}
+		t.errorf("expected empty file system but found files:\n%s", strings.Join(list, "\n"))
+	}
+	for _, name := range expected {
+		if !found[name] {
+			t.errorf("expected but not found: %s", name)
+		}
+	}
+	if len(t.errors) == 0 {
+		return nil
+	}
+	return fmt.Errorf("TestFS found errors:\n%w", errors.Join(t.errors...))
+}
+
+// An fsTester holds state for running the test.
+type fsTester struct {
+	fsys   fs.FS
+	errors []error
+	dirs   []string
+	files  []string
+}
+
+// errorf adds an error to the list of errors.
+func (t *fsTester) errorf(format string, args ...any) {
+	t.errors = append(t.errors, fmt.Errorf(format, args...))
+}
+
+func (t *fsTester) openDir(dir string) fs.ReadDirFile {
+	f, err := t.fsys.Open(dir)
+	if err != nil {
+		t.errorf("%s: Open: %w", dir, err)
+		return nil
+	}
+	d, ok := f.(fs.ReadDirFile)
+	if !ok {
+		f.Close()
+		t.errorf("%s: Open returned File type %T, not a fs.ReadDirFile", dir, f)
+		return nil
+	}
+	return d
+}
+
+// checkDir checks the directory dir, which is expected to exist
+// (it is either the root or was found in a directory listing with IsDir true).
+func (t *fsTester) checkDir(dir string) {
+	// Read entire directory.
+	t.dirs = append(t.dirs, dir)
+	d := t.openDir(dir)
+	if d == nil {
+		return
+	}
+	list, err := d.ReadDir(-1)
+	if err != nil {
+		d.Close()
+		t.errorf("%s: ReadDir(-1): %w", dir, err)
+		return
+	}
+
+	// Check all children.
+	var prefix string
+	if dir == "." {
+		prefix = ""
+	} else {
+		prefix = dir + "/"
+	}
+	for _, info := range list {
+		name := info.Name()
+		switch {
+		case name == ".", name == "..", name == "":
+			t.errorf("%s: ReadDir: child has invalid name: %#q", dir, name)
+			continue
+		case strings.Contains(name, "/"):
+			t.errorf("%s: ReadDir: child name contains slash: %#q", dir, name)
+			continue
+		case strings.Contains(name, `\`):
+			t.errorf("%s: ReadDir: child name contains backslash: %#q", dir, name)
+			continue
+		}
+		path := prefix + name
+		t.checkStat(path, info)
+		t.checkOpen(path)
+		if info.IsDir() {
+			t.checkDir(path)
+		} else {
+			t.checkFile(path)
+		}
+	}
+
+	// Check ReadDir(-1) at EOF.
+	list2, err := d.ReadDir(-1)
+	if len(list2) > 0 || err != nil {
+		d.Close()
+		t.errorf("%s: ReadDir(-1) at EOF = %d entries, %w, wanted 0 entries, nil", dir, len(list2), err)
+		return
+	}
+
+	// Check ReadDir(1) at EOF (different results).
+	list2, err = d.ReadDir(1)
+	if len(list2) > 0 || err != io.EOF {
+		d.Close()
+		t.errorf("%s: ReadDir(1) at EOF = %d entries, %w, wanted 0 entries, EOF", dir, len(list2), err)
+		return
+	}
+
+	// Check that close does not report an error.
+	if err := d.Close(); err != nil {
+		t.errorf("%s: Close: %w", dir, err)
+	}
+
+	// Check that closing twice doesn't crash.
+	// The return value doesn't matter.
+	d.Close()
+
+	// Reopen directory, read a second time, make sure contents match.
+	if d = t.openDir(dir); d == nil {
+		return
+	}
+	defer d.Close()
+	list2, err = d.ReadDir(-1)
+	if err != nil {
+		t.errorf("%s: second Open+ReadDir(-1): %w", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs second Open+ReadDir(-1)", list, list2)
+
+	// Reopen directory, read a third time in pieces, make sure contents match.
+	if d = t.openDir(dir); d == nil {
+		return
+	}
+	defer d.Close()
+	list2 = nil
+	for {
+		n := 1
+		if len(list2) > 0 {
+			n = 2
+		}
+		frag, err := d.ReadDir(n)
+		if len(frag) > n {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: %d entries (too many)", dir, n, len(list2), len(frag))
+			return
+		}
+		list2 = append(list2, frag...)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: %w", dir, n, len(list2), err)
+			return
+		}
+		if n == 0 {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: 0 entries but nil error", dir, n, len(list2))
+			return
+		}
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs third Open+ReadDir(1,2) loop", list, list2)
+
+	// If fsys has ReadDir, check that it matches and is sorted.
+	if fsys, ok := t.fsys.(fs.ReadDirFS); ok {
+		list2, err := fsys.ReadDir(dir)
+		if err != nil {
+			t.errorf("%s: fsys.ReadDir: %w", dir, err)
+			return
+		}
+		t.checkDirList(dir, "first Open+ReadDir(-1) vs fsys.ReadDir", list, list2)
+
+		for i := 0; i+1 < len(list2); i++ {
+			if list2[i].Name() >= list2[i+1].Name() {
+				t.errorf("%s: fsys.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+			}
+		}
+	}
+
+	// Check fs.ReadDir as well.
+	list2, err = fs.ReadDir(t.fsys, dir)
+	if err != nil {
+		t.errorf("%s: fs.ReadDir: %w", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs fs.ReadDir", list, list2)
+
+	for i := 0; i+1 < len(list2); i++ {
+		if list2[i].Name() >= list2[i+1].Name() {
+			t.errorf("%s: fs.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+		}
+	}
+
+	t.checkGlob(dir, list2)
+}
+
+// formatEntry formats an fs.DirEntry into a string for error messages and comparison.
+func formatEntry(entry fs.DirEntry) string {
+	return fmt.Sprintf("%s IsDir=%v Type=%v", entry.Name(), entry.IsDir(), entry.Type())
+}
+
+// formatInfoEntry formats an fs.FileInfo into a string like the result of formatEntry, for error messages and comparison.
+func formatInfoEntry(info fs.FileInfo) string {
+	return fmt.Sprintf("%s IsDir=%v Type=%v", info.Name(), info.IsDir(), info.Mode().Type())
+}
+
+// formatInfo formats an fs.FileInfo into a string for error messages and comparison.
+func formatInfo(info fs.FileInfo) string {
+	return fmt.Sprintf("%s IsDir=%v Mode=%v Size=%d ModTime=%v", info.Name(), info.IsDir(), info.Mode(), info.Size(), info.ModTime())
+}
+
+// checkGlob checks that various glob patterns work if the file system implements GlobFS.
+func (t *fsTester) checkGlob(dir string, list []fs.DirEntry) {
+	if _, ok := t.fsys.(fs.GlobFS); !ok {
+		return
+	}
+
+	// Make a complex glob pattern prefix that only matches dir.
+	var glob string
+	if dir != "." {
+		elem := strings.Split(dir, "/")
+		for i, e := range elem {
+			var pattern []rune
+			for j, r := range e {
+				if r == '*' || r == '?' || r == '\\' || r == '[' || r == '-' {
+					pattern = append(pattern, '\\', r)
+					continue
+				}
+				switch (i + j) % 5 {
+				case 0:
+					pattern = append(pattern, r)
+				case 1:
+					pattern = append(pattern, '[', r, ']')
+				case 2:
+					pattern = append(pattern, '[', r, '-', r, ']')
+				case 3:
+					pattern = append(pattern, '[', '\\', r, ']')
+				case 4:
+					pattern = append(pattern, '[', '\\', r, '-', '\\', r, ']')
+				}
+			}
+			elem[i] = string(pattern)
+		}
+		glob = strings.Join(elem, "/") + "/"
+	}
+
+	// Test that malformed patterns are detected.
+	// The error is likely path.ErrBadPattern but need not be.
+	if _, err := t.fsys.(fs.GlobFS).Glob(glob + "nonexist/[]"); err == nil {
+		t.errorf("%s: Glob(%#q): bad pattern not detected", dir, glob+"nonexist/[]")
+	}
+
+	// Try to find a letter that appears in only some of the final names.
+	c := rune('a')
+	for ; c <= 'z'; c++ {
+		have, haveNot := false, false
+		for _, d := range list {
+			if strings.ContainsRune(d.Name(), c) {
+				have = true
+			} else {
+				haveNot = true
+			}
+		}
+		if have && haveNot {
+			break
+		}
+	}
+	if c > 'z' {
+		c = 'a'
+	}
+	glob += "*" + string(c) + "*"
+
+	var want []string
+	for _, d := range list {
+		if strings.ContainsRune(d.Name(), c) {
+			want = append(want, path.Join(dir, d.Name()))
+		}
+	}
+
+	names, err := t.fsys.(fs.GlobFS).Glob(glob)
+	if err != nil {
+		t.errorf("%s: Glob(%#q): %w", dir, glob, err)
+		return
+	}
+	if reflect.DeepEqual(want, names) {
+		return
+	}
+
+	if !slices.IsSorted(names) {
+		t.errorf("%s: Glob(%#q): unsorted output:\n%s", dir, glob, strings.Join(names, "\n"))
+		slices.Sort(names)
+	}
+
+	var problems []string
+	for len(want) > 0 || len(names) > 0 {
+		switch {
+		case len(want) > 0 && len(names) > 0 && want[0] == names[0]:
+			want, names = want[1:], names[1:]
+		case len(want) > 0 && (len(names) == 0 || want[0] < names[0]):
+			problems = append(problems, "missing: "+want[0])
+			want = want[1:]
+		default:
+			problems = append(problems, "extra: "+names[0])
+			names = names[1:]
+		}
+	}
+	t.errorf("%s: Glob(%#q): wrong output:\n%s", dir, glob, strings.Join(problems, "\n"))
+}
+
+// checkStat checks that a direct stat of path matches entry,
+// which was found in the parent's directory listing.
+func (t *fsTester) checkStat(path string, entry fs.DirEntry) {
+	file, err := t.fsys.Open(path)
+	if err != nil {
+		t.errorf("%s: Open: %w", path, err)
+		return
+	}
+	info, err := file.Stat()
+	file.Close()
+	if err != nil {
+		t.errorf("%s: Stat: %w", path, err)
+		return
+	}
+	fentry := formatEntry(entry)
+	fientry := formatInfoEntry(info)
+	// Note: mismatch here is OK for symlink, because Open dereferences symlink.
+	if fentry != fientry && entry.Type()&fs.ModeSymlink == 0 {
+		t.errorf("%s: mismatch:\n\tentry = %s\n\tfile.Stat() = %s", path, fentry, fientry)
+	}
+
+	einfo, err := entry.Info()
+	if err != nil {
+		t.errorf("%s: entry.Info: %w", path, err)
+		return
+	}
+	finfo := formatInfo(info)
+	if entry.Type()&fs.ModeSymlink != 0 {
+		// For symlink, just check that entry.Info matches entry on common fields.
+		// Open deferences symlink, so info itself may differ.
+		feentry := formatInfoEntry(einfo)
+		if fentry != feentry {
+			t.errorf("%s: mismatch\n\tentry = %s\n\tentry.Info() = %s\n", path, fentry, feentry)
+		}
+	} else {
+		feinfo := formatInfo(einfo)
+		if feinfo != finfo {
+			t.errorf("%s: mismatch:\n\tentry.Info() = %s\n\tfile.Stat() = %s\n", path, feinfo, finfo)
+		}
+	}
+
+	// Stat should be the same as Open+Stat, even for symlinks.
+	info2, err := fs.Stat(t.fsys, path)
+	if err != nil {
+		t.errorf("%s: fs.Stat: %w", path, err)
+		return
+	}
+	finfo2 := formatInfo(info2)
+	if finfo2 != finfo {
+		t.errorf("%s: fs.Stat(...) = %s\n\twant %s", path, finfo2, finfo)
+	}
+
+	if fsys, ok := t.fsys.(fs.StatFS); ok {
+		info2, err := fsys.Stat(path)
+		if err != nil {
+			t.errorf("%s: fsys.Stat: %w", path, err)
+			return
+		}
+		finfo2 := formatInfo(info2)
+		if finfo2 != finfo {
+			t.errorf("%s: fsys.Stat(...) = %s\n\twant %s", path, finfo2, finfo)
+		}
+	}
+}
+
+// checkDirList checks that two directory lists contain the same files and file info.
+// The order of the lists need not match.
+func (t *fsTester) checkDirList(dir, desc string, list1, list2 []fs.DirEntry) {
+	old := make(map[string]fs.DirEntry)
+	checkMode := func(entry fs.DirEntry) {
+		if entry.IsDir() != (entry.Type()&fs.ModeDir != 0) {
+			if entry.IsDir() {
+				t.errorf("%s: ReadDir returned %s with IsDir() = true, Type() & ModeDir = 0", dir, entry.Name())
+			} else {
+				t.errorf("%s: ReadDir returned %s with IsDir() = false, Type() & ModeDir = ModeDir", dir, entry.Name())
+			}
+		}
+	}
+
+	for _, entry1 := range list1 {
+		old[entry1.Name()] = entry1
+		checkMode(entry1)
+	}
+
+	var diffs []string
+	for _, entry2 := range list2 {
+		entry1 := old[entry2.Name()]
+		if entry1 == nil {
+			checkMode(entry2)
+			diffs = append(diffs, "+ "+formatEntry(entry2))
+			continue
+		}
+		if formatEntry(entry1) != formatEntry(entry2) {
+			diffs = append(diffs, "- "+formatEntry(entry1), "+ "+formatEntry(entry2))
+		}
+		delete(old, entry2.Name())
+	}
+	for _, entry1 := range old {
+		diffs = append(diffs, "- "+formatEntry(entry1))
+	}
+
+	if len(diffs) == 0 {
+		return
+	}
+
+	slices.SortFunc(diffs, func(a, b string) int {
+		fa := strings.Fields(a)
+		fb := strings.Fields(b)
+		// sort by name (i < j) and then +/- (j < i, because + < -)
+		return strings.Compare(fa[1]+" "+fb[0], fb[1]+" "+fa[0])
+	})
+
+	t.errorf("%s: diff %s:\n\t%s", dir, desc, strings.Join(diffs, "\n\t"))
+}
+
+// checkFile checks that basic file reading works correctly.
+func (t *fsTester) checkFile(file string) {
+	t.files = append(t.files, file)
+
+	// Read entire file.
+	f, err := t.fsys.Open(file)
+	if err != nil {
+		t.errorf("%s: Open: %w", file, err)
+		return
+	}
+
+	data, err := io.ReadAll(f)
+	if err != nil {
+		f.Close()
+		t.errorf("%s: Open+ReadAll: %w", file, err)
+		return
+	}
+
+	if err := f.Close(); err != nil {
+		t.errorf("%s: Close: %w", file, err)
+	}
+
+	// Check that closing twice doesn't crash.
+	// The return value doesn't matter.
+	f.Close()
+
+	// Check that ReadFile works if present.
+	if fsys, ok := t.fsys.(fs.ReadFileFS); ok {
+		data2, err := fsys.ReadFile(file)
+		if err != nil {
+			t.errorf("%s: fsys.ReadFile: %w", file, err)
+			return
+		}
+		t.checkFileRead(file, "ReadAll vs fsys.ReadFile", data, data2)
+
+		// Modify the data and check it again. Modifying the
+		// returned byte slice should not affect the next call.
+		for i := range data2 {
+			data2[i]++
+		}
+		data2, err = fsys.ReadFile(file)
+		if err != nil {
+			t.errorf("%s: second call to fsys.ReadFile: %w", file, err)
+			return
+		}
+		t.checkFileRead(file, "Readall vs second fsys.ReadFile", data, data2)
+
+		t.checkBadPath(file, "ReadFile",
+			func(name string) error { _, err := fsys.ReadFile(name); return err })
+	}
+
+	// Check that fs.ReadFile works with t.fsys.
+	data2, err := fs.ReadFile(t.fsys, file)
+	if err != nil {
+		t.errorf("%s: fs.ReadFile: %w", file, err)
+		return
+	}
+	t.checkFileRead(file, "ReadAll vs fs.ReadFile", data, data2)
+
+	// Use iotest.TestReader to check small reads, Seek, ReadAt.
+	f, err = t.fsys.Open(file)
+	if err != nil {
+		t.errorf("%s: second Open: %w", file, err)
+		return
+	}
+	defer f.Close()
+	if err := iotest.TestReader(f, data); err != nil {
+		t.errorf("%s: failed TestReader:\n\t%s", file, strings.ReplaceAll(err.Error(), "\n", "\n\t"))
+	}
+}
+
+func (t *fsTester) checkFileRead(file, desc string, data1, data2 []byte) {
+	if string(data1) != string(data2) {
+		t.errorf("%s: %s: different data returned\n\t%q\n\t%q", file, desc, data1, data2)
+		return
+	}
+}
+
+// checkBadPath checks that various invalid forms of file's name cannot be opened using t.fsys.Open.
+func (t *fsTester) checkOpen(file string) {
+	t.checkBadPath(file, "Open", func(file string) error {
+		f, err := t.fsys.Open(file)
+		if err == nil {
+			f.Close()
+		}
+		return err
+	})
+}
+
+// checkBadPath checks that various invalid forms of file's name cannot be opened using open.
+func (t *fsTester) checkBadPath(file string, desc string, open func(string) error) {
+	bad := []string{
+		"/" + file,
+		file + "/.",
+	}
+	if file == "." {
+		bad = append(bad, "/")
+	}
+	if i := strings.Index(file, "/"); i >= 0 {
+		bad = append(bad,
+			file[:i]+"//"+file[i+1:],
+			file[:i]+"/./"+file[i+1:],
+			file[:i]+`\`+file[i+1:],
+			file[:i]+"/../"+file,
+		)
+	}
+	if i := strings.LastIndex(file, "/"); i >= 0 {
+		bad = append(bad,
+			file[:i]+"//"+file[i+1:],
+			file[:i]+"/./"+file[i+1:],
+			file[:i]+`\`+file[i+1:],
+			file+"/../"+file[i+1:],
+		)
+	}
+
+	for _, b := range bad {
+		if err := open(b); err == nil {
+			t.errorf("%s: %s(%s) succeeded, want error", file, desc, b)
+		}
+	}
+}

src/testing/fstest/testfs_test.go

@@ -0,0 +1,117 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"errors"
+	"internal/testenv"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+)
+
+func TestSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	tmp := t.TempDir()
+	tmpfs := os.DirFS(tmp)
+
+	if err := os.WriteFile(filepath.Join(tmp, "hello"), []byte("hello, world\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.Symlink(filepath.Join(tmp, "hello"), filepath.Join(tmp, "hello.link")); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := TestFS(tmpfs, "hello", "hello.link"); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestDash(t *testing.T) {
+	m := MapFS{
+		"a-b/a": {Data: []byte("a-b/a")},
+	}
+	if err := TestFS(m, "a-b/a"); err != nil {
+		t.Error(err)
+	}
+}
+
+type shuffledFS MapFS
+
+func (fsys shuffledFS) Open(name string) (fs.File, error) {
+	f, err := MapFS(fsys).Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return &shuffledFile{File: f}, nil
+}
+
+type shuffledFile struct{ fs.File }
+
+func (f *shuffledFile) ReadDir(n int) ([]fs.DirEntry, error) {
+	dirents, err := f.File.(fs.ReadDirFile).ReadDir(n)
+	// Shuffle in a deterministic way, all we care about is making sure that the
+	// list of directory entries is not is the lexicographic order.
+	//
+	// We do this to make sure that the TestFS test suite is not affected by the
+	// order of directory entries.
+	slices.SortFunc(dirents, func(a, b fs.DirEntry) int {
+		return strings.Compare(b.Name(), a.Name())
+	})
+	return dirents, err
+}
+
+func TestShuffledFS(t *testing.T) {
+	fsys := shuffledFS{
+		"tmp/one":   {Data: []byte("1")},
+		"tmp/two":   {Data: []byte("2")},
+		"tmp/three": {Data: []byte("3")},
+	}
+	if err := TestFS(fsys, "tmp/one", "tmp/two", "tmp/three"); err != nil {
+		t.Error(err)
+	}
+}
+
+// failPermFS is a filesystem that always fails with fs.ErrPermission.
+type failPermFS struct{}
+
+func (f failPermFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrInvalid}
+	}
+	return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrPermission}
+}
+
+func TestTestFSWrappedErrors(t *testing.T) {
+	err := TestFS(failPermFS{})
+	if err == nil {
+		t.Fatal("error expected")
+	}
+	t.Logf("Error (expecting wrapped fs.ErrPermission):\n%v", err)
+
+	if !errors.Is(err, fs.ErrPermission) {
+		t.Errorf("error should be a wrapped ErrPermission: %#v", err)
+	}
+
+	// TestFS is expected to return a list of errors.
+	// Enforce that the list can be extracted for browsing.
+	var errs interface{ Unwrap() []error }
+	if !errors.As(err, &errs) {
+		t.Errorf("caller should be able to extract the errors as a list: %#v", err)
+	} else {
+		for _, err := range errs.Unwrap() {
+			// ErrPermission is expected
+			// but any other error must be reported.
+			if !errors.Is(err, fs.ErrPermission) {
+				t.Errorf("unexpected error: %v", err)
+			}
+		}
+	}
+}

src/testing/fuzz.go

@@ -0,0 +1,731 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"time"
+)
+
+func initFuzzFlags() {
+	matchFuzz = flag.String("test.fuzz", "", "run the fuzz test matching `regexp`")
+	flag.Var(&fuzzDuration, "test.fuzztime", "time to spend fuzzing; default is to run indefinitely")
+	flag.Var(&minimizeDuration, "test.fuzzminimizetime", "time to spend minimizing a value after finding a failing input")
+
+	fuzzCacheDir = flag.String("test.fuzzcachedir", "", "directory where interesting fuzzing inputs are stored (for use only by cmd/go)")
+	isFuzzWorker = flag.Bool("test.fuzzworker", false, "coordinate with the parent process to fuzz random values (for use only by cmd/go)")
+}
+
+var (
+	matchFuzz        *string
+	fuzzDuration     durationOrCountFlag
+	minimizeDuration = durationOrCountFlag{d: 60 * time.Second, allowZero: true}
+	fuzzCacheDir     *string
+	isFuzzWorker     *bool
+
+	// corpusDir is the parent directory of the fuzz test's seed corpus within
+	// the package.
+	corpusDir = "testdata/fuzz"
+)
+
+// fuzzWorkerExitCode is used as an exit code by fuzz worker processes after an
+// internal error. This distinguishes internal errors from uncontrolled panics
+// and other failures. Keep in sync with internal/fuzz.workerExitCode.
+const fuzzWorkerExitCode = 70
+
+// InternalFuzzTarget is an internal type but exported because it is
+// cross-package; it is part of the implementation of the "go test" command.
+type InternalFuzzTarget struct {
+	Name string
+	Fn   func(f *F)
+}
+
+// F is a type passed to fuzz tests.
+//
+// Fuzz tests run generated inputs against a provided fuzz target, which can
+// find and report potential bugs in the code being tested.
+//
+// A fuzz test runs the seed corpus by default, which includes entries provided
+// by (*F).Add and entries in the testdata/fuzz/<FuzzTestName> directory. After
+// any necessary setup and calls to (*F).Add, the fuzz test must then call
+// (*F).Fuzz to provide the fuzz target. See the testing package documentation
+// for an example, and see the [F.Fuzz] and [F.Add] method documentation for
+// details.
+//
+// *F methods can only be called before (*F).Fuzz. Once the test is
+// executing the fuzz target, only (*T) methods can be used. The only *F methods
+// that are allowed in the (*F).Fuzz function are (*F).Failed and (*F).Name.
+type F struct {
+	common
+	fuzzContext *fuzzContext
+	testContext *testContext
+
+	// inFuzzFn is true when the fuzz function is running. Most F methods cannot
+	// be called when inFuzzFn is true.
+	inFuzzFn bool
+
+	// corpus is a set of seed corpus entries, added with F.Add and loaded
+	// from testdata.
+	corpus []corpusEntry
+
+	result     fuzzResult
+	fuzzCalled bool
+}
+
+var _ TB = (*F)(nil)
+
+// corpusEntry is an alias to the same type as internal/fuzz.CorpusEntry.
+// We use a type alias because we don't want to export this type, and we can't
+// import internal/fuzz from testing.
+type corpusEntry = struct {
+	Parent     string
+	Path       string
+	Data       []byte
+	Values     []any
+	Generation int
+	IsSeed     bool
+}
+
+// Helper marks the calling function as a test helper function.
+// When printing file and line information, that function will be skipped.
+// Helper may be called simultaneously from multiple goroutines.
+func (f *F) Helper() {
+	if f.inFuzzFn {
+		panic("testing: f.Helper was called inside the fuzz target, use t.Helper instead")
+	}
+
+	// common.Helper is inlined here.
+	// If we called it, it would mark F.Helper as the helper
+	// instead of the caller.
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if f.helperPCs == nil {
+		f.helperPCs = make(map[uintptr]struct{})
+	}
+	// repeating code from callerName here to save walking a stack frame
+	var pc [1]uintptr
+	n := runtime.Callers(2, pc[:]) // skip runtime.Callers + Helper
+	if n == 0 {
+		panic("testing: zero callers found")
+	}
+	if _, found := f.helperPCs[pc[0]]; !found {
+		f.helperPCs[pc[0]] = struct{}{}
+		f.helperNames = nil // map will be recreated next time it is needed
+	}
+}
+
+// Fail marks the function as having failed but continues execution.
+func (f *F) Fail() {
+	// (*F).Fail may be called by (*T).Fail, which we should allow. However, we
+	// shouldn't allow direct (*F).Fail calls from inside the (*F).Fuzz function.
+	if f.inFuzzFn {
+		panic("testing: f.Fail was called inside the fuzz target, use t.Fail instead")
+	}
+	f.common.Helper()
+	f.common.Fail()
+}
+
+// Skipped reports whether the test was skipped.
+func (f *F) Skipped() bool {
+	// (*F).Skipped may be called by tRunner, which we should allow. However, we
+	// shouldn't allow direct (*F).Skipped calls from inside the (*F).Fuzz function.
+	if f.inFuzzFn {
+		panic("testing: f.Skipped was called inside the fuzz target, use t.Skipped instead")
+	}
+	f.common.Helper()
+	return f.common.Skipped()
+}
+
+// Add will add the arguments to the seed corpus for the fuzz test. This will be
+// a no-op if called after or within the fuzz target, and args must match the
+// arguments for the fuzz target.
+func (f *F) Add(args ...any) {
+	var values []any
+	for i := range args {
+		if t := reflect.TypeOf(args[i]); !supportedTypes[t] {
+			panic(fmt.Sprintf("testing: unsupported type to Add %v", t))
+		}
+		values = append(values, args[i])
+	}
+	f.corpus = append(f.corpus, corpusEntry{Values: values, IsSeed: true, Path: fmt.Sprintf("seed#%d", len(f.corpus))})
+}
+
+// supportedTypes represents all of the supported types which can be fuzzed.
+var supportedTypes = map[reflect.Type]bool{
+	reflect.TypeOf(([]byte)("")):  true,
+	reflect.TypeOf((string)("")):  true,
+	reflect.TypeOf((bool)(false)): true,
+	reflect.TypeOf((byte)(0)):     true,
+	reflect.TypeOf((rune)(0)):     true,
+	reflect.TypeOf((float32)(0)):  true,
+	reflect.TypeOf((float64)(0)):  true,
+	reflect.TypeOf((int)(0)):      true,
+	reflect.TypeOf((int8)(0)):     true,
+	reflect.TypeOf((int16)(0)):    true,
+	reflect.TypeOf((int32)(0)):    true,
+	reflect.TypeOf((int64)(0)):    true,
+	reflect.TypeOf((uint)(0)):     true,
+	reflect.TypeOf((uint8)(0)):    true,
+	reflect.TypeOf((uint16)(0)):   true,
+	reflect.TypeOf((uint32)(0)):   true,
+	reflect.TypeOf((uint64)(0)):   true,
+}
+
+// Fuzz runs the fuzz function, ff, for fuzz testing. If ff fails for a set of
+// arguments, those arguments will be added to the seed corpus.
+//
+// ff must be a function with no return value whose first argument is *T and
+// whose remaining arguments are the types to be fuzzed.
+// For example:
+//
+//	f.Fuzz(func(t *testing.T, b []byte, i int) { ... })
+//
+// The following types are allowed: []byte, string, bool, byte, rune, float32,
+// float64, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64.
+// More types may be supported in the future.
+//
+// ff must not call any *F methods, e.g. (*F).Log, (*F).Error, (*F).Skip. Use
+// the corresponding *T method instead. The only *F methods that are allowed in
+// the (*F).Fuzz function are (*F).Failed and (*F).Name.
+//
+// This function should be fast and deterministic, and its behavior should not
+// depend on shared state. No mutable input arguments, or pointers to them,
+// should be retained between executions of the fuzz function, as the memory
+// backing them may be mutated during a subsequent invocation. ff must not
+// modify the underlying data of the arguments provided by the fuzzing engine.
+//
+// When fuzzing, F.Fuzz does not return until a problem is found, time runs out
+// (set with -fuzztime), or the test process is interrupted by a signal. F.Fuzz
+// should be called exactly once, unless F.Skip or [F.Fail] is called beforehand.
+func (f *F) Fuzz(ff any) {
+	if f.fuzzCalled {
+		panic("testing: F.Fuzz called more than once")
+	}
+	f.fuzzCalled = true
+	if f.failed {
+		return
+	}
+	f.Helper()
+
+	// ff should be in the form func(*testing.T, ...interface{})
+	fn := reflect.ValueOf(ff)
+	fnType := fn.Type()
+	if fnType.Kind() != reflect.Func {
+		panic("testing: F.Fuzz must receive a function")
+	}
+	if fnType.NumIn() < 2 || fnType.In(0) != reflect.TypeOf((*T)(nil)) {
+		panic("testing: fuzz target must receive at least two arguments, where the first argument is a *T")
+	}
+	if fnType.NumOut() != 0 {
+		panic("testing: fuzz target must not return a value")
+	}
+
+	// Save the types of the function to compare against the corpus.
+	var types []reflect.Type
+	for i := 1; i < fnType.NumIn(); i++ {
+		t := fnType.In(i)
+		if !supportedTypes[t] {
+			panic(fmt.Sprintf("testing: unsupported type for fuzzing %v", t))
+		}
+		types = append(types, t)
+	}
+
+	// Load the testdata seed corpus. Check types of entries in the testdata
+	// corpus and entries declared with F.Add.
+	//
+	// Don't load the seed corpus if this is a worker process; we won't use it.
+	if f.fuzzContext.mode != fuzzWorker {
+		for _, c := range f.corpus {
+			if err := f.fuzzContext.deps.CheckCorpus(c.Values, types); err != nil {
+				// TODO(#48302): Report the source location of the F.Add call.
+				f.Fatal(err)
+			}
+		}
+
+		// Load seed corpus
+		c, err := f.fuzzContext.deps.ReadCorpus(filepath.Join(corpusDir, f.name), types)
+		if err != nil {
+			f.Fatal(err)
+		}
+		for i := range c {
+			c[i].IsSeed = true // these are all seed corpus values
+			if f.fuzzContext.mode == fuzzCoordinator {
+				// If this is the coordinator process, zero the values, since we don't need
+				// to hold onto them.
+				c[i].Values = nil
+			}
+		}
+
+		f.corpus = append(f.corpus, c...)
+	}
+
+	// run calls fn on a given input, as a subtest with its own T.
+	// run is analogous to T.Run. The test filtering and cleanup works similarly.
+	// fn is called in its own goroutine.
+	run := func(captureOut io.Writer, e corpusEntry) (ok bool) {
+		if e.Values == nil {
+			// The corpusEntry must have non-nil Values in order to run the
+			// test. If Values is nil, it is a bug in our code.
+			panic(fmt.Sprintf("corpus file %q was not unmarshaled", e.Path))
+		}
+		if shouldFailFast() {
+			return true
+		}
+		testName := f.name
+		if e.Path != "" {
+			testName = fmt.Sprintf("%s/%s", testName, filepath.Base(e.Path))
+		}
+		if f.testContext.isFuzzing {
+			// Don't preserve subtest names while fuzzing. If fn calls T.Run,
+			// there will be a very large number of subtests with duplicate names,
+			// which will use a large amount of memory. The subtest names aren't
+			// useful since there's no way to re-run them deterministically.
+			f.testContext.match.clearSubNames()
+		}
+
+		// Record the stack trace at the point of this call so that if the subtest
+		// function - which runs in a separate stack - is marked as a helper, we can
+		// continue walking the stack into the parent test.
+		var pc [maxStackLen]uintptr
+		n := runtime.Callers(2, pc[:])
+		t := &T{
+			common: common{
+				barrier: make(chan bool),
+				signal:  make(chan bool),
+				name:    testName,
+				parent:  &f.common,
+				level:   f.level + 1,
+				creator: pc[:n],
+				chatty:  f.chatty,
+			},
+			context: f.testContext,
+		}
+		if captureOut != nil {
+			// t.parent aliases f.common.
+			t.parent.w = captureOut
+		}
+		t.w = indenter{&t.common}
+		if t.chatty != nil {
+			t.chatty.Updatef(t.name, "=== RUN   %s\n", t.name)
+		}
+		f.common.inFuzzFn, f.inFuzzFn = true, true
+		go tRunner(t, func(t *T) {
+			args := []reflect.Value{reflect.ValueOf(t)}
+			for _, v := range e.Values {
+				args = append(args, reflect.ValueOf(v))
+			}
+			// Before resetting the current coverage, defer the snapshot so that
+			// we make sure it is called right before the tRunner function
+			// exits, regardless of whether it was executed cleanly, panicked,
+			// or if the fuzzFn called t.Fatal.
+			if f.testContext.isFuzzing {
+				defer f.fuzzContext.deps.SnapshotCoverage()
+				f.fuzzContext.deps.ResetCoverage()
+			}
+			fn.Call(args)
+		})
+		<-t.signal
+		if t.chatty != nil && t.chatty.json {
+			t.chatty.Updatef(t.parent.name, "=== NAME  %s\n", t.parent.name)
+		}
+		f.common.inFuzzFn, f.inFuzzFn = false, false
+		return !t.Failed()
+	}
+
+	switch f.fuzzContext.mode {
+	case fuzzCoordinator:
+		// Fuzzing is enabled, and this is the test process started by 'go test'.
+		// Act as the coordinator process, and coordinate workers to perform the
+		// actual fuzzing.
+		corpusTargetDir := filepath.Join(corpusDir, f.name)
+		cacheTargetDir := filepath.Join(*fuzzCacheDir, f.name)
+		err := f.fuzzContext.deps.CoordinateFuzzing(
+			fuzzDuration.d,
+			int64(fuzzDuration.n),
+			minimizeDuration.d,
+			int64(minimizeDuration.n),
+			*parallel,
+			f.corpus,
+			types,
+			corpusTargetDir,
+			cacheTargetDir)
+		if err != nil {
+			f.result = fuzzResult{Error: err}
+			f.Fail()
+			fmt.Fprintf(f.w, "%v\n", err)
+			if crashErr, ok := err.(fuzzCrashError); ok {
+				crashPath := crashErr.CrashPath()
+				fmt.Fprintf(f.w, "Failing input written to %s\n", crashPath)
+				testName := filepath.Base(crashPath)
+				fmt.Fprintf(f.w, "To re-run:\ngo test -run=%s/%s\n", f.name, testName)
+			}
+		}
+		// TODO(jayconrod,katiehockman): Aggregate statistics across workers
+		// and add to FuzzResult (ie. time taken, num iterations)
+
+	case fuzzWorker:
+		// Fuzzing is enabled, and this is a worker process. Follow instructions
+		// from the coordinator.
+		if err := f.fuzzContext.deps.RunFuzzWorker(func(e corpusEntry) error {
+			// Don't write to f.w (which points to Stdout) if running from a
+			// fuzz worker. This would become very verbose, particularly during
+			// minimization. Return the error instead, and let the caller deal
+			// with the output.
+			var buf strings.Builder
+			if ok := run(&buf, e); !ok {
+				return errors.New(buf.String())
+			}
+			return nil
+		}); err != nil {
+			// Internal errors are marked with f.Fail; user code may call this too, before F.Fuzz.
+			// The worker will exit with fuzzWorkerExitCode, indicating this is a failure
+			// (and 'go test' should exit non-zero) but a failing input should not be recorded.
+			f.Errorf("communicating with fuzzing coordinator: %v", err)
+		}
+
+	default:
+		// Fuzzing is not enabled, or will be done later. Only run the seed
+		// corpus now.
+		for _, e := range f.corpus {
+			name := fmt.Sprintf("%s/%s", f.name, filepath.Base(e.Path))
+			if _, ok, _ := f.testContext.match.fullName(nil, name); ok {
+				run(f.w, e)
+			}
+		}
+	}
+}
+
+func (f *F) report() {
+	if *isFuzzWorker || f.parent == nil {
+		return
+	}
+	dstr := fmtDuration(f.duration)
+	format := "--- %s: %s (%s)\n"
+	if f.Failed() {
+		f.flushToParent(f.name, format, "FAIL", f.name, dstr)
+	} else if f.chatty != nil {
+		if f.Skipped() {
+			f.flushToParent(f.name, format, "SKIP", f.name, dstr)
+		} else {
+			f.flushToParent(f.name, format, "PASS", f.name, dstr)
+		}
+	}
+}
+
+// fuzzResult contains the results of a fuzz run.
+type fuzzResult struct {
+	N     int           // The number of iterations.
+	T     time.Duration // The total time taken.
+	Error error         // Error is the error from the failing input
+}
+
+func (r fuzzResult) String() string {
+	if r.Error == nil {
+		return ""
+	}
+	return r.Error.Error()
+}
+
+// fuzzCrashError is satisfied by a failing input detected while fuzzing.
+// These errors are written to the seed corpus and can be re-run with 'go test'.
+// Errors within the fuzzing framework (like I/O errors between coordinator
+// and worker processes) don't satisfy this interface.
+type fuzzCrashError interface {
+	error
+	Unwrap() error
+
+	// CrashPath returns the path of the subtest that corresponds to the saved
+	// crash input file in the seed corpus. The test can be re-run with go test
+	// -run=$test/$name $test is the fuzz test name, and $name is the
+	// filepath.Base of the string returned here.
+	CrashPath() string
+}
+
+// fuzzContext holds fields common to all fuzz tests.
+type fuzzContext struct {
+	deps testDeps
+	mode fuzzMode
+}
+
+type fuzzMode uint8
+
+const (
+	seedCorpusOnly fuzzMode = iota
+	fuzzCoordinator
+	fuzzWorker
+)
+
+// runFuzzTests runs the fuzz tests matching the pattern for -run. This will
+// only run the (*F).Fuzz function for each seed corpus without using the
+// fuzzing engine to generate or mutate inputs.
+func runFuzzTests(deps testDeps, fuzzTests []InternalFuzzTarget, deadline time.Time) (ran, ok bool) {
+	ok = true
+	if len(fuzzTests) == 0 || *isFuzzWorker {
+		return ran, ok
+	}
+	m := newMatcher(deps.MatchString, *match, "-test.run", *skip)
+	var mFuzz *matcher
+	if *matchFuzz != "" {
+		mFuzz = newMatcher(deps.MatchString, *matchFuzz, "-test.fuzz", *skip)
+	}
+
+	for _, procs := range cpuList {
+		runtime.GOMAXPROCS(procs)
+		for i := uint(0); i < *count; i++ {
+			if shouldFailFast() {
+				break
+			}
+
+			tctx := newTestContext(*parallel, m)
+			tctx.deadline = deadline
+			fctx := &fuzzContext{deps: deps, mode: seedCorpusOnly}
+			root := common{w: os.Stdout} // gather output in one place
+			if Verbose() {
+				root.chatty = newChattyPrinter(root.w)
+			}
+			for _, ft := range fuzzTests {
+				if shouldFailFast() {
+					break
+				}
+				testName, matched, _ := tctx.match.fullName(nil, ft.Name)
+				if !matched {
+					continue
+				}
+				if mFuzz != nil {
+					if _, fuzzMatched, _ := mFuzz.fullName(nil, ft.Name); fuzzMatched {
+						// If this will be fuzzed, then don't run the seed corpus
+						// right now. That will happen later.
+						continue
+					}
+				}
+				f := &F{
+					common: common{
+						signal:  make(chan bool),
+						barrier: make(chan bool),
+						name:    testName,
+						parent:  &root,
+						level:   root.level + 1,
+						chatty:  root.chatty,
+					},
+					testContext: tctx,
+					fuzzContext: fctx,
+				}
+				f.w = indenter{&f.common}
+				if f.chatty != nil {
+					f.chatty.Updatef(f.name, "=== RUN   %s\n", f.name)
+				}
+				go fRunner(f, ft.Fn)
+				<-f.signal
+				if f.chatty != nil && f.chatty.json {
+					f.chatty.Updatef(f.parent.name, "=== NAME  %s\n", f.parent.name)
+				}
+				ok = ok && !f.Failed()
+				ran = ran || f.ran
+			}
+			if !ran {
+				// There were no tests to run on this iteration.
+				// This won't change, so no reason to keep trying.
+				break
+			}
+		}
+	}
+
+	return ran, ok
+}
+
+// runFuzzing runs the fuzz test matching the pattern for -fuzz. Only one such
+// fuzz test must match. This will run the fuzzing engine to generate and
+// mutate new inputs against the fuzz target.
+//
+// If fuzzing is disabled (-test.fuzz is not set), runFuzzing
+// returns immediately.
+func runFuzzing(deps testDeps, fuzzTests []InternalFuzzTarget) (ok bool) {
+	if len(fuzzTests) == 0 || *matchFuzz == "" {
+		return true
+	}
+	m := newMatcher(deps.MatchString, *matchFuzz, "-test.fuzz", *skip)
+	tctx := newTestContext(1, m)
+	tctx.isFuzzing = true
+	fctx := &fuzzContext{
+		deps: deps,
+	}
+	root := common{w: os.Stdout}
+	if *isFuzzWorker {
+		root.w = io.Discard
+		fctx.mode = fuzzWorker
+	} else {
+		fctx.mode = fuzzCoordinator
+	}
+	if Verbose() && !*isFuzzWorker {
+		root.chatty = newChattyPrinter(root.w)
+	}
+	var fuzzTest *InternalFuzzTarget
+	var testName string
+	var matched []string
+	for i := range fuzzTests {
+		name, ok, _ := tctx.match.fullName(nil, fuzzTests[i].Name)
+		if !ok {
+			continue
+		}
+		matched = append(matched, name)
+		fuzzTest = &fuzzTests[i]
+		testName = name
+	}
+	if len(matched) == 0 {
+		fmt.Fprintln(os.Stderr, "testing: warning: no fuzz tests to fuzz")
+		return true
+	}
+	if len(matched) > 1 {
+		fmt.Fprintf(os.Stderr, "testing: will not fuzz, -fuzz matches more than one fuzz test: %v\n", matched)
+		return false
+	}
+
+	f := &F{
+		common: common{
+			signal:  make(chan bool),
+			barrier: nil, // T.Parallel has no effect when fuzzing.
+			name:    testName,
+			parent:  &root,
+			level:   root.level + 1,
+			chatty:  root.chatty,
+		},
+		fuzzContext: fctx,
+		testContext: tctx,
+	}
+	f.w = indenter{&f.common}
+	if f.chatty != nil {
+		f.chatty.Updatef(f.name, "=== RUN   %s\n", f.name)
+	}
+	go fRunner(f, fuzzTest.Fn)
+	<-f.signal
+	if f.chatty != nil {
+		f.chatty.Updatef(f.parent.name, "=== NAME  %s\n", f.parent.name)
+	}
+	return !f.failed
+}
+
+// fRunner wraps a call to a fuzz test and ensures that cleanup functions are
+// called and status flags are set. fRunner should be called in its own
+// goroutine. To wait for its completion, receive from f.signal.
+//
+// fRunner is analogous to tRunner, which wraps subtests started with T.Run.
+// Unit tests and fuzz tests work a little differently, so for now, these
+// functions aren't consolidated. In particular, because there are no F.Run and
+// F.Parallel methods, i.e., no fuzz sub-tests or parallel fuzz tests, a few
+// simplifications are made. We also require that F.Fuzz, F.Skip, or F.Fail is
+// called.
+func fRunner(f *F, fn func(*F)) {
+	// When this goroutine is done, either because runtime.Goexit was called, a
+	// panic started, or fn returned normally, record the duration and send
+	// t.signal, indicating the fuzz test is done.
+	defer func() {
+		// Detect whether the fuzz test panicked or called runtime.Goexit
+		// without calling F.Fuzz, F.Fail, or F.Skip. If it did, panic (possibly
+		// replacing a nil panic value). Nothing should recover after fRunner
+		// unwinds, so this should crash the process and print stack.
+		// Unfortunately, recovering here adds stack frames, but the location of
+		// the original panic should still be
+		// clear.
+		f.checkRaces()
+		if f.Failed() {
+			numFailed.Add(1)
+		}
+		err := recover()
+		if err == nil {
+			f.mu.RLock()
+			fuzzNotCalled := !f.fuzzCalled && !f.skipped && !f.failed
+			if !f.finished && !f.skipped && !f.failed {
+				err = errNilPanicOrGoexit
+			}
+			f.mu.RUnlock()
+			if fuzzNotCalled && err == nil {
+				f.Error("returned without calling F.Fuzz, F.Fail, or F.Skip")
+			}
+		}
+
+		// Use a deferred call to ensure that we report that the test is
+		// complete even if a cleanup function calls F.FailNow. See issue 41355.
+		didPanic := false
+		defer func() {
+			if !didPanic {
+				// Only report that the test is complete if it doesn't panic,
+				// as otherwise the test binary can exit before the panic is
+				// reported to the user. See issue 41479.
+				f.signal <- true
+			}
+		}()
+
+		// If we recovered a panic or inappropriate runtime.Goexit, fail the test,
+		// flush the output log up to the root, then panic.
+		doPanic := func(err any) {
+			f.Fail()
+			if r := f.runCleanup(recoverAndReturnPanic); r != nil {
+				f.Logf("cleanup panicked with %v", r)
+			}
+			for root := &f.common; root.parent != nil; root = root.parent {
+				root.mu.Lock()
+				root.duration += highPrecisionTimeSince(root.start)
+				d := root.duration
+				root.mu.Unlock()
+				root.flushToParent(root.name, "--- FAIL: %s (%s)\n", root.name, fmtDuration(d))
+			}
+			didPanic = true
+			panic(err)
+		}
+		if err != nil {
+			doPanic(err)
+		}
+
+		// No panic or inappropriate Goexit.
+		f.duration += highPrecisionTimeSince(f.start)
+
+		if len(f.sub) > 0 {
+			// Unblock inputs that called T.Parallel while running the seed corpus.
+			// This only affects fuzz tests run as normal tests.
+			// While fuzzing, T.Parallel has no effect, so f.sub is empty, and this
+			// branch is not taken. f.barrier is nil in that case.
+			f.testContext.release()
+			close(f.barrier)
+			// Wait for the subtests to complete.
+			for _, sub := range f.sub {
+				<-sub.signal
+			}
+			cleanupStart := highPrecisionTimeNow()
+			err := f.runCleanup(recoverAndReturnPanic)
+			f.duration += highPrecisionTimeSince(cleanupStart)
+			if err != nil {
+				doPanic(err)
+			}
+		}
+
+		// Report after all subtests have finished.
+		f.report()
+		f.done = true
+		f.setRan()
+	}()
+	defer func() {
+		if len(f.sub) == 0 {
+			f.runCleanup(normalPanic)
+		}
+	}()
+
+	f.start = highPrecisionTimeNow()
+	f.resetRaces()
+	fn(f)
+
+	// Code beyond this point will not be executed when FailNow or SkipNow
+	// is invoked.
+	f.mu.Lock()
+	f.finished = true
+	f.mu.Unlock()
+}

src/testing/helper_test.go

@@ -0,0 +1,116 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"internal/testenv"
+	"os"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+func TestTBHelper(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		testTestHelper(t)
+
+		// Check that calling Helper from inside a top-level test function
+		// has no effect.
+		t.Helper()
+		t.Error("8")
+		return
+	}
+
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, exe, "-test.run=^TestTBHelper$")
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
+	out, _ := cmd.CombinedOutput()
+
+	want := `--- FAIL: TestTBHelper \([^)]+\)
+    helperfuncs_test.go:15: 0
+    helperfuncs_test.go:47: 1
+    helperfuncs_test.go:24: 2
+    helperfuncs_test.go:49: 3
+    helperfuncs_test.go:56: 4
+    --- FAIL: TestTBHelper/sub \([^)]+\)
+        helperfuncs_test.go:59: 5
+        helperfuncs_test.go:24: 6
+        helperfuncs_test.go:58: 7
+    --- FAIL: TestTBHelper/sub2 \([^)]+\)
+        helperfuncs_test.go:80: 11
+    helperfuncs_test.go:84: recover 12
+    helperfuncs_test.go:86: GenericFloat64
+    helperfuncs_test.go:87: GenericInt
+    helper_test.go:22: 8
+    helperfuncs_test.go:73: 9
+    helperfuncs_test.go:69: 10
+`
+	if !regexp.MustCompile(want).Match(out) {
+		t.Errorf("got output:\n\n%s\nwant matching:\n\n%s", out, want)
+	}
+}
+
+func TestTBHelperParallel(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		parallelTestHelper(t)
+		return
+	}
+
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, exe, "-test.run=^TestTBHelperParallel$")
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
+	out, _ := cmd.CombinedOutput()
+
+	t.Logf("output:\n%s", out)
+
+	lines := strings.Split(strings.TrimSpace(string(out)), "\n")
+
+	// We expect to see one "--- FAIL" line at the start
+	// of the log, five lines of "parallel" logging,
+	// and a final "FAIL" line at the end of the test.
+	const wantLines = 7
+
+	if len(lines) != wantLines {
+		t.Fatalf("parallelTestHelper gave %d lines of output; want %d", len(lines), wantLines)
+	}
+	want := "helperfuncs_test.go:24: parallel"
+	if got := strings.TrimSpace(lines[1]); got != want {
+		t.Errorf("got second output line %q; want %q", got, want)
+	}
+}
+
+func BenchmarkTBHelper(b *testing.B) {
+	f1 := func() {
+		b.Helper()
+	}
+	f2 := func() {
+		b.Helper()
+	}
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		if i&1 == 0 {
+			f1()
+		} else {
+			f2()
+		}
+	}
+}

src/testing/helperfuncs_test.go

@@ -0,0 +1,124 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"sync"
+	"testing"
+)
+
+// The line numbering of this file is important for TestTBHelper.
+
+func notHelper(t *testing.T, msg string) {
+	t.Error(msg)
+}
+
+func helper(t *testing.T, msg string) {
+	t.Helper()
+	t.Error(msg)
+}
+
+func notHelperCallingHelper(t *testing.T, msg string) {
+	helper(t, msg)
+}
+
+func helperCallingHelper(t *testing.T, msg string) {
+	t.Helper()
+	helper(t, msg)
+}
+
+func genericHelper[G any](t *testing.T, msg string) {
+	t.Helper()
+	t.Error(msg)
+}
+
+var genericIntHelper = genericHelper[int]
+
+func testTestHelper(t *testing.T) {
+	testHelper(t)
+}
+
+func testHelper(t *testing.T) {
+	// Check combinations of directly and indirectly
+	// calling helper functions.
+	notHelper(t, "0")
+	helper(t, "1")
+	notHelperCallingHelper(t, "2")
+	helperCallingHelper(t, "3")
+
+	// Check a function literal closing over t that uses Helper.
+	fn := func(msg string) {
+		t.Helper()
+		t.Error(msg)
+	}
+	fn("4")
+
+	t.Run("sub", func(t *testing.T) {
+		helper(t, "5")
+		notHelperCallingHelper(t, "6")
+		// Check that calling Helper from inside a subtest entry function
+		// works as if it were in an ordinary function call.
+		t.Helper()
+		t.Error("7")
+	})
+
+	// Check that right caller is reported for func passed to Cleanup when
+	// multiple cleanup functions have been registered.
+	t.Cleanup(func() {
+		t.Helper()
+		t.Error("10")
+	})
+	t.Cleanup(func() {
+		t.Helper()
+		t.Error("9")
+	})
+
+	// Check that helper-ness propagates up through subtests
+	// to helpers above. See https://golang.org/issue/44887.
+	helperSubCallingHelper(t, "11")
+
+	// Check that helper-ness propagates up through panic/recover.
+	// See https://golang.org/issue/31154.
+	recoverHelper(t, "12")
+
+	genericHelper[float64](t, "GenericFloat64")
+	genericIntHelper(t, "GenericInt")
+}
+
+func parallelTestHelper(t *testing.T) {
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func() {
+			notHelperCallingHelper(t, "parallel")
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}
+
+func helperSubCallingHelper(t *testing.T, msg string) {
+	t.Helper()
+	t.Run("sub2", func(t *testing.T) {
+		t.Helper()
+		t.Fatal(msg)
+	})
+}
+
+func recoverHelper(t *testing.T, msg string) {
+	t.Helper()
+	defer func() {
+		t.Helper()
+		if err := recover(); err != nil {
+			t.Errorf("recover %s", err)
+		}
+	}()
+	doPanic(t, msg)
+}
+
+func doPanic(t *testing.T, msg string) {
+	t.Helper()
+	panic(msg)
+}

src/testing/internal/testdeps/deps.go

@@ -0,0 +1,240 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testdeps provides access to dependencies needed by test execution.
+//
+// This package is imported by the generated main package, which passes
+// TestDeps into testing.Main. This allows tests to use packages at run time
+// without making those packages direct dependencies of package testing.
+// Direct dependencies of package testing are harder to write tests for.
+package testdeps
+
+import (
+	"bufio"
+	"context"
+	"internal/fuzz"
+	"internal/testlog"
+	"io"
+	"os"
+	"os/signal"
+	"reflect"
+	"regexp"
+	"runtime/pprof"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Cover indicates whether coverage is enabled.
+var Cover bool
+
+// TestDeps is an implementation of the testing.testDeps interface,
+// suitable for passing to [testing.MainStart].
+type TestDeps struct{}
+
+var matchPat string
+var matchRe *regexp.Regexp
+
+func (TestDeps) MatchString(pat, str string) (result bool, err error) {
+	if matchRe == nil || matchPat != pat {
+		matchPat = pat
+		matchRe, err = regexp.Compile(matchPat)
+		if err != nil {
+			return
+		}
+	}
+	return matchRe.MatchString(str), nil
+}
+
+func (TestDeps) StartCPUProfile(w io.Writer) error {
+	return pprof.StartCPUProfile(w)
+}
+
+func (TestDeps) StopCPUProfile() {
+	pprof.StopCPUProfile()
+}
+
+func (TestDeps) WriteProfileTo(name string, w io.Writer, debug int) error {
+	return pprof.Lookup(name).WriteTo(w, debug)
+}
+
+// ImportPath is the import path of the testing binary, set by the generated main function.
+var ImportPath string
+
+func (TestDeps) ImportPath() string {
+	return ImportPath
+}
+
+// testLog implements testlog.Interface, logging actions by package os.
+type testLog struct {
+	mu  sync.Mutex
+	w   *bufio.Writer
+	set bool
+}
+
+func (l *testLog) Getenv(key string) {
+	l.add("getenv", key)
+}
+
+func (l *testLog) Open(name string) {
+	l.add("open", name)
+}
+
+func (l *testLog) Stat(name string) {
+	l.add("stat", name)
+}
+
+func (l *testLog) Chdir(name string) {
+	l.add("chdir", name)
+}
+
+// add adds the (op, name) pair to the test log.
+func (l *testLog) add(op, name string) {
+	if strings.Contains(name, "\n") || name == "" {
+		return
+	}
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	if l.w == nil {
+		return
+	}
+	l.w.WriteString(op)
+	l.w.WriteByte(' ')
+	l.w.WriteString(name)
+	l.w.WriteByte('\n')
+}
+
+var log testLog
+
+func (TestDeps) StartTestLog(w io.Writer) {
+	log.mu.Lock()
+	log.w = bufio.NewWriter(w)
+	if !log.set {
+		// Tests that define TestMain and then run m.Run multiple times
+		// will call StartTestLog/StopTestLog multiple times.
+		// Checking log.set avoids calling testlog.SetLogger multiple times
+		// (which will panic) and also avoids writing the header multiple times.
+		log.set = true
+		testlog.SetLogger(&log)
+		log.w.WriteString("# test log\n") // known to cmd/go/internal/test/test.go
+	}
+	log.mu.Unlock()
+}
+
+func (TestDeps) StopTestLog() error {
+	log.mu.Lock()
+	defer log.mu.Unlock()
+	err := log.w.Flush()
+	log.w = nil
+	return err
+}
+
+// SetPanicOnExit0 tells the os package whether to panic on os.Exit(0).
+func (TestDeps) SetPanicOnExit0(v bool) {
+	testlog.SetPanicOnExit0(v)
+}
+
+func (TestDeps) CoordinateFuzzing(
+	timeout time.Duration,
+	limit int64,
+	minimizeTimeout time.Duration,
+	minimizeLimit int64,
+	parallel int,
+	seed []fuzz.CorpusEntry,
+	types []reflect.Type,
+	corpusDir,
+	cacheDir string) (err error) {
+	// Fuzzing may be interrupted with a timeout or if the user presses ^C.
+	// In either case, we'll stop worker processes gracefully and save
+	// crashers and interesting values.
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
+	defer cancel()
+	err = fuzz.CoordinateFuzzing(ctx, fuzz.CoordinateFuzzingOpts{
+		Log:             os.Stderr,
+		Timeout:         timeout,
+		Limit:           limit,
+		MinimizeTimeout: minimizeTimeout,
+		MinimizeLimit:   minimizeLimit,
+		Parallel:        parallel,
+		Seed:            seed,
+		Types:           types,
+		CorpusDir:       corpusDir,
+		CacheDir:        cacheDir,
+	})
+	if err == ctx.Err() {
+		return nil
+	}
+	return err
+}
+
+func (TestDeps) RunFuzzWorker(fn func(fuzz.CorpusEntry) error) error {
+	// Worker processes may or may not receive a signal when the user presses ^C
+	// On POSIX operating systems, a signal sent to a process group is delivered
+	// to all processes in that group. This is not the case on Windows.
+	// If the worker is interrupted, return quickly and without error.
+	// If only the coordinator process is interrupted, it tells each worker
+	// process to stop by closing its "fuzz_in" pipe.
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
+	defer cancel()
+	err := fuzz.RunFuzzWorker(ctx, fn)
+	if err == ctx.Err() {
+		return nil
+	}
+	return err
+}
+
+func (TestDeps) ReadCorpus(dir string, types []reflect.Type) ([]fuzz.CorpusEntry, error) {
+	return fuzz.ReadCorpus(dir, types)
+}
+
+func (TestDeps) CheckCorpus(vals []any, types []reflect.Type) error {
+	return fuzz.CheckCorpus(vals, types)
+}
+
+func (TestDeps) ResetCoverage() {
+	fuzz.ResetCoverage()
+}
+
+func (TestDeps) SnapshotCoverage() {
+	fuzz.SnapshotCoverage()
+}
+
+var CoverMode string
+var Covered string
+var CoverSelectedPackages []string
+
+// These variables below are set at runtime (via code in testmain) to point
+// to the equivalent functions in package internal/coverage/cfile; doing
+// things this way allows us to have tests import internal/coverage/cfile
+// only when -cover is in effect (as opposed to importing for all tests).
+var (
+	CoverSnapshotFunc           func() float64
+	CoverProcessTestDirFunc     func(dir string, cfile string, cm string, cpkg string, w io.Writer, selpkgs []string) error
+	CoverMarkProfileEmittedFunc func(val bool)
+)
+
+func (TestDeps) InitRuntimeCoverage() (mode string, tearDown func(string, string) (string, error), snapcov func() float64) {
+	if CoverMode == "" {
+		return
+	}
+	return CoverMode, coverTearDown, CoverSnapshotFunc
+}
+
+func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
+	var err error
+	if gocoverdir == "" {
+		gocoverdir, err = os.MkdirTemp("", "gocoverdir")
+		if err != nil {
+			return "error setting GOCOVERDIR: bad os.MkdirTemp return", err
+		}
+		defer os.RemoveAll(gocoverdir)
+	}
+	CoverMarkProfileEmittedFunc(true)
+	cmode := CoverMode
+	if err := CoverProcessTestDirFunc(gocoverdir, coverprofile, cmode, Covered, os.Stdout, CoverSelectedPackages); err != nil {
+		return "error generating coverage report", err
+	}
+	return "", nil
+}

src/testing/iotest/example_test.go

@@ -0,0 +1,22 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest_test
+
+import (
+	"errors"
+	"fmt"
+	"testing/iotest"
+)
+
+func ExampleErrReader() {
+	// A reader that always returns a custom error.
+	r := iotest.ErrReader(errors.New("custom error"))
+	n, err := r.Read(nil)
+	fmt.Printf("n:   %d\nerr: %q\n", n, err)
+
+	// Output:
+	// n:   0
+	// err: "custom error"
+}

src/testing/iotest/logger.go

@@ -0,0 +1,54 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"io"
+	"log"
+)
+
+type writeLogger struct {
+	prefix string
+	w      io.Writer
+}
+
+func (l *writeLogger) Write(p []byte) (n int, err error) {
+	n, err = l.w.Write(p)
+	if err != nil {
+		log.Printf("%s %x: %v", l.prefix, p[0:n], err)
+	} else {
+		log.Printf("%s %x", l.prefix, p[0:n])
+	}
+	return
+}
+
+// NewWriteLogger returns a writer that behaves like w except
+// that it logs (using [log.Printf]) each write to standard error,
+// printing the prefix and the hexadecimal data written.
+func NewWriteLogger(prefix string, w io.Writer) io.Writer {
+	return &writeLogger{prefix, w}
+}
+
+type readLogger struct {
+	prefix string
+	r      io.Reader
+}
+
+func (l *readLogger) Read(p []byte) (n int, err error) {
+	n, err = l.r.Read(p)
+	if err != nil {
+		log.Printf("%s %x: %v", l.prefix, p[0:n], err)
+	} else {
+		log.Printf("%s %x", l.prefix, p[0:n])
+	}
+	return
+}
+
+// NewReadLogger returns a reader that behaves like r except
+// that it logs (using [log.Printf]) each read to standard error,
+// printing the prefix and the hexadecimal data read.
+func NewReadLogger(prefix string, r io.Reader) io.Reader {
+	return &readLogger{prefix, r}
+}

src/testing/iotest/logger_test.go

@@ -0,0 +1,153 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"log"
+	"strings"
+	"testing"
+)
+
+type errWriter struct {
+	err error
+}
+
+func (w errWriter) Write([]byte) (int, error) {
+	return 0, w.err
+}
+
+func TestWriteLogger(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(strings.Builder)
+	log.SetPrefix("lw: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	lw := new(strings.Builder)
+	wl := NewWriteLogger("write:", lw)
+	if _, err := wl.Write([]byte("Hello, World!")); err != nil {
+		t.Fatalf("Unexpectedly failed to write: %v", err)
+	}
+
+	if g, w := lw.String(), "Hello, World!"; g != w {
+		t.Errorf("WriteLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+	wantLogWithHex := fmt.Sprintf("lw: write: %x\n", "Hello, World!")
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("WriteLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
+
+func TestWriteLogger_errorOnWrite(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(strings.Builder)
+	log.SetPrefix("lw: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	lw := errWriter{err: errors.New("Write Error!")}
+	wl := NewWriteLogger("write:", lw)
+	if _, err := wl.Write([]byte("Hello, World!")); err == nil {
+		t.Fatalf("Unexpectedly succeeded to write: %v", err)
+	}
+
+	wantLogWithHex := fmt.Sprintf("lw: write: %x: %v\n", "", "Write Error!")
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("WriteLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
+
+func TestReadLogger(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(strings.Builder)
+	log.SetPrefix("lr: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	data := []byte("Hello, World!")
+	p := make([]byte, len(data))
+	lr := bytes.NewReader(data)
+	rl := NewReadLogger("read:", lr)
+
+	n, err := rl.Read(p)
+	if err != nil {
+		t.Fatalf("Unexpectedly failed to read: %v", err)
+	}
+
+	if g, w := p[:n], data; !bytes.Equal(g, w) {
+		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+
+	wantLogWithHex := fmt.Sprintf("lr: read: %x\n", "Hello, World!")
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
+
+func TestReadLogger_errorOnRead(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(strings.Builder)
+	log.SetPrefix("lr: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	data := []byte("Hello, World!")
+	p := make([]byte, len(data))
+
+	lr := ErrReader(errors.New("io failure"))
+	rl := NewReadLogger("read", lr)
+	n, err := rl.Read(p)
+	if err == nil {
+		t.Fatalf("Unexpectedly succeeded to read: %v", err)
+	}
+
+	wantLogWithHex := fmt.Sprintf("lr: read %x: io failure\n", p[:n])
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}

src/testing/iotest/reader.go

@@ -0,0 +1,268 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package iotest implements Readers and Writers useful mainly for testing.
+package iotest
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// OneByteReader returns a Reader that implements
+// each non-empty Read by reading one byte from r.
+func OneByteReader(r io.Reader) io.Reader { return &oneByteReader{r} }
+
+type oneByteReader struct {
+	r io.Reader
+}
+
+func (r *oneByteReader) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	return r.r.Read(p[0:1])
+}
+
+// HalfReader returns a Reader that implements Read
+// by reading half as many requested bytes from r.
+func HalfReader(r io.Reader) io.Reader { return &halfReader{r} }
+
+type halfReader struct {
+	r io.Reader
+}
+
+func (r *halfReader) Read(p []byte) (int, error) {
+	return r.r.Read(p[0 : (len(p)+1)/2])
+}
+
+// DataErrReader changes the way errors are handled by a Reader. Normally, a
+// Reader returns an error (typically EOF) from the first Read call after the
+// last piece of data is read. DataErrReader wraps a Reader and changes its
+// behavior so the final error is returned along with the final data, instead
+// of in the first call after the final data.
+func DataErrReader(r io.Reader) io.Reader { return &dataErrReader{r, nil, make([]byte, 1024)} }
+
+type dataErrReader struct {
+	r      io.Reader
+	unread []byte
+	data   []byte
+}
+
+func (r *dataErrReader) Read(p []byte) (n int, err error) {
+	// loop because first call needs two reads:
+	// one to get data and a second to look for an error.
+	for {
+		if len(r.unread) == 0 {
+			n1, err1 := r.r.Read(r.data)
+			r.unread = r.data[0:n1]
+			err = err1
+		}
+		if n > 0 || err != nil {
+			break
+		}
+		n = copy(p, r.unread)
+		r.unread = r.unread[n:]
+	}
+	return
+}
+
+// ErrTimeout is a fake timeout error.
+var ErrTimeout = errors.New("timeout")
+
+// TimeoutReader returns [ErrTimeout] on the second read
+// with no data. Subsequent calls to read succeed.
+func TimeoutReader(r io.Reader) io.Reader { return &timeoutReader{r, 0} }
+
+type timeoutReader struct {
+	r     io.Reader
+	count int
+}
+
+func (r *timeoutReader) Read(p []byte) (int, error) {
+	r.count++
+	if r.count == 2 {
+		return 0, ErrTimeout
+	}
+	return r.r.Read(p)
+}
+
+// ErrReader returns an [io.Reader] that returns 0, err from all Read calls.
+func ErrReader(err error) io.Reader {
+	return &errReader{err: err}
+}
+
+type errReader struct {
+	err error
+}
+
+func (r *errReader) Read(p []byte) (int, error) {
+	return 0, r.err
+}
+
+type smallByteReader struct {
+	r   io.Reader
+	off int
+	n   int
+}
+
+func (r *smallByteReader) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	r.n = r.n%3 + 1
+	n := r.n
+	if n > len(p) {
+		n = len(p)
+	}
+	n, err := r.r.Read(p[0:n])
+	if err != nil && err != io.EOF {
+		err = fmt.Errorf("Read(%d bytes at offset %d): %v", n, r.off, err)
+	}
+	r.off += n
+	return n, err
+}
+
+// TestReader tests that reading from r returns the expected file content.
+// It does reads of different sizes, until EOF.
+// If r implements [io.ReaderAt] or [io.Seeker], TestReader also checks
+// that those operations behave as they should.
+//
+// If TestReader finds any misbehaviors, it returns an error reporting them.
+// The error text may span multiple lines.
+func TestReader(r io.Reader, content []byte) error {
+	if len(content) > 0 {
+		n, err := r.Read(nil)
+		if n != 0 || err != nil {
+			return fmt.Errorf("Read(0) = %d, %v, want 0, nil", n, err)
+		}
+	}
+
+	data, err := io.ReadAll(&smallByteReader{r: r})
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(data, content) {
+		return fmt.Errorf("ReadAll(small amounts) = %q\n\twant %q", data, content)
+	}
+	n, err := r.Read(make([]byte, 10))
+	if n != 0 || err != io.EOF {
+		return fmt.Errorf("Read(10) at EOF = %v, %v, want 0, EOF", n, err)
+	}
+
+	if r, ok := r.(io.ReadSeeker); ok {
+		// Seek(0, 1) should report the current file position (EOF).
+		if off, err := r.Seek(0, 1); off != int64(len(content)) || err != nil {
+			return fmt.Errorf("Seek(0, 1) from EOF = %d, %v, want %d, nil", off, err, len(content))
+		}
+
+		// Seek backward partway through file, in two steps.
+		// If middle == 0, len(content) == 0, can't use the -1 and +1 seeks.
+		middle := len(content) - len(content)/3
+		if middle > 0 {
+			if off, err := r.Seek(-1, 1); off != int64(len(content)-1) || err != nil {
+				return fmt.Errorf("Seek(-1, 1) from EOF = %d, %v, want %d, nil", -off, err, len(content)-1)
+			}
+			if off, err := r.Seek(int64(-len(content)/3), 1); off != int64(middle-1) || err != nil {
+				return fmt.Errorf("Seek(%d, 1) from %d = %d, %v, want %d, nil", -len(content)/3, len(content)-1, off, err, middle-1)
+			}
+			if off, err := r.Seek(+1, 1); off != int64(middle) || err != nil {
+				return fmt.Errorf("Seek(+1, 1) from %d = %d, %v, want %d, nil", middle-1, off, err, middle)
+			}
+		}
+
+		// Seek(0, 1) should report the current file position (middle).
+		if off, err := r.Seek(0, 1); off != int64(middle) || err != nil {
+			return fmt.Errorf("Seek(0, 1) from %d = %d, %v, want %d, nil", middle, off, err, middle)
+		}
+
+		// Reading forward should return the last part of the file.
+		data, err := io.ReadAll(&smallByteReader{r: r})
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle, err)
+		}
+		if !bytes.Equal(data, content[middle:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle, data, content[middle:])
+		}
+
+		// Seek relative to end of file, but start elsewhere.
+		if off, err := r.Seek(int64(middle/2), 0); off != int64(middle/2) || err != nil {
+			return fmt.Errorf("Seek(%d, 0) from EOF = %d, %v, want %d, nil", middle/2, off, err, middle/2)
+		}
+		if off, err := r.Seek(int64(-len(content)/3), 2); off != int64(middle) || err != nil {
+			return fmt.Errorf("Seek(%d, 2) from %d = %d, %v, want %d, nil", -len(content)/3, middle/2, off, err, middle)
+		}
+
+		// Reading forward should return the last part of the file (again).
+		data, err = io.ReadAll(&smallByteReader{r: r})
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle, err)
+		}
+		if !bytes.Equal(data, content[middle:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle, data, content[middle:])
+		}
+
+		// Absolute seek & read forward.
+		if off, err := r.Seek(int64(middle/2), 0); off != int64(middle/2) || err != nil {
+			return fmt.Errorf("Seek(%d, 0) from EOF = %d, %v, want %d, nil", middle/2, off, err, middle/2)
+		}
+		data, err = io.ReadAll(r)
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle/2, err)
+		}
+		if !bytes.Equal(data, content[middle/2:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle/2, data, content[middle/2:])
+		}
+	}
+
+	if r, ok := r.(io.ReaderAt); ok {
+		data := make([]byte, len(content), len(content)+1)
+		for i := range data {
+			data[i] = 0xfe
+		}
+		n, err := r.ReadAt(data, 0)
+		if n != len(data) || err != nil && err != io.EOF {
+			return fmt.Errorf("ReadAt(%d, 0) = %v, %v, want %d, nil or EOF", len(data), n, err, len(data))
+		}
+		if !bytes.Equal(data, content) {
+			return fmt.Errorf("ReadAt(%d, 0) = %q\n\twant %q", len(data), data, content)
+		}
+
+		n, err = r.ReadAt(data[:1], int64(len(data)))
+		if n != 0 || err != io.EOF {
+			return fmt.Errorf("ReadAt(1, %d) = %v, %v, want 0, EOF", len(data), n, err)
+		}
+
+		for i := range data {
+			data[i] = 0xfe
+		}
+		n, err = r.ReadAt(data[:cap(data)], 0)
+		if n != len(data) || err != io.EOF {
+			return fmt.Errorf("ReadAt(%d, 0) = %v, %v, want %d, EOF", cap(data), n, err, len(data))
+		}
+		if !bytes.Equal(data, content) {
+			return fmt.Errorf("ReadAt(%d, 0) = %q\n\twant %q", len(data), data, content)
+		}
+
+		for i := range data {
+			data[i] = 0xfe
+		}
+		for i := range data {
+			n, err = r.ReadAt(data[i:i+1], int64(i))
+			if n != 1 || err != nil && (i != len(data)-1 || err != io.EOF) {
+				want := "nil"
+				if i == len(data)-1 {
+					want = "nil or EOF"
+				}
+				return fmt.Errorf("ReadAt(1, %d) = %v, %v, want 1, %s", i, n, err, want)
+			}
+			if data[i] != content[i] {
+				return fmt.Errorf("ReadAt(1, %d) = %q want %q", i, data[i:i+1], content[i:i+1])
+			}
+		}
+	}
+	return nil
+}

src/testing/iotest/reader_test.go

@@ -0,0 +1,261 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestOneByteReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+
+	obr := OneByteReader(buf)
+	var b []byte
+	n, err := obr.Read(b)
+	if err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 3)
+	// Read from obr until EOF.
+	got := new(strings.Builder)
+	for i := 0; ; i++ {
+		n, err = obr.Read(b)
+		if err != nil {
+			break
+		}
+		if g, w := n, 1; g != w {
+			t.Errorf("Iteration #%d read %d bytes, want %d", i, g, w)
+		}
+		got.Write(b[:n])
+	}
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Unexpected error after reading all bytes\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := got.String(), "Hello, World!"; g != w {
+		t.Errorf("Read mismatch\n\tGot:  %q\n\tWant: %q", g, w)
+	}
+}
+
+func TestOneByteReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+
+	obr := OneByteReader(r)
+	var b []byte
+	if n, err := obr.Read(b); err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 5)
+	n, err := obr.Read(b)
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestHalfReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+	// empty read buffer
+	hr := HalfReader(buf)
+	var b []byte
+	n, err := hr.Read(b)
+	if err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// non empty read buffer
+	b = make([]byte, 2)
+	got := new(strings.Builder)
+	for i := 0; ; i++ {
+		n, err = hr.Read(b)
+		if err != nil {
+			break
+		}
+		if g, w := n, 1; g != w {
+			t.Errorf("Iteration #%d read %d bytes, want %d", i, g, w)
+		}
+		got.Write(b[:n])
+	}
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Unexpected error after reading all bytes\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := got.String(), "Hello, World!"; g != w {
+		t.Errorf("Read mismatch\n\tGot:  %q\n\tWant: %q", g, w)
+	}
+}
+
+func TestHalfReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+
+	hr := HalfReader(r)
+	var b []byte
+	if n, err := hr.Read(b); err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 5)
+	n, err := hr.Read(b)
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestTimeOutReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+	// empty read buffer
+	tor := TimeoutReader(buf)
+	var b []byte
+	n, err := tor.Read(b)
+	if err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err = tor.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+	// non empty read buffer
+	tor2 := TimeoutReader(buf)
+	b = make([]byte, 3)
+	if n, err := tor2.Read(b); err != nil || n == 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err = tor2.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestTimeOutReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+	// empty read buffer
+	tor := TimeoutReader(r)
+	var b []byte
+	if n, err := tor.Read(b); err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err := tor.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+	// non empty read buffer
+	tor2 := TimeoutReader(r)
+	b = make([]byte, 5)
+	if n, err := tor2.Read(b); err != io.EOF || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err = tor2.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestDataErrReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+
+	der := DataErrReader(buf)
+
+	b := make([]byte, 3)
+	got := new(strings.Builder)
+	var n int
+	var err error
+	for {
+		n, err = der.Read(b)
+		got.Write(b[:n])
+		if err != nil {
+			break
+		}
+	}
+	if err != io.EOF || n == 0 {
+		t.Errorf("Last Read returned n=%d err=%v", n, err)
+	}
+	if g, w := got.String(), "Hello, World!"; g != w {
+		t.Errorf("Read mismatch\n\tGot:  %q\n\tWant: %q", g, w)
+	}
+}
+
+func TestDataErrReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+
+	der := DataErrReader(r)
+	var b []byte
+	if n, err := der.Read(b); err != io.EOF || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 5)
+	n, err := der.Read(b)
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestErrReader(t *testing.T) {
+	cases := []struct {
+		name string
+		err  error
+	}{
+		{"nil error", nil},
+		{"non-nil error", errors.New("io failure")},
+		{"io.EOF", io.EOF},
+	}
+
+	for _, tt := range cases {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			n, err := ErrReader(tt.err).Read(nil)
+			if err != tt.err {
+				t.Fatalf("Error mismatch\nGot:  %v\nWant: %v", err, tt.err)
+			}
+			if n != 0 {
+				t.Fatalf("Byte count mismatch: got %d want 0", n)
+			}
+		})
+	}
+}
+
+func TestStringsReader(t *testing.T) {
+	const msg = "Now is the time for all good gophers."
+
+	r := strings.NewReader(msg)
+	if err := TestReader(r, []byte(msg)); err != nil {
+		t.Fatal(err)
+	}
+}

src/testing/iotest/writer.go

@@ -0,0 +1,35 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import "io"
+
+// TruncateWriter returns a Writer that writes to w
+// but stops silently after n bytes.
+func TruncateWriter(w io.Writer, n int64) io.Writer {
+	return &truncateWriter{w, n}
+}
+
+type truncateWriter struct {
+	w io.Writer
+	n int64
+}
+
+func (t *truncateWriter) Write(p []byte) (n int, err error) {
+	if t.n <= 0 {
+		return len(p), nil
+	}
+	// real write
+	n = len(p)
+	if int64(n) > t.n {
+		n = int(t.n)
+	}
+	n, err = t.w.Write(p[0:n])
+	t.n -= int64(n)
+	if err == nil {
+		n = len(p)
+	}
+	return
+}

src/testing/iotest/writer_test.go

@@ -0,0 +1,39 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"strings"
+	"testing"
+)
+
+var truncateWriterTests = []struct {
+	in    string
+	want  string
+	trunc int64
+	n     int
+}{
+	{"hello", "", -1, 5},
+	{"world", "", 0, 5},
+	{"abcde", "abc", 3, 5},
+	{"edcba", "edcba", 7, 5},
+}
+
+func TestTruncateWriter(t *testing.T) {
+	for _, tt := range truncateWriterTests {
+		buf := new(strings.Builder)
+		tw := TruncateWriter(buf, tt.trunc)
+		n, err := tw.Write([]byte(tt.in))
+		if err != nil {
+			t.Errorf("Unexpected error %v for\n\t%+v", err, tt)
+		}
+		if g, w := buf.String(), tt.want; g != w {
+			t.Errorf("got %q, expected %q", g, w)
+		}
+		if g, w := n, tt.n; g != w {
+			t.Errorf("read %d bytes, but expected to have read %d bytes for\n\t%+v", g, w, tt)
+		}
+	}
+}

src/testing/match.go

@@ -0,0 +1,317 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+// matcher sanitizes, uniques, and filters names of subtests and subbenchmarks.
+type matcher struct {
+	filter    filterMatch
+	skip      filterMatch
+	matchFunc func(pat, str string) (bool, error)
+
+	mu sync.Mutex
+
+	// subNames is used to deduplicate subtest names.
+	// Each key is the subtest name joined to the deduplicated name of the parent test.
+	// Each value is the count of the number of occurrences of the given subtest name
+	// already seen.
+	subNames map[string]int32
+}
+
+type filterMatch interface {
+	// matches checks the name against the receiver's pattern strings using the
+	// given match function.
+	matches(name []string, matchString func(pat, str string) (bool, error)) (ok, partial bool)
+
+	// verify checks that the receiver's pattern strings are valid filters by
+	// calling the given match function.
+	verify(name string, matchString func(pat, str string) (bool, error)) error
+}
+
+// simpleMatch matches a test name if all of the pattern strings match in
+// sequence.
+type simpleMatch []string
+
+// alternationMatch matches a test name if one of the alternations match.
+type alternationMatch []filterMatch
+
+// TODO: fix test_main to avoid race and improve caching, also allowing to
+// eliminate this Mutex.
+var matchMutex sync.Mutex
+
+func allMatcher() *matcher {
+	return newMatcher(nil, "", "", "")
+}
+
+func newMatcher(matchString func(pat, str string) (bool, error), patterns, name, skips string) *matcher {
+	var filter, skip filterMatch
+	if patterns == "" {
+		filter = simpleMatch{} // always partial true
+	} else {
+		filter = splitRegexp(patterns)
+		if err := filter.verify(name, matchString); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: invalid regexp for %s\n", err)
+			os.Exit(1)
+		}
+	}
+	if skips == "" {
+		skip = alternationMatch{} // always false
+	} else {
+		skip = splitRegexp(skips)
+		if err := skip.verify("-test.skip", matchString); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: invalid regexp for %v\n", err)
+			os.Exit(1)
+		}
+	}
+	return &matcher{
+		filter:    filter,
+		skip:      skip,
+		matchFunc: matchString,
+		subNames:  map[string]int32{},
+	}
+}
+
+func (m *matcher) fullName(c *common, subname string) (name string, ok, partial bool) {
+	name = subname
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if c != nil && c.level > 0 {
+		name = m.unique(c.name, rewrite(subname))
+	}
+
+	matchMutex.Lock()
+	defer matchMutex.Unlock()
+
+	// We check the full array of paths each time to allow for the case that a pattern contains a '/'.
+	elem := strings.Split(name, "/")
+
+	// filter must match.
+	// accept partial match that may produce full match later.
+	ok, partial = m.filter.matches(elem, m.matchFunc)
+	if !ok {
+		return name, false, false
+	}
+
+	// skip must not match.
+	// ignore partial match so we can get to more precise match later.
+	skip, partialSkip := m.skip.matches(elem, m.matchFunc)
+	if skip && !partialSkip {
+		return name, false, false
+	}
+
+	return name, ok, partial
+}
+
+// clearSubNames clears the matcher's internal state, potentially freeing
+// memory. After this is called, T.Name may return the same strings as it did
+// for earlier subtests.
+func (m *matcher) clearSubNames() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	clear(m.subNames)
+}
+
+func (m simpleMatch) matches(name []string, matchString func(pat, str string) (bool, error)) (ok, partial bool) {
+	for i, s := range name {
+		if i >= len(m) {
+			break
+		}
+		if ok, _ := matchString(m[i], s); !ok {
+			return false, false
+		}
+	}
+	return true, len(name) < len(m)
+}
+
+func (m simpleMatch) verify(name string, matchString func(pat, str string) (bool, error)) error {
+	for i, s := range m {
+		m[i] = rewrite(s)
+	}
+	// Verify filters before doing any processing.
+	for i, s := range m {
+		if _, err := matchString(s, "non-empty"); err != nil {
+			return fmt.Errorf("element %d of %s (%q): %s", i, name, s, err)
+		}
+	}
+	return nil
+}
+
+func (m alternationMatch) matches(name []string, matchString func(pat, str string) (bool, error)) (ok, partial bool) {
+	for _, m := range m {
+		if ok, partial = m.matches(name, matchString); ok {
+			return ok, partial
+		}
+	}
+	return false, false
+}
+
+func (m alternationMatch) verify(name string, matchString func(pat, str string) (bool, error)) error {
+	for i, m := range m {
+		if err := m.verify(name, matchString); err != nil {
+			return fmt.Errorf("alternation %d of %s", i, err)
+		}
+	}
+	return nil
+}
+
+func splitRegexp(s string) filterMatch {
+	a := make(simpleMatch, 0, strings.Count(s, "/"))
+	b := make(alternationMatch, 0, strings.Count(s, "|"))
+	cs := 0
+	cp := 0
+	for i := 0; i < len(s); {
+		switch s[i] {
+		case '[':
+			cs++
+		case ']':
+			if cs--; cs < 0 { // An unmatched ']' is legal.
+				cs = 0
+			}
+		case '(':
+			if cs == 0 {
+				cp++
+			}
+		case ')':
+			if cs == 0 {
+				cp--
+			}
+		case '\\':
+			i++
+		case '/':
+			if cs == 0 && cp == 0 {
+				a = append(a, s[:i])
+				s = s[i+1:]
+				i = 0
+				continue
+			}
+		case '|':
+			if cs == 0 && cp == 0 {
+				a = append(a, s[:i])
+				s = s[i+1:]
+				i = 0
+				b = append(b, a)
+				a = make(simpleMatch, 0, len(a))
+				continue
+			}
+		}
+		i++
+	}
+
+	a = append(a, s)
+	if len(b) == 0 {
+		return a
+	}
+	return append(b, a)
+}
+
+// unique creates a unique name for the given parent and subname by affixing it
+// with one or more counts, if necessary.
+func (m *matcher) unique(parent, subname string) string {
+	base := parent + "/" + subname
+
+	for {
+		n := m.subNames[base]
+		if n < 0 {
+			panic("subtest count overflow")
+		}
+		m.subNames[base] = n + 1
+
+		if n == 0 && subname != "" {
+			prefix, nn := parseSubtestNumber(base)
+			if len(prefix) < len(base) && nn < m.subNames[prefix] {
+				// This test is explicitly named like "parent/subname#NN",
+				// and #NN was already used for the NNth occurrence of "parent/subname".
+				// Loop to add a disambiguating suffix.
+				continue
+			}
+			return base
+		}
+
+		name := fmt.Sprintf("%s#%02d", base, n)
+		if m.subNames[name] != 0 {
+			// This is the nth occurrence of base, but the name "parent/subname#NN"
+			// collides with the first occurrence of a subtest *explicitly* named
+			// "parent/subname#NN". Try the next number.
+			continue
+		}
+
+		return name
+	}
+}
+
+// parseSubtestNumber splits a subtest name into a "#%02d"-formatted int32
+// suffix (if present), and a prefix preceding that suffix (always).
+func parseSubtestNumber(s string) (prefix string, nn int32) {
+	i := strings.LastIndex(s, "#")
+	if i < 0 {
+		return s, 0
+	}
+
+	prefix, suffix := s[:i], s[i+1:]
+	if len(suffix) < 2 || (len(suffix) > 2 && suffix[0] == '0') {
+		// Even if suffix is numeric, it is not a possible output of a "%02" format
+		// string: it has either too few digits or too many leading zeroes.
+		return s, 0
+	}
+	if suffix == "00" {
+		if !strings.HasSuffix(prefix, "/") {
+			// We only use "#00" as a suffix for subtests named with the empty
+			// string — it isn't a valid suffix if the subtest name is non-empty.
+			return s, 0
+		}
+	}
+
+	n, err := strconv.ParseInt(suffix, 10, 32)
+	if err != nil || n < 0 {
+		return s, 0
+	}
+	return prefix, int32(n)
+}
+
+// rewrite rewrites a subname to having only printable characters and no white
+// space.
+func rewrite(s string) string {
+	b := []byte{}
+	for _, r := range s {
+		switch {
+		case isSpace(r):
+			b = append(b, '_')
+		case !strconv.IsPrint(r):
+			s := strconv.QuoteRune(r)
+			b = append(b, s[1:len(s)-1]...)
+		default:
+			b = append(b, string(r)...)
+		}
+	}
+	return string(b)
+}
+
+func isSpace(r rune) bool {
+	if r < 0x2000 {
+		switch r {
+		// Note: not the same as Unicode Z class.
+		case '\t', '\n', '\v', '\f', '\r', ' ', 0x85, 0xA0, 0x1680:
+			return true
+		}
+	} else {
+		if r <= 0x200a {
+			return true
+		}
+		switch r {
+		case 0x2028, 0x2029, 0x202f, 0x205f, 0x3000:
+			return true
+		}
+	}
+	return false
+}

src/testing/match_test.go

@@ -0,0 +1,263 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"fmt"
+	"reflect"
+	"regexp"
+	"strings"
+	"unicode"
+)
+
+func init() {
+	testingTesting = true
+}
+
+// Verify that our IsSpace agrees with unicode.IsSpace.
+func TestIsSpace(t *T) {
+	n := 0
+	for r := rune(0); r <= unicode.MaxRune; r++ {
+		if isSpace(r) != unicode.IsSpace(r) {
+			t.Errorf("IsSpace(%U)=%t incorrect", r, isSpace(r))
+			n++
+			if n > 10 {
+				return
+			}
+		}
+	}
+}
+
+func TestSplitRegexp(t *T) {
+	res := func(s ...string) filterMatch { return simpleMatch(s) }
+	alt := func(m ...filterMatch) filterMatch { return alternationMatch(m) }
+	testCases := []struct {
+		pattern string
+		result  filterMatch
+	}{
+		// Correct patterns
+		// If a regexp pattern is correct, all split regexps need to be correct
+		// as well.
+		{"", res("")},
+		{"/", res("", "")},
+		{"//", res("", "", "")},
+		{"A", res("A")},
+		{"A/B", res("A", "B")},
+		{"A/B/", res("A", "B", "")},
+		{"/A/B/", res("", "A", "B", "")},
+		{"[A]/(B)", res("[A]", "(B)")},
+		{"[/]/[/]", res("[/]", "[/]")},
+		{"[/]/[:/]", res("[/]", "[:/]")},
+		{"/]", res("", "]")},
+		{"]/", res("]", "")},
+		{"]/[/]", res("]", "[/]")},
+		{`([)/][(])`, res(`([)/][(])`)},
+		{"[(]/[)]", res("[(]", "[)]")},
+
+		{"A/B|C/D", alt(res("A", "B"), res("C", "D"))},
+
+		// Faulty patterns
+		// Errors in original should produce at least one faulty regexp in results.
+		{")/", res(")/")},
+		{")/(/)", res(")/(", ")")},
+		{"a[/)b", res("a[/)b")},
+		{"(/]", res("(/]")},
+		{"(/", res("(/")},
+		{"[/]/[/", res("[/]", "[/")},
+		{`\p{/}`, res(`\p{`, "}")},
+		{`\p/`, res(`\p`, "")},
+		{`[[:/:]]`, res(`[[:/:]]`)},
+	}
+	for _, tc := range testCases {
+		a := splitRegexp(tc.pattern)
+		if !reflect.DeepEqual(a, tc.result) {
+			t.Errorf("splitRegexp(%q) = %#v; want %#v", tc.pattern, a, tc.result)
+		}
+
+		// If there is any error in the pattern, one of the returned subpatterns
+		// needs to have an error as well.
+		if _, err := regexp.Compile(tc.pattern); err != nil {
+			ok := true
+			if err := a.verify("", regexp.MatchString); err != nil {
+				ok = false
+			}
+			if ok {
+				t.Errorf("%s: expected error in any of %q", tc.pattern, a)
+			}
+		}
+	}
+}
+
+func TestMatcher(t *T) {
+	testCases := []struct {
+		pattern     string
+		skip        string
+		parent, sub string
+		ok          bool
+		partial     bool
+	}{
+		// Behavior without subtests.
+		{"", "", "", "TestFoo", true, false},
+		{"TestFoo", "", "", "TestFoo", true, false},
+		{"TestFoo/", "", "", "TestFoo", true, true},
+		{"TestFoo/bar/baz", "", "", "TestFoo", true, true},
+		{"TestFoo", "", "", "TestBar", false, false},
+		{"TestFoo/", "", "", "TestBar", false, false},
+		{"TestFoo/bar/baz", "", "", "TestBar/bar/baz", false, false},
+		{"", "TestBar", "", "TestFoo", true, false},
+		{"", "TestBar", "", "TestBar", false, false},
+
+		// Skipping a non-existent test doesn't change anything.
+		{"", "TestFoo/skipped", "", "TestFoo", true, false},
+		{"TestFoo", "TestFoo/skipped", "", "TestFoo", true, false},
+		{"TestFoo/", "TestFoo/skipped", "", "TestFoo", true, true},
+		{"TestFoo/bar/baz", "TestFoo/skipped", "", "TestFoo", true, true},
+		{"TestFoo", "TestFoo/skipped", "", "TestBar", false, false},
+		{"TestFoo/", "TestFoo/skipped", "", "TestBar", false, false},
+		{"TestFoo/bar/baz", "TestFoo/skipped", "", "TestBar/bar/baz", false, false},
+
+		// with subtests
+		{"", "", "TestFoo", "x", true, false},
+		{"TestFoo", "", "TestFoo", "x", true, false},
+		{"TestFoo/", "", "TestFoo", "x", true, false},
+		{"TestFoo/bar/baz", "", "TestFoo", "bar", true, true},
+
+		{"", "TestFoo/skipped", "TestFoo", "x", true, false},
+		{"TestFoo", "TestFoo/skipped", "TestFoo", "x", true, false},
+		{"TestFoo", "TestFoo/skipped", "TestFoo", "skipped", false, false},
+		{"TestFoo/", "TestFoo/skipped", "TestFoo", "x", true, false},
+		{"TestFoo/bar/baz", "TestFoo/skipped", "TestFoo", "bar", true, true},
+
+		// Subtest with a '/' in its name still allows for copy and pasted names
+		// to match.
+		{"TestFoo/bar/baz", "", "TestFoo", "bar/baz", true, false},
+		{"TestFoo/bar/baz", "TestFoo/bar/baz", "TestFoo", "bar/baz", false, false},
+		{"TestFoo/bar/baz", "TestFoo/bar/baz/skip", "TestFoo", "bar/baz", true, false},
+		{"TestFoo/bar/baz", "", "TestFoo/bar", "baz", true, false},
+		{"TestFoo/bar/baz", "", "TestFoo", "x", false, false},
+		{"TestFoo", "", "TestBar", "x", false, false},
+		{"TestFoo/", "", "TestBar", "x", false, false},
+		{"TestFoo/bar/baz", "", "TestBar", "x/bar/baz", false, false},
+
+		{"A/B|C/D", "", "TestA", "B", true, false},
+		{"A/B|C/D", "", "TestC", "D", true, false},
+		{"A/B|C/D", "", "TestA", "C", false, false},
+
+		// subtests only
+		{"", "", "TestFoo", "x", true, false},
+		{"/", "", "TestFoo", "x", true, false},
+		{"./", "", "TestFoo", "x", true, false},
+		{"./.", "", "TestFoo", "x", true, false},
+		{"/bar/baz", "", "TestFoo", "bar", true, true},
+		{"/bar/baz", "", "TestFoo", "bar/baz", true, false},
+		{"//baz", "", "TestFoo", "bar/baz", true, false},
+		{"//", "", "TestFoo", "bar/baz", true, false},
+		{"/bar/baz", "", "TestFoo/bar", "baz", true, false},
+		{"//foo", "", "TestFoo", "bar/baz", false, false},
+		{"/bar/baz", "", "TestFoo", "x", false, false},
+		{"/bar/baz", "", "TestBar", "x/bar/baz", false, false},
+	}
+
+	for _, tc := range testCases {
+		m := newMatcher(regexp.MatchString, tc.pattern, "-test.run", tc.skip)
+
+		parent := &common{name: tc.parent}
+		if tc.parent != "" {
+			parent.level = 1
+		}
+		if n, ok, partial := m.fullName(parent, tc.sub); ok != tc.ok || partial != tc.partial {
+			t.Errorf("for pattern %q, fullName(parent=%q, sub=%q) = %q, ok %v partial %v; want ok %v partial %v",
+				tc.pattern, tc.parent, tc.sub, n, ok, partial, tc.ok, tc.partial)
+		}
+	}
+}
+
+var namingTestCases = []struct{ name, want string }{
+	// Uniqueness
+	{"", "x/#00"},
+	{"", "x/#01"},
+	{"#0", "x/#0"},      // Doesn't conflict with #00 because the number of digits differs.
+	{"#00", "x/#00#01"}, // Conflicts with implicit #00 (used above), so add a suffix.
+	{"#", "x/#"},
+	{"#", "x/##01"},
+
+	{"t", "x/t"},
+	{"t", "x/t#01"},
+	{"t", "x/t#02"},
+	{"t#00", "x/t#00"}, // Explicit "#00" doesn't conflict with the unsuffixed first subtest.
+
+	{"a#01", "x/a#01"},    // user has subtest with this name.
+	{"a", "x/a"},          // doesn't conflict with this name.
+	{"a", "x/a#02"},       // This string is claimed now, so resume
+	{"a", "x/a#03"},       // with counting.
+	{"a#02", "x/a#02#01"}, // We already used a#02 once, so add a suffix.
+
+	{"b#00", "x/b#00"},
+	{"b", "x/b"}, // Implicit 0 doesn't conflict with explicit "#00".
+	{"b", "x/b#01"},
+	{"b#9223372036854775807", "x/b#9223372036854775807"}, // MaxInt64
+	{"b", "x/b#02"},
+	{"b", "x/b#03"},
+
+	// Sanitizing
+	{"A:1 B:2", "x/A:1_B:2"},
+	{"s\t\r\u00a0", "x/s___"},
+	{"\x01", `x/\x01`},
+	{"\U0010ffff", `x/\U0010ffff`},
+}
+
+func TestNaming(t *T) {
+	m := newMatcher(regexp.MatchString, "", "", "")
+	parent := &common{name: "x", level: 1} // top-level test.
+
+	for i, tc := range namingTestCases {
+		if got, _, _ := m.fullName(parent, tc.name); got != tc.want {
+			t.Errorf("%d:%s: got %q; want %q", i, tc.name, got, tc.want)
+		}
+	}
+}
+
+func FuzzNaming(f *F) {
+	for _, tc := range namingTestCases {
+		f.Add(tc.name)
+	}
+	parent := &common{name: "x", level: 1}
+	var m *matcher
+	var seen map[string]string
+	reset := func() {
+		m = allMatcher()
+		seen = make(map[string]string)
+	}
+	reset()
+
+	f.Fuzz(func(t *T, subname string) {
+		if len(subname) > 10 {
+			// Long names attract the OOM killer.
+			t.Skip()
+		}
+		name := m.unique(parent.name, subname)
+		if !strings.Contains(name, "/"+subname) {
+			t.Errorf("name %q does not contain subname %q", name, subname)
+		}
+		if prev, ok := seen[name]; ok {
+			t.Errorf("name %q generated by both %q and %q", name, prev, subname)
+		}
+		if len(seen) > 1e6 {
+			// Free up memory.
+			reset()
+		}
+		seen[name] = subname
+	})
+}
+
+// GoString returns a string that is more readable than the default, which makes
+// it easier to read test errors.
+func (m alternationMatch) GoString() string {
+	s := make([]string, len(m))
+	for i, m := range m {
+		s[i] = fmt.Sprintf("%#v", m)
+	}
+	return fmt.Sprintf("(%s)", strings.Join(s, " | "))
+}

src/testing/newcover.go

@@ -0,0 +1,56 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Support for test coverage with redesigned coverage implementation.
+
+package testing
+
+import (
+	"fmt"
+	"internal/goexperiment"
+	"os"
+	_ "unsafe" // for linkname
+)
+
+// cover2 variable stores the current coverage mode and a
+// tear-down function to be called at the end of the testing run.
+var cover2 struct {
+	mode        string
+	tearDown    func(coverprofile string, gocoverdir string) (string, error)
+	snapshotcov func() float64
+}
+
+// registerCover2 is invoked during "go test -cover" runs.
+// It is used to record a 'tear down' function
+// (to be called when the test is complete) and the coverage mode.
+func registerCover2(mode string, tearDown func(coverprofile string, gocoverdir string) (string, error), snapcov func() float64) {
+	if mode == "" {
+		return
+	}
+	cover2.mode = mode
+	cover2.tearDown = tearDown
+	cover2.snapshotcov = snapcov
+}
+
+// coverReport2 invokes a callback in _testmain.go that will
+// emit coverage data at the point where test execution is complete,
+// for "go test -cover" runs.
+func coverReport2() {
+	if !goexperiment.CoverageRedesign {
+		panic("unexpected")
+	}
+	if errmsg, err := cover2.tearDown(*coverProfile, *gocoverdir); err != nil {
+		fmt.Fprintf(os.Stderr, "%s: %v\n", errmsg, err)
+		os.Exit(2)
+	}
+}
+
+// coverage2 returns a rough "coverage percentage so far"
+// number to support the testing.Coverage() function.
+func coverage2() float64 {
+	if cover2.mode == "" {
+		return 0.0
+	}
+	return cover2.snapshotcov()
+}

src/testing/panic_test.go

@@ -0,0 +1,267 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"flag"
+	"fmt"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"regexp"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+var testPanicTest = flag.String("test_panic_test", "", "TestPanic: indicates which test should panic")
+var testPanicParallel = flag.Bool("test_panic_parallel", false, "TestPanic: run subtests in parallel")
+var testPanicCleanup = flag.Bool("test_panic_cleanup", false, "TestPanic: indicates whether test should call Cleanup")
+var testPanicCleanupPanic = flag.String("test_panic_cleanup_panic", "", "TestPanic: indicate whether test should call Cleanup function that panics")
+
+func TestPanic(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	testCases := []struct {
+		desc  string
+		flags []string
+		want  string
+	}{{
+		desc:  "root test panics",
+		flags: []string{"-test_panic_test=TestPanicHelper"},
+		want: `
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+`,
+	}, {
+		desc:  "subtest panics",
+		flags: []string{"-test_panic_test=TestPanicHelper/1"},
+		want: `
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}, {
+		desc:  "subtest panics with cleanup",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}, {
+		desc:  "subtest panics with outer cleanup panic",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_cleanup_panic=outer"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+`,
+	}, {
+		desc:  "subtest panics with middle cleanup panic",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_cleanup_panic=middle"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}, {
+		desc:  "subtest panics with inner cleanup panic",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_cleanup_panic=inner"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}, {
+		desc:  "parallel subtest panics with cleanup",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_parallel"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}, {
+		desc:  "parallel subtest panics with outer cleanup panic",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_cleanup_panic=outer", "-test_panic_parallel"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+`,
+	}, {
+		desc:  "parallel subtest panics with middle cleanup panic",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_cleanup_panic=middle", "-test_panic_parallel"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}, {
+		desc:  "parallel subtest panics with inner cleanup panic",
+		flags: []string{"-test_panic_test=TestPanicHelper/1", "-test_panic_cleanup", "-test_panic_cleanup_panic=inner", "-test_panic_parallel"},
+		want: `
+ran inner cleanup 1
+ran middle cleanup 1
+ran outer cleanup
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}}
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			cmd := exec.Command(os.Args[0], "-test.run=^TestPanicHelper$")
+			cmd.Args = append(cmd.Args, tc.flags...)
+			cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+			b, _ := cmd.CombinedOutput()
+			got := string(b)
+			want := strings.TrimSpace(tc.want)
+			re := makeRegexp(want)
+			if ok, err := regexp.MatchString(re, got); !ok || err != nil {
+				t.Errorf("output:\ngot:\n%s\nwant:\n%s", got, want)
+			}
+		})
+	}
+}
+
+func makeRegexp(s string) string {
+	s = regexp.QuoteMeta(s)
+	s = strings.ReplaceAll(s, ":NNN:", `:\d+:`)
+	s = strings.ReplaceAll(s, "N\\.NNs", `\d*\.\d*s`)
+	return s
+}
+
+func TestPanicHelper(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+	t.Log(t.Name())
+	if t.Name() == *testPanicTest {
+		panic("panic")
+	}
+	switch *testPanicCleanupPanic {
+	case "", "outer", "middle", "inner":
+	default:
+		t.Fatalf("bad -test_panic_cleanup_panic: %s", *testPanicCleanupPanic)
+	}
+	t.Cleanup(func() {
+		fmt.Println("ran outer cleanup")
+		if *testPanicCleanupPanic == "outer" {
+			panic("outer cleanup")
+		}
+	})
+	for i := 0; i < 3; i++ {
+		i := i
+		t.Run(fmt.Sprintf("%v", i), func(t *testing.T) {
+			chosen := t.Name() == *testPanicTest
+			if chosen && *testPanicCleanup {
+				t.Cleanup(func() {
+					fmt.Printf("ran middle cleanup %d\n", i)
+					if *testPanicCleanupPanic == "middle" {
+						panic("middle cleanup")
+					}
+				})
+			}
+			if chosen && *testPanicParallel {
+				t.Parallel()
+			}
+			t.Log(t.Name())
+			if chosen {
+				if *testPanicCleanup {
+					t.Cleanup(func() {
+						fmt.Printf("ran inner cleanup %d\n", i)
+						if *testPanicCleanupPanic == "inner" {
+							panic("inner cleanup")
+						}
+					})
+				}
+				panic("panic")
+			}
+		})
+	}
+}
+
+func TestMorePanic(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	testCases := []struct {
+		desc  string
+		flags []string
+		want  string
+	}{
+		{
+			desc:  "Issue 48502: call runtime.Goexit in t.Cleanup after panic",
+			flags: []string{"-test.run=^TestGoexitInCleanupAfterPanicHelper$"},
+			want: `panic: die
+	panic: test executed panic(nil) or runtime.Goexit`,
+		},
+		{
+			desc:  "Issue 48515: call t.Run in t.Cleanup should trigger panic",
+			flags: []string{"-test.run=^TestCallRunInCleanupHelper$"},
+			want:  `panic: testing: t.Run called during t.Cleanup`,
+		},
+	}
+
+	for _, tc := range testCases {
+		cmd := exec.Command(os.Args[0], tc.flags...)
+		cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+		b, _ := cmd.CombinedOutput()
+		got := string(b)
+		want := tc.want
+		re := makeRegexp(want)
+		if ok, err := regexp.MatchString(re, got); !ok || err != nil {
+			t.Errorf("output:\ngot:\n%s\nwant:\n%s", got, want)
+		}
+	}
+}
+
+func TestCallRunInCleanupHelper(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+
+	t.Cleanup(func() {
+		t.Run("in-cleanup", func(t *testing.T) {
+			t.Log("must not be executed")
+		})
+	})
+}
+
+func TestGoexitInCleanupAfterPanicHelper(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+
+	t.Cleanup(func() { runtime.Goexit() })
+	t.Parallel()
+	panic("die")
+}

src/testing/quick/quick.go

@@ -0,0 +1,385 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quick implements utility functions to help with black box testing.
+//
+// The testing/quick package is frozen and is not accepting new features.
+package quick
+
+import (
+	"flag"
+	"fmt"
+	"math"
+	"math/rand"
+	"reflect"
+	"strings"
+	"time"
+)
+
+var defaultMaxCount *int = flag.Int("quickchecks", 100, "The default number of iterations for each check")
+
+// A Generator can generate random values of its own type.
+type Generator interface {
+	// Generate returns a random instance of the type on which it is a
+	// method using the size as a size hint.
+	Generate(rand *rand.Rand, size int) reflect.Value
+}
+
+// randFloat32 generates a random float taking the full range of a float32.
+func randFloat32(rand *rand.Rand) float32 {
+	f := rand.Float64() * math.MaxFloat32
+	if rand.Int()&1 == 1 {
+		f = -f
+	}
+	return float32(f)
+}
+
+// randFloat64 generates a random float taking the full range of a float64.
+func randFloat64(rand *rand.Rand) float64 {
+	f := rand.Float64() * math.MaxFloat64
+	if rand.Int()&1 == 1 {
+		f = -f
+	}
+	return f
+}
+
+// randInt64 returns a random int64.
+func randInt64(rand *rand.Rand) int64 {
+	return int64(rand.Uint64())
+}
+
+// complexSize is the maximum length of arbitrary values that contain other
+// values.
+const complexSize = 50
+
+// Value returns an arbitrary value of the given type.
+// If the type implements the [Generator] interface, that will be used.
+// Note: To create arbitrary values for structs, all the fields must be exported.
+func Value(t reflect.Type, rand *rand.Rand) (value reflect.Value, ok bool) {
+	return sizedValue(t, rand, complexSize)
+}
+
+// sizedValue returns an arbitrary value of the given type. The size
+// hint is used for shrinking as a function of indirection level so
+// that recursive data structures will terminate.
+func sizedValue(t reflect.Type, rand *rand.Rand, size int) (value reflect.Value, ok bool) {
+	if m, ok := reflect.Zero(t).Interface().(Generator); ok {
+		return m.Generate(rand, size), true
+	}
+
+	v := reflect.New(t).Elem()
+	switch concrete := t; concrete.Kind() {
+	case reflect.Bool:
+		v.SetBool(rand.Int()&1 == 0)
+	case reflect.Float32:
+		v.SetFloat(float64(randFloat32(rand)))
+	case reflect.Float64:
+		v.SetFloat(randFloat64(rand))
+	case reflect.Complex64:
+		v.SetComplex(complex(float64(randFloat32(rand)), float64(randFloat32(rand))))
+	case reflect.Complex128:
+		v.SetComplex(complex(randFloat64(rand), randFloat64(rand)))
+	case reflect.Int16:
+		v.SetInt(randInt64(rand))
+	case reflect.Int32:
+		v.SetInt(randInt64(rand))
+	case reflect.Int64:
+		v.SetInt(randInt64(rand))
+	case reflect.Int8:
+		v.SetInt(randInt64(rand))
+	case reflect.Int:
+		v.SetInt(randInt64(rand))
+	case reflect.Uint16:
+		v.SetUint(uint64(randInt64(rand)))
+	case reflect.Uint32:
+		v.SetUint(uint64(randInt64(rand)))
+	case reflect.Uint64:
+		v.SetUint(uint64(randInt64(rand)))
+	case reflect.Uint8:
+		v.SetUint(uint64(randInt64(rand)))
+	case reflect.Uint:
+		v.SetUint(uint64(randInt64(rand)))
+	case reflect.Uintptr:
+		v.SetUint(uint64(randInt64(rand)))
+	case reflect.Map:
+		numElems := rand.Intn(size)
+		v.Set(reflect.MakeMap(concrete))
+		for i := 0; i < numElems; i++ {
+			key, ok1 := sizedValue(concrete.Key(), rand, size)
+			value, ok2 := sizedValue(concrete.Elem(), rand, size)
+			if !ok1 || !ok2 {
+				return reflect.Value{}, false
+			}
+			v.SetMapIndex(key, value)
+		}
+	case reflect.Pointer:
+		if rand.Intn(size) == 0 {
+			v.SetZero() // Generate nil pointer.
+		} else {
+			elem, ok := sizedValue(concrete.Elem(), rand, size)
+			if !ok {
+				return reflect.Value{}, false
+			}
+			v.Set(reflect.New(concrete.Elem()))
+			v.Elem().Set(elem)
+		}
+	case reflect.Slice:
+		numElems := rand.Intn(size)
+		sizeLeft := size - numElems
+		v.Set(reflect.MakeSlice(concrete, numElems, numElems))
+		for i := 0; i < numElems; i++ {
+			elem, ok := sizedValue(concrete.Elem(), rand, sizeLeft)
+			if !ok {
+				return reflect.Value{}, false
+			}
+			v.Index(i).Set(elem)
+		}
+	case reflect.Array:
+		for i := 0; i < v.Len(); i++ {
+			elem, ok := sizedValue(concrete.Elem(), rand, size)
+			if !ok {
+				return reflect.Value{}, false
+			}
+			v.Index(i).Set(elem)
+		}
+	case reflect.String:
+		numChars := rand.Intn(complexSize)
+		codePoints := make([]rune, numChars)
+		for i := 0; i < numChars; i++ {
+			codePoints[i] = rune(rand.Intn(0x10ffff))
+		}
+		v.SetString(string(codePoints))
+	case reflect.Struct:
+		n := v.NumField()
+		// Divide sizeLeft evenly among the struct fields.
+		sizeLeft := size
+		if n > sizeLeft {
+			sizeLeft = 1
+		} else if n > 0 {
+			sizeLeft /= n
+		}
+		for i := 0; i < n; i++ {
+			elem, ok := sizedValue(concrete.Field(i).Type, rand, sizeLeft)
+			if !ok {
+				return reflect.Value{}, false
+			}
+			v.Field(i).Set(elem)
+		}
+	default:
+		return reflect.Value{}, false
+	}
+
+	return v, true
+}
+
+// A Config structure contains options for running a test.
+type Config struct {
+	// MaxCount sets the maximum number of iterations.
+	// If zero, MaxCountScale is used.
+	MaxCount int
+	// MaxCountScale is a non-negative scale factor applied to the
+	// default maximum.
+	// A count of zero implies the default, which is usually 100
+	// but can be set by the -quickchecks flag.
+	MaxCountScale float64
+	// Rand specifies a source of random numbers.
+	// If nil, a default pseudo-random source will be used.
+	Rand *rand.Rand
+	// Values specifies a function to generate a slice of
+	// arbitrary reflect.Values that are congruent with the
+	// arguments to the function being tested.
+	// If nil, the top-level Value function is used to generate them.
+	Values func([]reflect.Value, *rand.Rand)
+}
+
+var defaultConfig Config
+
+// getRand returns the *rand.Rand to use for a given Config.
+func (c *Config) getRand() *rand.Rand {
+	if c.Rand == nil {
+		return rand.New(rand.NewSource(time.Now().UnixNano()))
+	}
+	return c.Rand
+}
+
+// getMaxCount returns the maximum number of iterations to run for a given
+// Config.
+func (c *Config) getMaxCount() (maxCount int) {
+	maxCount = c.MaxCount
+	if maxCount == 0 {
+		if c.MaxCountScale != 0 {
+			maxCount = int(c.MaxCountScale * float64(*defaultMaxCount))
+		} else {
+			maxCount = *defaultMaxCount
+		}
+	}
+
+	return
+}
+
+// A SetupError is the result of an error in the way that check is being
+// used, independent of the functions being tested.
+type SetupError string
+
+func (s SetupError) Error() string { return string(s) }
+
+// A CheckError is the result of Check finding an error.
+type CheckError struct {
+	Count int
+	In    []any
+}
+
+func (s *CheckError) Error() string {
+	return fmt.Sprintf("#%d: failed on input %s", s.Count, toString(s.In))
+}
+
+// A CheckEqualError is the result [CheckEqual] finding an error.
+type CheckEqualError struct {
+	CheckError
+	Out1 []any
+	Out2 []any
+}
+
+func (s *CheckEqualError) Error() string {
+	return fmt.Sprintf("#%d: failed on input %s. Output 1: %s. Output 2: %s", s.Count, toString(s.In), toString(s.Out1), toString(s.Out2))
+}
+
+// Check looks for an input to f, any function that returns bool,
+// such that f returns false. It calls f repeatedly, with arbitrary
+// values for each argument. If f returns false on a given input,
+// Check returns that input as a *[CheckError].
+// For example:
+//
+//	func TestOddMultipleOfThree(t *testing.T) {
+//		f := func(x int) bool {
+//			y := OddMultipleOfThree(x)
+//			return y%2 == 1 && y%3 == 0
+//		}
+//		if err := quick.Check(f, nil); err != nil {
+//			t.Error(err)
+//		}
+//	}
+func Check(f any, config *Config) error {
+	if config == nil {
+		config = &defaultConfig
+	}
+
+	fVal, fType, ok := functionAndType(f)
+	if !ok {
+		return SetupError("argument is not a function")
+	}
+
+	if fType.NumOut() != 1 {
+		return SetupError("function does not return one value")
+	}
+	if fType.Out(0).Kind() != reflect.Bool {
+		return SetupError("function does not return a bool")
+	}
+
+	arguments := make([]reflect.Value, fType.NumIn())
+	rand := config.getRand()
+	maxCount := config.getMaxCount()
+
+	for i := 0; i < maxCount; i++ {
+		err := arbitraryValues(arguments, fType, config, rand)
+		if err != nil {
+			return err
+		}
+
+		if !fVal.Call(arguments)[0].Bool() {
+			return &CheckError{i + 1, toInterfaces(arguments)}
+		}
+	}
+
+	return nil
+}
+
+// CheckEqual looks for an input on which f and g return different results.
+// It calls f and g repeatedly with arbitrary values for each argument.
+// If f and g return different answers, CheckEqual returns a *[CheckEqualError]
+// describing the input and the outputs.
+func CheckEqual(f, g any, config *Config) error {
+	if config == nil {
+		config = &defaultConfig
+	}
+
+	x, xType, ok := functionAndType(f)
+	if !ok {
+		return SetupError("f is not a function")
+	}
+	y, yType, ok := functionAndType(g)
+	if !ok {
+		return SetupError("g is not a function")
+	}
+
+	if xType != yType {
+		return SetupError("functions have different types")
+	}
+
+	arguments := make([]reflect.Value, xType.NumIn())
+	rand := config.getRand()
+	maxCount := config.getMaxCount()
+
+	for i := 0; i < maxCount; i++ {
+		err := arbitraryValues(arguments, xType, config, rand)
+		if err != nil {
+			return err
+		}
+
+		xOut := toInterfaces(x.Call(arguments))
+		yOut := toInterfaces(y.Call(arguments))
+
+		if !reflect.DeepEqual(xOut, yOut) {
+			return &CheckEqualError{CheckError{i + 1, toInterfaces(arguments)}, xOut, yOut}
+		}
+	}
+
+	return nil
+}
+
+// arbitraryValues writes Values to args such that args contains Values
+// suitable for calling f.
+func arbitraryValues(args []reflect.Value, f reflect.Type, config *Config, rand *rand.Rand) (err error) {
+	if config.Values != nil {
+		config.Values(args, rand)
+		return
+	}
+
+	for j := 0; j < len(args); j++ {
+		var ok bool
+		args[j], ok = Value(f.In(j), rand)
+		if !ok {
+			err = SetupError(fmt.Sprintf("cannot create arbitrary value of type %s for argument %d", f.In(j), j))
+			return
+		}
+	}
+
+	return
+}
+
+func functionAndType(f any) (v reflect.Value, t reflect.Type, ok bool) {
+	v = reflect.ValueOf(f)
+	ok = v.Kind() == reflect.Func
+	if !ok {
+		return
+	}
+	t = v.Type()
+	return
+}
+
+func toInterfaces(values []reflect.Value) []any {
+	ret := make([]any, len(values))
+	for i, v := range values {
+		ret[i] = v.Interface()
+	}
+	return ret
+}
+
+func toString(interfaces []any) string {
+	s := make([]string, len(interfaces))
+	for i, v := range interfaces {
+		s[i] = fmt.Sprintf("%#v", v)
+	}
+	return strings.Join(s, ", ")
+}

src/testing/quick/quick_test.go

@@ -0,0 +1,327 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quick
+
+import (
+	"math/rand"
+	"reflect"
+	"testing"
+)
+
+func fArray(a [4]byte) [4]byte { return a }
+
+type TestArrayAlias [4]byte
+
+func fArrayAlias(a TestArrayAlias) TestArrayAlias { return a }
+
+func fBool(a bool) bool { return a }
+
+type TestBoolAlias bool
+
+func fBoolAlias(a TestBoolAlias) TestBoolAlias { return a }
+
+func fFloat32(a float32) float32 { return a }
+
+type TestFloat32Alias float32
+
+func fFloat32Alias(a TestFloat32Alias) TestFloat32Alias { return a }
+
+func fFloat64(a float64) float64 { return a }
+
+type TestFloat64Alias float64
+
+func fFloat64Alias(a TestFloat64Alias) TestFloat64Alias { return a }
+
+func fComplex64(a complex64) complex64 { return a }
+
+type TestComplex64Alias complex64
+
+func fComplex64Alias(a TestComplex64Alias) TestComplex64Alias { return a }
+
+func fComplex128(a complex128) complex128 { return a }
+
+type TestComplex128Alias complex128
+
+func fComplex128Alias(a TestComplex128Alias) TestComplex128Alias { return a }
+
+func fInt16(a int16) int16 { return a }
+
+type TestInt16Alias int16
+
+func fInt16Alias(a TestInt16Alias) TestInt16Alias { return a }
+
+func fInt32(a int32) int32 { return a }
+
+type TestInt32Alias int32
+
+func fInt32Alias(a TestInt32Alias) TestInt32Alias { return a }
+
+func fInt64(a int64) int64 { return a }
+
+type TestInt64Alias int64
+
+func fInt64Alias(a TestInt64Alias) TestInt64Alias { return a }
+
+func fInt8(a int8) int8 { return a }
+
+type TestInt8Alias int8
+
+func fInt8Alias(a TestInt8Alias) TestInt8Alias { return a }
+
+func fInt(a int) int { return a }
+
+type TestIntAlias int
+
+func fIntAlias(a TestIntAlias) TestIntAlias { return a }
+
+func fMap(a map[int]int) map[int]int { return a }
+
+type TestMapAlias map[int]int
+
+func fMapAlias(a TestMapAlias) TestMapAlias { return a }
+
+func fPtr(a *int) *int {
+	if a == nil {
+		return nil
+	}
+	b := *a
+	return &b
+}
+
+type TestPtrAlias *int
+
+func fPtrAlias(a TestPtrAlias) TestPtrAlias { return a }
+
+func fSlice(a []byte) []byte { return a }
+
+type TestSliceAlias []byte
+
+func fSliceAlias(a TestSliceAlias) TestSliceAlias { return a }
+
+func fString(a string) string { return a }
+
+type TestStringAlias string
+
+func fStringAlias(a TestStringAlias) TestStringAlias { return a }
+
+type TestStruct struct {
+	A int
+	B string
+}
+
+func fStruct(a TestStruct) TestStruct { return a }
+
+type TestStructAlias TestStruct
+
+func fStructAlias(a TestStructAlias) TestStructAlias { return a }
+
+func fUint16(a uint16) uint16 { return a }
+
+type TestUint16Alias uint16
+
+func fUint16Alias(a TestUint16Alias) TestUint16Alias { return a }
+
+func fUint32(a uint32) uint32 { return a }
+
+type TestUint32Alias uint32
+
+func fUint32Alias(a TestUint32Alias) TestUint32Alias { return a }
+
+func fUint64(a uint64) uint64 { return a }
+
+type TestUint64Alias uint64
+
+func fUint64Alias(a TestUint64Alias) TestUint64Alias { return a }
+
+func fUint8(a uint8) uint8 { return a }
+
+type TestUint8Alias uint8
+
+func fUint8Alias(a TestUint8Alias) TestUint8Alias { return a }
+
+func fUint(a uint) uint { return a }
+
+type TestUintAlias uint
+
+func fUintAlias(a TestUintAlias) TestUintAlias { return a }
+
+func fUintptr(a uintptr) uintptr { return a }
+
+type TestUintptrAlias uintptr
+
+func fUintptrAlias(a TestUintptrAlias) TestUintptrAlias { return a }
+
+func reportError(property string, err error, t *testing.T) {
+	if err != nil {
+		t.Errorf("%s: %s", property, err)
+	}
+}
+
+func TestCheckEqual(t *testing.T) {
+	reportError("fArray", CheckEqual(fArray, fArray, nil), t)
+	reportError("fArrayAlias", CheckEqual(fArrayAlias, fArrayAlias, nil), t)
+	reportError("fBool", CheckEqual(fBool, fBool, nil), t)
+	reportError("fBoolAlias", CheckEqual(fBoolAlias, fBoolAlias, nil), t)
+	reportError("fFloat32", CheckEqual(fFloat32, fFloat32, nil), t)
+	reportError("fFloat32Alias", CheckEqual(fFloat32Alias, fFloat32Alias, nil), t)
+	reportError("fFloat64", CheckEqual(fFloat64, fFloat64, nil), t)
+	reportError("fFloat64Alias", CheckEqual(fFloat64Alias, fFloat64Alias, nil), t)
+	reportError("fComplex64", CheckEqual(fComplex64, fComplex64, nil), t)
+	reportError("fComplex64Alias", CheckEqual(fComplex64Alias, fComplex64Alias, nil), t)
+	reportError("fComplex128", CheckEqual(fComplex128, fComplex128, nil), t)
+	reportError("fComplex128Alias", CheckEqual(fComplex128Alias, fComplex128Alias, nil), t)
+	reportError("fInt16", CheckEqual(fInt16, fInt16, nil), t)
+	reportError("fInt16Alias", CheckEqual(fInt16Alias, fInt16Alias, nil), t)
+	reportError("fInt32", CheckEqual(fInt32, fInt32, nil), t)
+	reportError("fInt32Alias", CheckEqual(fInt32Alias, fInt32Alias, nil), t)
+	reportError("fInt64", CheckEqual(fInt64, fInt64, nil), t)
+	reportError("fInt64Alias", CheckEqual(fInt64Alias, fInt64Alias, nil), t)
+	reportError("fInt8", CheckEqual(fInt8, fInt8, nil), t)
+	reportError("fInt8Alias", CheckEqual(fInt8Alias, fInt8Alias, nil), t)
+	reportError("fInt", CheckEqual(fInt, fInt, nil), t)
+	reportError("fIntAlias", CheckEqual(fIntAlias, fIntAlias, nil), t)
+	reportError("fInt32", CheckEqual(fInt32, fInt32, nil), t)
+	reportError("fInt32Alias", CheckEqual(fInt32Alias, fInt32Alias, nil), t)
+	reportError("fMap", CheckEqual(fMap, fMap, nil), t)
+	reportError("fMapAlias", CheckEqual(fMapAlias, fMapAlias, nil), t)
+	reportError("fPtr", CheckEqual(fPtr, fPtr, nil), t)
+	reportError("fPtrAlias", CheckEqual(fPtrAlias, fPtrAlias, nil), t)
+	reportError("fSlice", CheckEqual(fSlice, fSlice, nil), t)
+	reportError("fSliceAlias", CheckEqual(fSliceAlias, fSliceAlias, nil), t)
+	reportError("fString", CheckEqual(fString, fString, nil), t)
+	reportError("fStringAlias", CheckEqual(fStringAlias, fStringAlias, nil), t)
+	reportError("fStruct", CheckEqual(fStruct, fStruct, nil), t)
+	reportError("fStructAlias", CheckEqual(fStructAlias, fStructAlias, nil), t)
+	reportError("fUint16", CheckEqual(fUint16, fUint16, nil), t)
+	reportError("fUint16Alias", CheckEqual(fUint16Alias, fUint16Alias, nil), t)
+	reportError("fUint32", CheckEqual(fUint32, fUint32, nil), t)
+	reportError("fUint32Alias", CheckEqual(fUint32Alias, fUint32Alias, nil), t)
+	reportError("fUint64", CheckEqual(fUint64, fUint64, nil), t)
+	reportError("fUint64Alias", CheckEqual(fUint64Alias, fUint64Alias, nil), t)
+	reportError("fUint8", CheckEqual(fUint8, fUint8, nil), t)
+	reportError("fUint8Alias", CheckEqual(fUint8Alias, fUint8Alias, nil), t)
+	reportError("fUint", CheckEqual(fUint, fUint, nil), t)
+	reportError("fUintAlias", CheckEqual(fUintAlias, fUintAlias, nil), t)
+	reportError("fUintptr", CheckEqual(fUintptr, fUintptr, nil), t)
+	reportError("fUintptrAlias", CheckEqual(fUintptrAlias, fUintptrAlias, nil), t)
+}
+
+// This tests that ArbitraryValue is working by checking that all the arbitrary
+// values of type MyStruct have x = 42.
+type myStruct struct {
+	x int
+}
+
+func (m myStruct) Generate(r *rand.Rand, _ int) reflect.Value {
+	return reflect.ValueOf(myStruct{x: 42})
+}
+
+func myStructProperty(in myStruct) bool { return in.x == 42 }
+
+func TestCheckProperty(t *testing.T) {
+	reportError("myStructProperty", Check(myStructProperty, nil), t)
+}
+
+func TestFailure(t *testing.T) {
+	f := func(x int) bool { return false }
+	err := Check(f, nil)
+	if err == nil {
+		t.Errorf("Check didn't return an error")
+	}
+	if _, ok := err.(*CheckError); !ok {
+		t.Errorf("Error was not a CheckError: %s", err)
+	}
+
+	err = CheckEqual(fUint, fUint32, nil)
+	if err == nil {
+		t.Errorf("#1 CheckEqual didn't return an error")
+	}
+	if _, ok := err.(SetupError); !ok {
+		t.Errorf("#1 Error was not a SetupError: %s", err)
+	}
+
+	err = CheckEqual(func(x, y int) {}, func(x int) {}, nil)
+	if err == nil {
+		t.Errorf("#2 CheckEqual didn't return an error")
+	}
+	if _, ok := err.(SetupError); !ok {
+		t.Errorf("#2 Error was not a SetupError: %s", err)
+	}
+
+	err = CheckEqual(func(x int) int { return 0 }, func(x int) int32 { return 0 }, nil)
+	if err == nil {
+		t.Errorf("#3 CheckEqual didn't return an error")
+	}
+	if _, ok := err.(SetupError); !ok {
+		t.Errorf("#3 Error was not a SetupError: %s", err)
+	}
+}
+
+// Recursive data structures didn't terminate.
+// Issues 8818 and 11148.
+func TestRecursive(t *testing.T) {
+	type R struct {
+		Ptr      *R
+		SliceP   []*R
+		Slice    []R
+		Map      map[int]R
+		MapP     map[int]*R
+		MapR     map[*R]*R
+		SliceMap []map[int]R
+	}
+
+	f := func(r R) bool { return true }
+	Check(f, nil)
+}
+
+func TestEmptyStruct(t *testing.T) {
+	f := func(struct{}) bool { return true }
+	Check(f, nil)
+}
+
+type (
+	A struct{ B *B }
+	B struct{ A *A }
+)
+
+func TestMutuallyRecursive(t *testing.T) {
+	f := func(a A) bool { return true }
+	Check(f, nil)
+}
+
+// Some serialization formats (e.g. encoding/pem) cannot distinguish
+// between a nil and an empty map or slice, so avoid generating the
+// zero value for these.
+func TestNonZeroSliceAndMap(t *testing.T) {
+	type Q struct {
+		M map[int]int
+		S []int
+	}
+	f := func(q Q) bool {
+		return q.M != nil && q.S != nil
+	}
+	err := Check(f, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestInt64(t *testing.T) {
+	var lo, hi int64
+	f := func(x int64) bool {
+		if x < lo {
+			lo = x
+		}
+		if x > hi {
+			hi = x
+		}
+		return true
+	}
+	cfg := &Config{MaxCount: 10000}
+	Check(f, cfg)
+	if uint64(lo)>>62 == 0 || uint64(hi)>>62 == 0 {
+		t.Errorf("int64 returned range %#016x,%#016x; does not look like full range", lo, hi)
+	}
+}

src/testing/run_example.go

@@ -0,0 +1,66 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !js && !wasip1
+
+// TODO(@musiol, @odeke-em): re-unify this entire file back into
+// example.go when js/wasm gets an os.Pipe implementation
+// and no longer needs this separation.
+
+package testing
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+func runExample(eg InternalExample) (ok bool) {
+	if chatty.on {
+		fmt.Printf("%s=== RUN   %s\n", chatty.prefix(), eg.Name)
+	}
+
+	// Capture stdout.
+	stdout := os.Stdout
+	r, w, err := os.Pipe()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	os.Stdout = w
+	outC := make(chan string)
+	go func() {
+		var buf strings.Builder
+		_, err := io.Copy(&buf, r)
+		r.Close()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: copying pipe: %v\n", err)
+			os.Exit(1)
+		}
+		outC <- buf.String()
+	}()
+
+	finished := false
+	start := time.Now()
+
+	// Clean up in a deferred call so we can recover if the example panics.
+	defer func() {
+		timeSpent := time.Since(start)
+
+		// Close pipe, restore stdout, get output.
+		w.Close()
+		os.Stdout = stdout
+		out := <-outC
+
+		err := recover()
+		ok = eg.processRunResult(out, timeSpent, finished, err)
+	}()
+
+	// Run example.
+	eg.F()
+	finished = true
+	return
+}

src/testing/run_example_wasm.go

@@ -0,0 +1,76 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js || wasip1
+
+package testing
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+// TODO(@musiol, @odeke-em): unify this code back into
+// example.go when js/wasm gets an os.Pipe implementation.
+func runExample(eg InternalExample) (ok bool) {
+	if chatty.on {
+		fmt.Printf("%s=== RUN   %s\n", chatty.prefix(), eg.Name)
+	}
+
+	// Capture stdout to temporary file. We're not using
+	// os.Pipe because it is not supported on js/wasm.
+	stdout := os.Stdout
+	f := createTempFile(eg.Name)
+	os.Stdout = f
+	finished := false
+	start := time.Now()
+
+	// Clean up in a deferred call so we can recover if the example panics.
+	defer func() {
+		timeSpent := time.Since(start)
+
+		// Restore stdout, get output and remove temporary file.
+		os.Stdout = stdout
+		var buf strings.Builder
+		_, seekErr := f.Seek(0, io.SeekStart)
+		_, readErr := io.Copy(&buf, f)
+		out := buf.String()
+		f.Close()
+		os.Remove(f.Name())
+		if seekErr != nil {
+			fmt.Fprintf(os.Stderr, "testing: seek temp file: %v\n", seekErr)
+			os.Exit(1)
+		}
+		if readErr != nil {
+			fmt.Fprintf(os.Stderr, "testing: read temp file: %v\n", readErr)
+			os.Exit(1)
+		}
+
+		err := recover()
+		ok = eg.processRunResult(out, timeSpent, finished, err)
+	}()
+
+	// Run example.
+	eg.F()
+	finished = true
+	return
+}
+
+func createTempFile(exampleName string) *os.File {
+	for i := 0; ; i++ {
+		name := fmt.Sprintf("%s/go-example-stdout-%s-%d.txt", os.TempDir(), exampleName, i)
+		f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
+		if err != nil {
+			if os.IsExist(err) {
+				continue
+			}
+			fmt.Fprintf(os.Stderr, "testing: open temp file: %v\n", err)
+			os.Exit(1)
+		}
+		return f
+	}
+}

src/testing/slogtest/example_test.go

@@ -0,0 +1,44 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slogtest_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"log"
+	"log/slog"
+	"testing/slogtest"
+)
+
+// This example demonstrates one technique for testing a handler with this
+// package. The handler is given a [bytes.Buffer] to write to, and each line
+// of the resulting output is parsed.
+// For JSON output, [encoding/json.Unmarshal] produces a result in the desired
+// format when given a pointer to a map[string]any.
+func Example_parsing() {
+	var buf bytes.Buffer
+	h := slog.NewJSONHandler(&buf, nil)
+
+	results := func() []map[string]any {
+		var ms []map[string]any
+		for _, line := range bytes.Split(buf.Bytes(), []byte{'\n'}) {
+			if len(line) == 0 {
+				continue
+			}
+			var m map[string]any
+			if err := json.Unmarshal(line, &m); err != nil {
+				panic(err) // In a real test, use t.Fatal.
+			}
+			ms = append(ms, m)
+		}
+		return ms
+	}
+	err := slogtest.TestHandler(h, results)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+}

src/testing/slogtest/run_test.go

@@ -0,0 +1,31 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slogtest_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"testing"
+	"testing/slogtest"
+)
+
+func TestRun(t *testing.T) {
+	var buf bytes.Buffer
+
+	newHandler := func(*testing.T) slog.Handler {
+		buf.Reset()
+		return slog.NewJSONHandler(&buf, nil)
+	}
+	result := func(t *testing.T) map[string]any {
+		m := map[string]any{}
+		if err := json.Unmarshal(buf.Bytes(), &m); err != nil {
+			t.Fatal(err)
+		}
+		return m
+	}
+
+	slogtest.Run(t, newHandler, result)
+}

src/testing/slogtest/slogtest.go

@@ -0,0 +1,375 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package slogtest implements support for testing implementations of log/slog.Handler.
+package slogtest
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"reflect"
+	"runtime"
+	"testing"
+	"time"
+)
+
+type testCase struct {
+	// Subtest name.
+	name string
+	// If non-empty, explanation explains the violated constraint.
+	explanation string
+	// f executes a single log event using its argument logger.
+	// So that mkdescs.sh can generate the right description,
+	// the body of f must appear on a single line whose first
+	// non-whitespace characters are "l.".
+	f func(*slog.Logger)
+	// If mod is not nil, it is called to modify the Record
+	// generated by the Logger before it is passed to the Handler.
+	mod func(*slog.Record)
+	// checks is a list of checks to run on the result.
+	checks []check
+}
+
+var cases = []testCase{
+	{
+		name:        "built-ins",
+		explanation: withSource("this test expects slog.TimeKey, slog.LevelKey and slog.MessageKey"),
+		f: func(l *slog.Logger) {
+			l.Info("message")
+		},
+		checks: []check{
+			hasKey(slog.TimeKey),
+			hasKey(slog.LevelKey),
+			hasAttr(slog.MessageKey, "message"),
+		},
+	},
+	{
+		name:        "attrs",
+		explanation: withSource("a Handler should output attributes passed to the logging function"),
+		f: func(l *slog.Logger) {
+			l.Info("message", "k", "v")
+		},
+		checks: []check{
+			hasAttr("k", "v"),
+		},
+	},
+	{
+		name:        "empty-attr",
+		explanation: withSource("a Handler should ignore an empty Attr"),
+		f: func(l *slog.Logger) {
+			l.Info("msg", "a", "b", "", nil, "c", "d")
+		},
+		checks: []check{
+			hasAttr("a", "b"),
+			missingKey(""),
+			hasAttr("c", "d"),
+		},
+	},
+	{
+		name:        "zero-time",
+		explanation: withSource("a Handler should ignore a zero Record.Time"),
+		f: func(l *slog.Logger) {
+			l.Info("msg", "k", "v")
+		},
+		mod: func(r *slog.Record) { r.Time = time.Time{} },
+		checks: []check{
+			missingKey(slog.TimeKey),
+		},
+	},
+	{
+		name:        "WithAttrs",
+		explanation: withSource("a Handler should include the attributes from the WithAttrs method"),
+		f: func(l *slog.Logger) {
+			l.With("a", "b").Info("msg", "k", "v")
+		},
+		checks: []check{
+			hasAttr("a", "b"),
+			hasAttr("k", "v"),
+		},
+	},
+	{
+		name:        "groups",
+		explanation: withSource("a Handler should handle Group attributes"),
+		f: func(l *slog.Logger) {
+			l.Info("msg", "a", "b", slog.Group("G", slog.String("c", "d")), "e", "f")
+		},
+		checks: []check{
+			hasAttr("a", "b"),
+			inGroup("G", hasAttr("c", "d")),
+			hasAttr("e", "f"),
+		},
+	},
+	{
+		name:        "empty-group",
+		explanation: withSource("a Handler should ignore an empty group"),
+		f: func(l *slog.Logger) {
+			l.Info("msg", "a", "b", slog.Group("G"), "e", "f")
+		},
+		checks: []check{
+			hasAttr("a", "b"),
+			missingKey("G"),
+			hasAttr("e", "f"),
+		},
+	},
+	{
+		name:        "inline-group",
+		explanation: withSource("a Handler should inline the Attrs of a group with an empty key"),
+		f: func(l *slog.Logger) {
+			l.Info("msg", "a", "b", slog.Group("", slog.String("c", "d")), "e", "f")
+
+		},
+		checks: []check{
+			hasAttr("a", "b"),
+			hasAttr("c", "d"),
+			hasAttr("e", "f"),
+		},
+	},
+	{
+		name:        "WithGroup",
+		explanation: withSource("a Handler should handle the WithGroup method"),
+		f: func(l *slog.Logger) {
+			l.WithGroup("G").Info("msg", "a", "b")
+		},
+		checks: []check{
+			hasKey(slog.TimeKey),
+			hasKey(slog.LevelKey),
+			hasAttr(slog.MessageKey, "msg"),
+			missingKey("a"),
+			inGroup("G", hasAttr("a", "b")),
+		},
+	},
+	{
+		name:        "multi-With",
+		explanation: withSource("a Handler should handle multiple WithGroup and WithAttr calls"),
+		f: func(l *slog.Logger) {
+			l.With("a", "b").WithGroup("G").With("c", "d").WithGroup("H").Info("msg", "e", "f")
+		},
+		checks: []check{
+			hasKey(slog.TimeKey),
+			hasKey(slog.LevelKey),
+			hasAttr(slog.MessageKey, "msg"),
+			hasAttr("a", "b"),
+			inGroup("G", hasAttr("c", "d")),
+			inGroup("G", inGroup("H", hasAttr("e", "f"))),
+		},
+	},
+	{
+		name:        "empty-group-record",
+		explanation: withSource("a Handler should not output groups if there are no attributes"),
+		f: func(l *slog.Logger) {
+			l.With("a", "b").WithGroup("G").With("c", "d").WithGroup("H").Info("msg")
+		},
+		checks: []check{
+			hasKey(slog.TimeKey),
+			hasKey(slog.LevelKey),
+			hasAttr(slog.MessageKey, "msg"),
+			hasAttr("a", "b"),
+			inGroup("G", hasAttr("c", "d")),
+			inGroup("G", missingKey("H")),
+		},
+	},
+	{
+		name:        "resolve",
+		explanation: withSource("a Handler should call Resolve on attribute values"),
+		f: func(l *slog.Logger) {
+			l.Info("msg", "k", &replace{"replaced"})
+		},
+		checks: []check{hasAttr("k", "replaced")},
+	},
+	{
+		name:        "resolve-groups",
+		explanation: withSource("a Handler should call Resolve on attribute values in groups"),
+		f: func(l *slog.Logger) {
+			l.Info("msg",
+				slog.Group("G",
+					slog.String("a", "v1"),
+					slog.Any("b", &replace{"v2"})))
+		},
+		checks: []check{
+			inGroup("G", hasAttr("a", "v1")),
+			inGroup("G", hasAttr("b", "v2")),
+		},
+	},
+	{
+		name:        "resolve-WithAttrs",
+		explanation: withSource("a Handler should call Resolve on attribute values from WithAttrs"),
+		f: func(l *slog.Logger) {
+			l = l.With("k", &replace{"replaced"})
+			l.Info("msg")
+		},
+		checks: []check{hasAttr("k", "replaced")},
+	},
+	{
+		name:        "resolve-WithAttrs-groups",
+		explanation: withSource("a Handler should call Resolve on attribute values in groups from WithAttrs"),
+		f: func(l *slog.Logger) {
+			l = l.With(slog.Group("G",
+				slog.String("a", "v1"),
+				slog.Any("b", &replace{"v2"})))
+			l.Info("msg")
+		},
+		checks: []check{
+			inGroup("G", hasAttr("a", "v1")),
+			inGroup("G", hasAttr("b", "v2")),
+		},
+	},
+	{
+		name:        "empty-PC",
+		explanation: withSource("a Handler should not output SourceKey if the PC is zero"),
+		f: func(l *slog.Logger) {
+			l.Info("message")
+		},
+		mod: func(r *slog.Record) { r.PC = 0 },
+		checks: []check{
+			missingKey(slog.SourceKey),
+		},
+	},
+}
+
+// TestHandler tests a [slog.Handler].
+// If TestHandler finds any misbehaviors, it returns an error for each,
+// combined into a single error with [errors.Join].
+//
+// TestHandler installs the given Handler in a [slog.Logger] and
+// makes several calls to the Logger's output methods.
+// The Handler should be enabled for levels Info and above.
+//
+// The results function is invoked after all such calls.
+// It should return a slice of map[string]any, one for each call to a Logger output method.
+// The keys and values of the map should correspond to the keys and values of the Handler's
+// output. Each group in the output should be represented as its own nested map[string]any.
+// The standard keys [slog.TimeKey], [slog.LevelKey] and [slog.MessageKey] should be used.
+//
+// If the Handler outputs JSON, then calling [encoding/json.Unmarshal] with a `map[string]any`
+// will create the right data structure.
+//
+// If a Handler intentionally drops an attribute that is checked by a test,
+// then the results function should check for its absence and add it to the map it returns.
+func TestHandler(h slog.Handler, results func() []map[string]any) error {
+	// Run the handler on the test cases.
+	for _, c := range cases {
+		ht := h
+		if c.mod != nil {
+			ht = &wrapper{h, c.mod}
+		}
+		l := slog.New(ht)
+		c.f(l)
+	}
+
+	// Collect and check the results.
+	var errs []error
+	res := results()
+	if g, w := len(res), len(cases); g != w {
+		return fmt.Errorf("got %d results, want %d", g, w)
+	}
+	for i, got := range results() {
+		c := cases[i]
+		for _, check := range c.checks {
+			if problem := check(got); problem != "" {
+				errs = append(errs, fmt.Errorf("%s: %s", problem, c.explanation))
+			}
+		}
+	}
+	return errors.Join(errs...)
+}
+
+// Run exercises a [slog.Handler] on the same test cases as [TestHandler], but
+// runs each case in a subtest. For each test case, it first calls newHandler to
+// get an instance of the handler under test, then runs the test case, then
+// calls result to get the result. If the test case fails, it calls t.Error.
+func Run(t *testing.T, newHandler func(*testing.T) slog.Handler, result func(*testing.T) map[string]any) {
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			h := newHandler(t)
+			if c.mod != nil {
+				h = &wrapper{h, c.mod}
+			}
+			l := slog.New(h)
+			c.f(l)
+			got := result(t)
+			for _, check := range c.checks {
+				if p := check(got); p != "" {
+					t.Errorf("%s: %s", p, c.explanation)
+				}
+			}
+		})
+	}
+}
+
+type check func(map[string]any) string
+
+func hasKey(key string) check {
+	return func(m map[string]any) string {
+		if _, ok := m[key]; !ok {
+			return fmt.Sprintf("missing key %q", key)
+		}
+		return ""
+	}
+}
+
+func missingKey(key string) check {
+	return func(m map[string]any) string {
+		if _, ok := m[key]; ok {
+			return fmt.Sprintf("unexpected key %q", key)
+		}
+		return ""
+	}
+}
+
+func hasAttr(key string, wantVal any) check {
+	return func(m map[string]any) string {
+		if s := hasKey(key)(m); s != "" {
+			return s
+		}
+		gotVal := m[key]
+		if !reflect.DeepEqual(gotVal, wantVal) {
+			return fmt.Sprintf("%q: got %#v, want %#v", key, gotVal, wantVal)
+		}
+		return ""
+	}
+}
+
+func inGroup(name string, c check) check {
+	return func(m map[string]any) string {
+		v, ok := m[name]
+		if !ok {
+			return fmt.Sprintf("missing group %q", name)
+		}
+		g, ok := v.(map[string]any)
+		if !ok {
+			return fmt.Sprintf("value for group %q is not map[string]any", name)
+		}
+		return c(g)
+	}
+}
+
+type wrapper struct {
+	slog.Handler
+	mod func(*slog.Record)
+}
+
+func (h *wrapper) Handle(ctx context.Context, r slog.Record) error {
+	h.mod(&r)
+	return h.Handler.Handle(ctx, r)
+}
+
+func withSource(s string) string {
+	_, file, line, ok := runtime.Caller(1)
+	if !ok {
+		panic("runtime.Caller failed")
+	}
+	return fmt.Sprintf("%s (%s:%d)", s, file, line)
+}
+
+type replace struct {
+	v any
+}
+
+func (r *replace) LogValue() slog.Value { return slog.AnyValue(r.v) }
+
+func (r *replace) String() string {
+	return fmt.Sprintf("<replace(%v)>", r.v)
+}

src/testing/sub_test.go

@@ -0,0 +1,992 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"regexp"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+func init() {
+	// Make benchmark tests run 10x faster.
+	benchTime.d = 100 * time.Millisecond
+}
+
+func TestTestContext(t *T) {
+	const (
+		add1 = 0
+		done = 1
+	)
+	// After each of the calls are applied to the context, the
+	type call struct {
+		typ int // run or done
+		// result from applying the call
+		running int
+		waiting int
+		started bool
+	}
+	testCases := []struct {
+		max int
+		run []call
+	}{{
+		max: 1,
+		run: []call{
+			{typ: add1, running: 1, waiting: 0, started: true},
+			{typ: done, running: 0, waiting: 0, started: false},
+		},
+	}, {
+		max: 1,
+		run: []call{
+			{typ: add1, running: 1, waiting: 0, started: true},
+			{typ: add1, running: 1, waiting: 1, started: false},
+			{typ: done, running: 1, waiting: 0, started: true},
+			{typ: done, running: 0, waiting: 0, started: false},
+			{typ: add1, running: 1, waiting: 0, started: true},
+		},
+	}, {
+		max: 3,
+		run: []call{
+			{typ: add1, running: 1, waiting: 0, started: true},
+			{typ: add1, running: 2, waiting: 0, started: true},
+			{typ: add1, running: 3, waiting: 0, started: true},
+			{typ: add1, running: 3, waiting: 1, started: false},
+			{typ: add1, running: 3, waiting: 2, started: false},
+			{typ: add1, running: 3, waiting: 3, started: false},
+			{typ: done, running: 3, waiting: 2, started: true},
+			{typ: add1, running: 3, waiting: 3, started: false},
+			{typ: done, running: 3, waiting: 2, started: true},
+			{typ: done, running: 3, waiting: 1, started: true},
+			{typ: done, running: 3, waiting: 0, started: true},
+			{typ: done, running: 2, waiting: 0, started: false},
+			{typ: done, running: 1, waiting: 0, started: false},
+			{typ: done, running: 0, waiting: 0, started: false},
+		},
+	}}
+	for i, tc := range testCases {
+		ctx := &testContext{
+			startParallel: make(chan bool),
+			maxParallel:   tc.max,
+		}
+		for j, call := range tc.run {
+			doCall := func(f func()) chan bool {
+				done := make(chan bool)
+				go func() {
+					f()
+					done <- true
+				}()
+				return done
+			}
+			started := false
+			switch call.typ {
+			case add1:
+				signal := doCall(ctx.waitParallel)
+				select {
+				case <-signal:
+					started = true
+				case ctx.startParallel <- true:
+					<-signal
+				}
+			case done:
+				signal := doCall(ctx.release)
+				select {
+				case <-signal:
+				case <-ctx.startParallel:
+					started = true
+					<-signal
+				}
+			}
+			if started != call.started {
+				t.Errorf("%d:%d:started: got %v; want %v", i, j, started, call.started)
+			}
+			if ctx.running != call.running {
+				t.Errorf("%d:%d:running: got %v; want %v", i, j, ctx.running, call.running)
+			}
+			if ctx.numWaiting != call.waiting {
+				t.Errorf("%d:%d:waiting: got %v; want %v", i, j, ctx.numWaiting, call.waiting)
+			}
+		}
+	}
+}
+
+func TestTRun(t *T) {
+	realTest := t
+	testCases := []struct {
+		desc   string
+		ok     bool
+		maxPar int
+		chatty bool
+		json   bool
+		output string
+		f      func(*T)
+	}{{
+		desc:   "failnow skips future sequential and parallel tests at same level",
+		ok:     false,
+		maxPar: 1,
+		output: `
+--- FAIL: failnow skips future sequential and parallel tests at same level (N.NNs)
+    --- FAIL: failnow skips future sequential and parallel tests at same level/#00 (N.NNs)
+    `,
+		f: func(t *T) {
+			ranSeq := false
+			ranPar := false
+			t.Run("", func(t *T) {
+				t.Run("par", func(t *T) {
+					t.Parallel()
+					ranPar = true
+				})
+				t.Run("seq", func(t *T) {
+					ranSeq = true
+				})
+				t.FailNow()
+				t.Run("seq", func(t *T) {
+					realTest.Error("test must be skipped")
+				})
+				t.Run("par", func(t *T) {
+					t.Parallel()
+					realTest.Error("test must be skipped.")
+				})
+			})
+			if !ranPar {
+				realTest.Error("parallel test was not run")
+			}
+			if !ranSeq {
+				realTest.Error("sequential test was not run")
+			}
+		},
+	}, {
+		desc:   "failure in parallel test propagates upwards",
+		ok:     false,
+		maxPar: 1,
+		output: `
+--- FAIL: failure in parallel test propagates upwards (N.NNs)
+    --- FAIL: failure in parallel test propagates upwards/#00 (N.NNs)
+        --- FAIL: failure in parallel test propagates upwards/#00/par (N.NNs)
+        `,
+		f: func(t *T) {
+			t.Run("", func(t *T) {
+				t.Parallel()
+				t.Run("par", func(t *T) {
+					t.Parallel()
+					t.Fail()
+				})
+			})
+		},
+	}, {
+		desc:   "skipping without message, chatty",
+		ok:     true,
+		chatty: true,
+		output: `
+=== RUN   skipping without message, chatty
+--- SKIP: skipping without message, chatty (N.NNs)`,
+		f: func(t *T) { t.SkipNow() },
+	}, {
+		desc:   "chatty with recursion",
+		ok:     true,
+		chatty: true,
+		output: `
+=== RUN   chatty with recursion
+=== RUN   chatty with recursion/#00
+=== RUN   chatty with recursion/#00/#00
+--- PASS: chatty with recursion (N.NNs)
+    --- PASS: chatty with recursion/#00 (N.NNs)
+        --- PASS: chatty with recursion/#00/#00 (N.NNs)`,
+		f: func(t *T) {
+			t.Run("", func(t *T) {
+				t.Run("", func(t *T) {})
+			})
+		},
+	}, {
+		desc:   "chatty with recursion and json",
+		ok:     false,
+		chatty: true,
+		json:   true,
+		output: `
+^V=== RUN   chatty with recursion and json
+^V=== RUN   chatty with recursion and json/#00
+^V=== RUN   chatty with recursion and json/#00/#00
+^V--- PASS: chatty with recursion and json/#00/#00 (N.NNs)
+^V=== NAME  chatty with recursion and json/#00
+^V=== RUN   chatty with recursion and json/#00/#01
+    sub_test.go:NNN: skip
+^V--- SKIP: chatty with recursion and json/#00/#01 (N.NNs)
+^V=== NAME  chatty with recursion and json/#00
+^V=== RUN   chatty with recursion and json/#00/#02
+    sub_test.go:NNN: fail
+^V--- FAIL: chatty with recursion and json/#00/#02 (N.NNs)
+^V=== NAME  chatty with recursion and json/#00
+^V--- FAIL: chatty with recursion and json/#00 (N.NNs)
+^V=== NAME  chatty with recursion and json
+^V--- FAIL: chatty with recursion and json (N.NNs)
+^V=== NAME  `,
+		f: func(t *T) {
+			t.Run("", func(t *T) {
+				t.Run("", func(t *T) {})
+				t.Run("", func(t *T) { t.Skip("skip") })
+				t.Run("", func(t *T) { t.Fatal("fail") })
+			})
+		},
+	}, {
+		desc: "skipping without message, not chatty",
+		ok:   true,
+		f:    func(t *T) { t.SkipNow() },
+	}, {
+		desc: "skipping after error",
+		output: `
+--- FAIL: skipping after error (N.NNs)
+    sub_test.go:NNN: an error
+    sub_test.go:NNN: skipped`,
+		f: func(t *T) {
+			t.Error("an error")
+			t.Skip("skipped")
+		},
+	}, {
+		desc:   "use Run to locally synchronize parallelism",
+		ok:     true,
+		maxPar: 1,
+		f: func(t *T) {
+			var count uint32
+			t.Run("waitGroup", func(t *T) {
+				for i := 0; i < 4; i++ {
+					t.Run("par", func(t *T) {
+						t.Parallel()
+						atomic.AddUint32(&count, 1)
+					})
+				}
+			})
+			if count != 4 {
+				t.Errorf("count was %d; want 4", count)
+			}
+		},
+	}, {
+		desc: "alternate sequential and parallel",
+		// Sequential tests should partake in the counting of running threads.
+		// Otherwise, if one runs parallel subtests in sequential tests that are
+		// itself subtests of parallel tests, the counts can get askew.
+		ok:     true,
+		maxPar: 1,
+		f: func(t *T) {
+			t.Run("a", func(t *T) {
+				t.Parallel()
+				t.Run("b", func(t *T) {
+					// Sequential: ensure running count is decremented.
+					t.Run("c", func(t *T) {
+						t.Parallel()
+					})
+
+				})
+			})
+		},
+	}, {
+		desc: "alternate sequential and parallel 2",
+		// Sequential tests should partake in the counting of running threads.
+		// Otherwise, if one runs parallel subtests in sequential tests that are
+		// itself subtests of parallel tests, the counts can get askew.
+		ok:     true,
+		maxPar: 2,
+		f: func(t *T) {
+			for i := 0; i < 2; i++ {
+				t.Run("a", func(t *T) {
+					t.Parallel()
+					time.Sleep(time.Nanosecond)
+					for i := 0; i < 2; i++ {
+						t.Run("b", func(t *T) {
+							time.Sleep(time.Nanosecond)
+							for i := 0; i < 2; i++ {
+								t.Run("c", func(t *T) {
+									t.Parallel()
+									time.Sleep(time.Nanosecond)
+								})
+							}
+
+						})
+					}
+				})
+			}
+		},
+	}, {
+		desc:   "stress test",
+		ok:     true,
+		maxPar: 4,
+		f: func(t *T) {
+			t.Parallel()
+			for i := 0; i < 12; i++ {
+				t.Run("a", func(t *T) {
+					t.Parallel()
+					time.Sleep(time.Nanosecond)
+					for i := 0; i < 12; i++ {
+						t.Run("b", func(t *T) {
+							time.Sleep(time.Nanosecond)
+							for i := 0; i < 12; i++ {
+								t.Run("c", func(t *T) {
+									t.Parallel()
+									time.Sleep(time.Nanosecond)
+									t.Run("d1", func(t *T) {})
+									t.Run("d2", func(t *T) {})
+									t.Run("d3", func(t *T) {})
+									t.Run("d4", func(t *T) {})
+								})
+							}
+						})
+					}
+				})
+			}
+		},
+	}, {
+		desc:   "skip output",
+		ok:     true,
+		maxPar: 4,
+		f: func(t *T) {
+			t.Skip()
+		},
+	}, {
+		desc: "subtest calls error on parent",
+		ok:   false,
+		output: `
+--- FAIL: subtest calls error on parent (N.NNs)
+    sub_test.go:NNN: first this
+    sub_test.go:NNN: and now this!
+    sub_test.go:NNN: oh, and this too`,
+		maxPar: 1,
+		f: func(t *T) {
+			t.Errorf("first this")
+			outer := t
+			t.Run("", func(t *T) {
+				outer.Errorf("and now this!")
+			})
+			t.Errorf("oh, and this too")
+		},
+	}, {
+		desc: "subtest calls fatal on parent",
+		ok:   false,
+		output: `
+--- FAIL: subtest calls fatal on parent (N.NNs)
+    sub_test.go:NNN: first this
+    sub_test.go:NNN: and now this!
+    --- FAIL: subtest calls fatal on parent/#00 (N.NNs)
+        testing.go:NNN: test executed panic(nil) or runtime.Goexit: subtest may have called FailNow on a parent test`,
+		maxPar: 1,
+		f: func(t *T) {
+			outer := t
+			t.Errorf("first this")
+			t.Run("", func(t *T) {
+				outer.Fatalf("and now this!")
+			})
+			t.Errorf("Should not reach here.")
+		},
+	}, {
+		desc: "subtest calls error on ancestor",
+		ok:   false,
+		output: `
+--- FAIL: subtest calls error on ancestor (N.NNs)
+    sub_test.go:NNN: Report to ancestor
+    --- FAIL: subtest calls error on ancestor/#00 (N.NNs)
+        sub_test.go:NNN: Still do this
+    sub_test.go:NNN: Also do this`,
+		maxPar: 1,
+		f: func(t *T) {
+			outer := t
+			t.Run("", func(t *T) {
+				t.Run("", func(t *T) {
+					outer.Errorf("Report to ancestor")
+				})
+				t.Errorf("Still do this")
+			})
+			t.Errorf("Also do this")
+		},
+	}, {
+		desc: "subtest calls fatal on ancestor",
+		ok:   false,
+		output: `
+--- FAIL: subtest calls fatal on ancestor (N.NNs)
+    sub_test.go:NNN: Nope`,
+		maxPar: 1,
+		f: func(t *T) {
+			outer := t
+			t.Run("", func(t *T) {
+				for i := 0; i < 4; i++ {
+					t.Run("", func(t *T) {
+						outer.Fatalf("Nope")
+					})
+					t.Errorf("Don't do this")
+				}
+				t.Errorf("And neither do this")
+			})
+			t.Errorf("Nor this")
+		},
+	}, {
+		desc:   "panic on goroutine fail after test exit",
+		ok:     false,
+		maxPar: 4,
+		f: func(t *T) {
+			ch := make(chan bool)
+			t.Run("", func(t *T) {
+				go func() {
+					<-ch
+					defer func() {
+						if r := recover(); r == nil {
+							realTest.Errorf("expected panic")
+						}
+						ch <- true
+					}()
+					t.Errorf("failed after success")
+				}()
+			})
+			ch <- true
+			<-ch
+		},
+	}, {
+		desc: "log in finished sub test logs to parent",
+		ok:   false,
+		output: `
+		--- FAIL: log in finished sub test logs to parent (N.NNs)
+    sub_test.go:NNN: message2
+    sub_test.go:NNN: message1
+    sub_test.go:NNN: error`,
+		maxPar: 1,
+		f: func(t *T) {
+			ch := make(chan bool)
+			t.Run("sub", func(t2 *T) {
+				go func() {
+					<-ch
+					t2.Log("message1")
+					ch <- true
+				}()
+			})
+			t.Log("message2")
+			ch <- true
+			<-ch
+			t.Errorf("error")
+		},
+	}, {
+		// A chatty test should always log with fmt.Print, even if the
+		// parent test has completed.
+		desc:   "log in finished sub test with chatty",
+		ok:     false,
+		chatty: true,
+		output: `
+		--- FAIL: log in finished sub test with chatty (N.NNs)`,
+		maxPar: 1,
+		f: func(t *T) {
+			ch := make(chan bool)
+			t.Run("sub", func(t2 *T) {
+				go func() {
+					<-ch
+					t2.Log("message1")
+					ch <- true
+				}()
+			})
+			t.Log("message2")
+			ch <- true
+			<-ch
+			t.Errorf("error")
+		},
+	}, {
+		// If a subtest panics we should run cleanups.
+		desc:   "cleanup when subtest panics",
+		ok:     false,
+		chatty: false,
+		output: `
+--- FAIL: cleanup when subtest panics (N.NNs)
+    --- FAIL: cleanup when subtest panics/sub (N.NNs)
+    sub_test.go:NNN: running cleanup`,
+		f: func(t *T) {
+			t.Cleanup(func() { t.Log("running cleanup") })
+			t.Run("sub", func(t2 *T) {
+				t2.FailNow()
+			})
+		},
+	}}
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *T) {
+			ctx := newTestContext(tc.maxPar, allMatcher())
+			buf := &strings.Builder{}
+			root := &T{
+				common: common{
+					signal:  make(chan bool),
+					barrier: make(chan bool),
+					name:    "",
+					w:       buf,
+				},
+				context: ctx,
+			}
+			if tc.chatty {
+				root.chatty = newChattyPrinter(root.w)
+				root.chatty.json = tc.json
+			}
+			ok := root.Run(tc.desc, tc.f)
+			ctx.release()
+
+			if ok != tc.ok {
+				t.Errorf("%s:ok: got %v; want %v", tc.desc, ok, tc.ok)
+			}
+			if ok != !root.Failed() {
+				t.Errorf("%s:root failed: got %v; want %v", tc.desc, !ok, root.Failed())
+			}
+			if ctx.running != 0 || ctx.numWaiting != 0 {
+				t.Errorf("%s:running and waiting non-zero: got %d and %d", tc.desc, ctx.running, ctx.numWaiting)
+			}
+			got := strings.TrimSpace(buf.String())
+			want := strings.TrimSpace(tc.output)
+			re := makeRegexp(want)
+			if ok, err := regexp.MatchString(re, got); !ok || err != nil {
+				t.Errorf("%s:output:\ngot:\n%s\nwant:\n%s", tc.desc, got, want)
+			}
+		})
+	}
+}
+
+func TestBRun(t *T) {
+	work := func(b *B) {
+		for i := 0; i < b.N; i++ {
+			time.Sleep(time.Nanosecond)
+		}
+	}
+	testCases := []struct {
+		desc   string
+		failed bool
+		chatty bool
+		output string
+		f      func(*B)
+	}{{
+		desc: "simulate sequential run of subbenchmarks.",
+		f: func(b *B) {
+			b.Run("", func(b *B) { work(b) })
+			time1 := b.result.NsPerOp()
+			b.Run("", func(b *B) { work(b) })
+			time2 := b.result.NsPerOp()
+			if time1 >= time2 {
+				t.Errorf("no time spent in benchmark t1 >= t2 (%d >= %d)", time1, time2)
+			}
+		},
+	}, {
+		desc: "bytes set by all benchmarks",
+		f: func(b *B) {
+			b.Run("", func(b *B) { b.SetBytes(10); work(b) })
+			b.Run("", func(b *B) { b.SetBytes(10); work(b) })
+			if b.result.Bytes != 20 {
+				t.Errorf("bytes: got: %d; want 20", b.result.Bytes)
+			}
+		},
+	}, {
+		desc: "bytes set by some benchmarks",
+		// In this case the bytes result is meaningless, so it must be 0.
+		f: func(b *B) {
+			b.Run("", func(b *B) { b.SetBytes(10); work(b) })
+			b.Run("", func(b *B) { work(b) })
+			b.Run("", func(b *B) { b.SetBytes(10); work(b) })
+			if b.result.Bytes != 0 {
+				t.Errorf("bytes: got: %d; want 0", b.result.Bytes)
+			}
+		},
+	}, {
+		desc:   "failure carried over to root",
+		failed: true,
+		output: "--- FAIL: root",
+		f:      func(b *B) { b.Fail() },
+	}, {
+		desc:   "skipping without message, chatty",
+		chatty: true,
+		output: "--- SKIP: root",
+		f:      func(b *B) { b.SkipNow() },
+	}, {
+		desc:   "chatty with recursion",
+		chatty: true,
+		f: func(b *B) {
+			b.Run("", func(b *B) {
+				b.Run("", func(b *B) {})
+			})
+		},
+	}, {
+		desc: "skipping without message, not chatty",
+		f:    func(b *B) { b.SkipNow() },
+	}, {
+		desc:   "skipping after error",
+		failed: true,
+		output: `
+--- FAIL: root
+    sub_test.go:NNN: an error
+    sub_test.go:NNN: skipped`,
+		f: func(b *B) {
+			b.Error("an error")
+			b.Skip("skipped")
+		},
+	}, {
+		desc: "memory allocation",
+		f: func(b *B) {
+			const bufSize = 256
+			alloc := func(b *B) {
+				var buf [bufSize]byte
+				for i := 0; i < b.N; i++ {
+					_ = append([]byte(nil), buf[:]...)
+				}
+			}
+			b.Run("", func(b *B) {
+				alloc(b)
+				b.ReportAllocs()
+			})
+			b.Run("", func(b *B) {
+				alloc(b)
+				b.ReportAllocs()
+			})
+			// runtime.MemStats sometimes reports more allocations than the
+			// benchmark is responsible for. Luckily the point of this test is
+			// to ensure that the results are not underreported, so we can
+			// simply verify the lower bound.
+			if got := b.result.MemAllocs; got < 2 {
+				t.Errorf("MemAllocs was %v; want 2", got)
+			}
+			if got := b.result.MemBytes; got < 2*bufSize {
+				t.Errorf("MemBytes was %v; want %v", got, 2*bufSize)
+			}
+		},
+	}, {
+		desc: "cleanup is called",
+		f: func(b *B) {
+			var calls, cleanups, innerCalls, innerCleanups int
+			b.Run("", func(b *B) {
+				calls++
+				b.Cleanup(func() {
+					cleanups++
+				})
+				b.Run("", func(b *B) {
+					b.Cleanup(func() {
+						innerCleanups++
+					})
+					innerCalls++
+				})
+				work(b)
+			})
+			if calls == 0 || calls != cleanups {
+				t.Errorf("mismatched cleanups; got %d want %d", cleanups, calls)
+			}
+			if innerCalls == 0 || innerCalls != innerCleanups {
+				t.Errorf("mismatched cleanups; got %d want %d", cleanups, calls)
+			}
+		},
+	}, {
+		desc:   "cleanup is called on failure",
+		failed: true,
+		f: func(b *B) {
+			var calls, cleanups int
+			b.Run("", func(b *B) {
+				calls++
+				b.Cleanup(func() {
+					cleanups++
+				})
+				b.Fatalf("failure")
+			})
+			if calls == 0 || calls != cleanups {
+				t.Errorf("mismatched cleanups; got %d want %d", cleanups, calls)
+			}
+		},
+	}}
+	hideStdoutForTesting = true
+	defer func() {
+		hideStdoutForTesting = false
+	}()
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *T) {
+			var ok bool
+			buf := &strings.Builder{}
+			// This is almost like the Benchmark function, except that we override
+			// the benchtime and catch the failure result of the subbenchmark.
+			root := &B{
+				common: common{
+					signal: make(chan bool),
+					name:   "root",
+					w:      buf,
+				},
+				benchFunc: func(b *B) { ok = b.Run("test", tc.f) }, // Use Run to catch failure.
+				benchTime: durationOrCountFlag{d: 1 * time.Microsecond},
+			}
+			if tc.chatty {
+				root.chatty = newChattyPrinter(root.w)
+			}
+			root.runN(1)
+			if ok != !tc.failed {
+				t.Errorf("%s:ok: got %v; want %v", tc.desc, ok, !tc.failed)
+			}
+			if !ok != root.Failed() {
+				t.Errorf("%s:root failed: got %v; want %v", tc.desc, !ok, root.Failed())
+			}
+			// All tests are run as subtests
+			if root.result.N != 1 {
+				t.Errorf("%s: N for parent benchmark was %d; want 1", tc.desc, root.result.N)
+			}
+			got := strings.TrimSpace(buf.String())
+			want := strings.TrimSpace(tc.output)
+			re := makeRegexp(want)
+			if ok, err := regexp.MatchString(re, got); !ok || err != nil {
+				t.Errorf("%s:output:\ngot:\n%s\nwant:\n%s", tc.desc, got, want)
+			}
+		})
+	}
+}
+
+func makeRegexp(s string) string {
+	s = regexp.QuoteMeta(s)
+	s = strings.ReplaceAll(s, "^V", "\x16")
+	s = strings.ReplaceAll(s, ":NNN:", `:\d\d\d\d?:`)
+	s = strings.ReplaceAll(s, "N\\.NNs", `\d*\.\d*s`)
+	return s
+}
+
+func TestBenchmarkOutput(t *T) {
+	// Ensure Benchmark initialized common.w by invoking it with an error and
+	// normal case.
+	Benchmark(func(b *B) { b.Error("do not print this output") })
+	Benchmark(func(b *B) {})
+}
+
+func TestBenchmarkStartsFrom1(t *T) {
+	var first = true
+	Benchmark(func(b *B) {
+		if first && b.N != 1 {
+			panic(fmt.Sprintf("Benchmark() first N=%v; want 1", b.N))
+		}
+		first = false
+	})
+}
+
+func TestBenchmarkReadMemStatsBeforeFirstRun(t *T) {
+	var first = true
+	Benchmark(func(b *B) {
+		if first && (b.startAllocs == 0 || b.startBytes == 0) {
+			panic("ReadMemStats not called before first run")
+		}
+		first = false
+	})
+}
+
+type funcWriter struct {
+	write func([]byte) (int, error)
+}
+
+func (fw *funcWriter) Write(b []byte) (int, error) {
+	return fw.write(b)
+}
+
+func TestRacyOutput(t *T) {
+	var runs int32  // The number of running Writes
+	var races int32 // Incremented for each race detected
+	raceDetector := func(b []byte) (int, error) {
+		// Check if some other goroutine is concurrently calling Write.
+		if atomic.LoadInt32(&runs) > 0 {
+			atomic.AddInt32(&races, 1) // Race detected!
+		}
+		atomic.AddInt32(&runs, 1)
+		defer atomic.AddInt32(&runs, -1)
+		runtime.Gosched() // Increase probability of a race
+		return len(b), nil
+	}
+
+	root := &T{
+		common:  common{w: &funcWriter{raceDetector}},
+		context: newTestContext(1, allMatcher()),
+	}
+	root.chatty = newChattyPrinter(root.w)
+	root.Run("", func(t *T) {
+		var wg sync.WaitGroup
+		for i := 0; i < 100; i++ {
+			wg.Add(1)
+			go func(i int) {
+				defer wg.Done()
+				t.Run(fmt.Sprint(i), func(t *T) {
+					t.Logf("testing run %d", i)
+				})
+			}(i)
+		}
+		wg.Wait()
+	})
+
+	if races > 0 {
+		t.Errorf("detected %d racy Writes", races)
+	}
+}
+
+// The late log message did not include the test name.  Issue 29388.
+func TestLogAfterComplete(t *T) {
+	ctx := newTestContext(1, allMatcher())
+	var buf bytes.Buffer
+	t1 := &T{
+		common: common{
+			// Use a buffered channel so that tRunner can write
+			// to it although nothing is reading from it.
+			signal: make(chan bool, 1),
+			w:      &buf,
+		},
+		context: ctx,
+	}
+
+	c1 := make(chan bool)
+	c2 := make(chan string)
+	tRunner(t1, func(t *T) {
+		t.Run("TestLateLog", func(t *T) {
+			go func() {
+				defer close(c2)
+				defer func() {
+					p := recover()
+					if p == nil {
+						c2 <- "subtest did not panic"
+						return
+					}
+					s, ok := p.(string)
+					if !ok {
+						c2 <- fmt.Sprintf("subtest panic with unexpected value %v", p)
+						return
+					}
+					const want = "Log in goroutine after TestLateLog has completed: log after test"
+					if !strings.Contains(s, want) {
+						c2 <- fmt.Sprintf("subtest panic %q does not contain %q", s, want)
+					}
+				}()
+
+				<-c1
+				t.Log("log after test")
+			}()
+		})
+	})
+	close(c1)
+
+	if s := <-c2; s != "" {
+		t.Error(s)
+	}
+}
+
+func TestBenchmark(t *T) {
+	if Short() {
+		t.Skip("skipping in short mode")
+	}
+	res := Benchmark(func(b *B) {
+		for i := 0; i < 5; i++ {
+			b.Run("", func(b *B) {
+				for i := 0; i < b.N; i++ {
+					time.Sleep(time.Millisecond)
+				}
+			})
+		}
+	})
+	if res.NsPerOp() < 4000000 {
+		t.Errorf("want >5ms; got %v", time.Duration(res.NsPerOp()))
+	}
+}
+
+func TestCleanup(t *T) {
+	var cleanups []int
+	t.Run("test", func(t *T) {
+		t.Cleanup(func() { cleanups = append(cleanups, 1) })
+		t.Cleanup(func() { cleanups = append(cleanups, 2) })
+	})
+	if got, want := cleanups, []int{2, 1}; !reflect.DeepEqual(got, want) {
+		t.Errorf("unexpected cleanup record; got %v want %v", got, want)
+	}
+}
+
+func TestConcurrentCleanup(t *T) {
+	cleanups := 0
+	t.Run("test", func(t *T) {
+		var wg sync.WaitGroup
+		wg.Add(2)
+		for i := 0; i < 2; i++ {
+			i := i
+			go func() {
+				t.Cleanup(func() {
+					// Although the calls to Cleanup are concurrent, the functions passed
+					// to Cleanup should be called sequentially, in some nondeterministic
+					// order based on when the Cleanup calls happened to be scheduled.
+					// So these assignments to the cleanups variable should not race.
+					cleanups |= 1 << i
+				})
+				wg.Done()
+			}()
+		}
+		wg.Wait()
+	})
+	if cleanups != 1|2 {
+		t.Errorf("unexpected cleanup; got %d want 3", cleanups)
+	}
+}
+
+func TestCleanupCalledEvenAfterGoexit(t *T) {
+	cleanups := 0
+	t.Run("test", func(t *T) {
+		t.Cleanup(func() {
+			cleanups++
+		})
+		t.Cleanup(func() {
+			runtime.Goexit()
+		})
+	})
+	if cleanups != 1 {
+		t.Errorf("unexpected cleanup count; got %d want 1", cleanups)
+	}
+}
+
+func TestRunCleanup(t *T) {
+	outerCleanup := 0
+	innerCleanup := 0
+	t.Run("test", func(t *T) {
+		t.Cleanup(func() { outerCleanup++ })
+		t.Run("x", func(t *T) {
+			t.Cleanup(func() { innerCleanup++ })
+		})
+	})
+	if innerCleanup != 1 {
+		t.Errorf("unexpected inner cleanup count; got %d want 1", innerCleanup)
+	}
+	if outerCleanup != 1 {
+		t.Errorf("unexpected outer cleanup count; got %d want 0", outerCleanup)
+	}
+}
+
+func TestCleanupParallelSubtests(t *T) {
+	ranCleanup := 0
+	t.Run("test", func(t *T) {
+		t.Cleanup(func() { ranCleanup++ })
+		t.Run("x", func(t *T) {
+			t.Parallel()
+			if ranCleanup > 0 {
+				t.Error("outer cleanup ran before parallel subtest")
+			}
+		})
+	})
+	if ranCleanup != 1 {
+		t.Errorf("unexpected cleanup count; got %d want 1", ranCleanup)
+	}
+}
+
+func TestNestedCleanup(t *T) {
+	ranCleanup := 0
+	t.Run("test", func(t *T) {
+		t.Cleanup(func() {
+			if ranCleanup != 2 {
+				t.Errorf("unexpected cleanup count in first cleanup: got %d want 2", ranCleanup)
+			}
+			ranCleanup++
+		})
+		t.Cleanup(func() {
+			if ranCleanup != 0 {
+				t.Errorf("unexpected cleanup count in second cleanup: got %d want 0", ranCleanup)
+			}
+			ranCleanup++
+			t.Cleanup(func() {
+				if ranCleanup != 1 {
+					t.Errorf("unexpected cleanup count in nested cleanup: got %d want 1", ranCleanup)
+				}
+				ranCleanup++
+			})
+		})
+	})
+	if ranCleanup != 3 {
+		t.Errorf("unexpected cleanup count: got %d want 3", ranCleanup)
+	}
+}

src/testing/testing_other.go

@@ -0,0 +1,31 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package testing
+
+import "time"
+
+// isWindowsRetryable reports whether err is a Windows error code
+// that may be fixed by retrying a failed filesystem operation.
+func isWindowsRetryable(err error) bool {
+	return false
+}
+
+// highPrecisionTime represents a single point in time.
+// On all systems except Windows, using time.Time is fine.
+type highPrecisionTime struct {
+	now time.Time
+}
+
+// highPrecisionTimeNow returns high precision time for benchmarking.
+func highPrecisionTimeNow() highPrecisionTime {
+	return highPrecisionTime{now: time.Now()}
+}
+
+// highPrecisionTimeSince returns duration since b.
+func highPrecisionTimeSince(b highPrecisionTime) time.Duration {
+	return time.Since(b.now)
+}

src/testing/testing_test.go

@@ -0,0 +1,815 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"bytes"
+	"fmt"
+	"internal/race"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// This is exactly what a test would do without a TestMain.
+// It's here only so that there is at least one package in the
+// standard library with a TestMain, so that code is executed.
+
+func TestMain(m *testing.M) {
+	if os.Getenv("GO_WANT_RACE_BEFORE_TESTS") == "1" {
+		doRace()
+	}
+
+	m.Run()
+
+	// Note: m.Run currently prints the final "PASS" line, so if any race is
+	// reported here (after m.Run but before the process exits), it will print
+	// "PASS", then print the stack traces for the race, then exit with nonzero
+	// status.
+	//
+	// This is a somewhat fundamental race: because the race detector hooks into
+	// the runtime at a very low level, no matter where we put the printing it
+	// would be possible to report a race that occurs afterward. However, we could
+	// theoretically move the printing after TestMain, which would at least do a
+	// better job of diagnosing races in cleanup functions within TestMain itself.
+}
+
+func TestTempDirInCleanup(t *testing.T) {
+	var dir string
+
+	t.Run("test", func(t *testing.T) {
+		t.Cleanup(func() {
+			dir = t.TempDir()
+		})
+		_ = t.TempDir()
+	})
+
+	fi, err := os.Stat(dir)
+	if fi != nil {
+		t.Fatalf("Directory %q from user Cleanup still exists", dir)
+	}
+	if !os.IsNotExist(err) {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+}
+
+func TestTempDirInBenchmark(t *testing.T) {
+	testing.Benchmark(func(b *testing.B) {
+		if !b.Run("test", func(b *testing.B) {
+			// Add a loop so that the test won't fail. See issue 38677.
+			for i := 0; i < b.N; i++ {
+				_ = b.TempDir()
+			}
+		}) {
+			t.Fatal("Sub test failure in a benchmark")
+		}
+	})
+}
+
+func TestTempDir(t *testing.T) {
+	testTempDir(t)
+	t.Run("InSubtest", testTempDir)
+	t.Run("test/subtest", testTempDir)
+	t.Run("test\\subtest", testTempDir)
+	t.Run("test:subtest", testTempDir)
+	t.Run("test/..", testTempDir)
+	t.Run("../test", testTempDir)
+	t.Run("test[]", testTempDir)
+	t.Run("test*", testTempDir)
+	t.Run("äöüéè", testTempDir)
+}
+
+func testTempDir(t *testing.T) {
+	dirCh := make(chan string, 1)
+	t.Cleanup(func() {
+		// Verify directory has been removed.
+		select {
+		case dir := <-dirCh:
+			fi, err := os.Stat(dir)
+			if os.IsNotExist(err) {
+				// All good
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			t.Errorf("directory %q still exists: %v, isDir=%v", dir, fi, fi.IsDir())
+		default:
+			if !t.Failed() {
+				t.Fatal("never received dir channel")
+			}
+		}
+	})
+
+	dir := t.TempDir()
+	if dir == "" {
+		t.Fatal("expected dir")
+	}
+	dir2 := t.TempDir()
+	if dir == dir2 {
+		t.Fatal("subsequent calls to TempDir returned the same directory")
+	}
+	if filepath.Dir(dir) != filepath.Dir(dir2) {
+		t.Fatalf("calls to TempDir do not share a parent; got %q, %q", dir, dir2)
+	}
+	dirCh <- dir
+	fi, err := os.Stat(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !fi.IsDir() {
+		t.Errorf("dir %q is not a dir", dir)
+	}
+	files, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) > 0 {
+		t.Errorf("unexpected %d files in TempDir: %v", len(files), files)
+	}
+
+	glob := filepath.Join(dir, "*.txt")
+	if _, err := filepath.Glob(glob); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestSetenv(t *testing.T) {
+	tests := []struct {
+		name               string
+		key                string
+		initialValueExists bool
+		initialValue       string
+		newValue           string
+	}{
+		{
+			name:               "initial value exists",
+			key:                "GO_TEST_KEY_1",
+			initialValueExists: true,
+			initialValue:       "111",
+			newValue:           "222",
+		},
+		{
+			name:               "initial value exists but empty",
+			key:                "GO_TEST_KEY_2",
+			initialValueExists: true,
+			initialValue:       "",
+			newValue:           "222",
+		},
+		{
+			name:               "initial value is not exists",
+			key:                "GO_TEST_KEY_3",
+			initialValueExists: false,
+			initialValue:       "",
+			newValue:           "222",
+		},
+	}
+
+	for _, test := range tests {
+		if test.initialValueExists {
+			if err := os.Setenv(test.key, test.initialValue); err != nil {
+				t.Fatalf("unable to set env: got %v", err)
+			}
+		} else {
+			os.Unsetenv(test.key)
+		}
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Setenv(test.key, test.newValue)
+			if os.Getenv(test.key) != test.newValue {
+				t.Fatalf("unexpected value after t.Setenv: got %s, want %s", os.Getenv(test.key), test.newValue)
+			}
+		})
+
+		got, exists := os.LookupEnv(test.key)
+		if got != test.initialValue {
+			t.Fatalf("unexpected value after t.Setenv cleanup: got %s, want %s", got, test.initialValue)
+		}
+		if exists != test.initialValueExists {
+			t.Fatalf("unexpected value after t.Setenv cleanup: got %t, want %t", exists, test.initialValueExists)
+		}
+	}
+}
+
+func TestSetenvWithParallelAfterSetenv(t *testing.T) {
+	defer func() {
+		want := "testing: t.Parallel called after t.Setenv; cannot set environment variables in parallel tests"
+		if got := recover(); got != want {
+			t.Fatalf("expected panic; got %#v want %q", got, want)
+		}
+	}()
+
+	t.Setenv("GO_TEST_KEY_1", "value")
+
+	t.Parallel()
+}
+
+func TestSetenvWithParallelBeforeSetenv(t *testing.T) {
+	defer func() {
+		want := "testing: t.Setenv called after t.Parallel; cannot set environment variables in parallel tests"
+		if got := recover(); got != want {
+			t.Fatalf("expected panic; got %#v want %q", got, want)
+		}
+	}()
+
+	t.Parallel()
+
+	t.Setenv("GO_TEST_KEY_1", "value")
+}
+
+func TestSetenvWithParallelParentBeforeSetenv(t *testing.T) {
+	t.Parallel()
+
+	t.Run("child", func(t *testing.T) {
+		defer func() {
+			want := "testing: t.Setenv called after t.Parallel; cannot set environment variables in parallel tests"
+			if got := recover(); got != want {
+				t.Fatalf("expected panic; got %#v want %q", got, want)
+			}
+		}()
+
+		t.Setenv("GO_TEST_KEY_1", "value")
+	})
+}
+
+func TestSetenvWithParallelGrandParentBeforeSetenv(t *testing.T) {
+	t.Parallel()
+
+	t.Run("child", func(t *testing.T) {
+		t.Run("grand-child", func(t *testing.T) {
+			defer func() {
+				want := "testing: t.Setenv called after t.Parallel; cannot set environment variables in parallel tests"
+				if got := recover(); got != want {
+					t.Fatalf("expected panic; got %#v want %q", got, want)
+				}
+			}()
+
+			t.Setenv("GO_TEST_KEY_1", "value")
+		})
+	})
+}
+
+// testingTrueInInit is part of TestTesting.
+var testingTrueInInit = false
+
+// testingTrueInPackageVarInit is part of TestTesting.
+var testingTrueInPackageVarInit = testing.Testing()
+
+// init is part of TestTesting.
+func init() {
+	if testing.Testing() {
+		testingTrueInInit = true
+	}
+}
+
+var testingProg = `
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+func main() {
+	fmt.Println(testing.Testing())
+}
+`
+
+func TestTesting(t *testing.T) {
+	if !testing.Testing() {
+		t.Errorf("testing.Testing() == %t, want %t", testing.Testing(), true)
+	}
+	if !testingTrueInInit {
+		t.Errorf("testing.Testing() called by init function == %t, want %t", testingTrueInInit, true)
+	}
+	if !testingTrueInPackageVarInit {
+		t.Errorf("testing.Testing() variable initialized as %t, want %t", testingTrueInPackageVarInit, true)
+	}
+
+	if testing.Short() {
+		t.Skip("skipping building a binary in short mode")
+	}
+	testenv.MustHaveGoRun(t)
+
+	fn := filepath.Join(t.TempDir(), "x.go")
+	if err := os.WriteFile(fn, []byte(testingProg), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, testenv.GoToolPath(t), "run", fn)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("%v failed: %v\n%s", cmd, err, out)
+	}
+
+	s := string(bytes.TrimSpace(out))
+	if s != "false" {
+		t.Errorf("in non-test testing.Test() returned %q, want %q", s, "false")
+	}
+}
+
+// runTest runs a helper test with -test.v, ignoring its exit status.
+// runTest both logs and returns the test output.
+func runTest(t *testing.T, test string) []byte {
+	t.Helper()
+
+	testenv.MustHaveExec(t)
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Skipf("can't find test executable: %v", err)
+	}
+
+	cmd := testenv.Command(t, exe, "-test.run=^"+test+"$", "-test.bench="+test, "-test.v", "-test.parallel=2", "-test.benchtime=2x")
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
+	out, err := cmd.CombinedOutput()
+	t.Logf("%v: %v\n%s", cmd, err, out)
+
+	return out
+}
+
+// doRace provokes a data race that generates a race detector report if run
+// under the race detector and is otherwise benign.
+func doRace() {
+	var x int
+	c1 := make(chan bool)
+	go func() {
+		x = 1 // racy write
+		c1 <- true
+	}()
+	_ = x // racy read
+	<-c1
+}
+
+func TestRaceReports(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		// Generate a race detector report in a sub test.
+		t.Run("Sub", func(t *testing.T) {
+			doRace()
+		})
+		return
+	}
+
+	out := runTest(t, "TestRaceReports")
+
+	// We should see at most one race detector report.
+	c := bytes.Count(out, []byte("race detected"))
+	want := 0
+	if race.Enabled {
+		want = 1
+	}
+	if c != want {
+		t.Errorf("got %d race reports, want %d", c, want)
+	}
+}
+
+// Issue #60083. This used to fail on the race builder.
+func TestRaceName(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		doRace()
+		return
+	}
+
+	out := runTest(t, "TestRaceName")
+
+	if regexp.MustCompile(`=== NAME\s*$`).Match(out) {
+		t.Errorf("incorrectly reported test with no name")
+	}
+}
+
+func TestRaceSubReports(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		t.Parallel()
+		c1 := make(chan bool, 1)
+		t.Run("sub", func(t *testing.T) {
+			t.Run("subsub1", func(t *testing.T) {
+				t.Parallel()
+				doRace()
+				c1 <- true
+			})
+			t.Run("subsub2", func(t *testing.T) {
+				t.Parallel()
+				doRace()
+				<-c1
+			})
+		})
+		doRace()
+		return
+	}
+
+	out := runTest(t, "TestRaceSubReports")
+
+	// There should be three race reports: one for each subtest, and one for the
+	// race after the subtests complete. Note that because the subtests run in
+	// parallel, the race stacks may both be printed in with one or the other
+	// test's logs.
+	cReport := bytes.Count(out, []byte("race detected during execution of test"))
+	wantReport := 0
+	if race.Enabled {
+		wantReport = 3
+	}
+	if cReport != wantReport {
+		t.Errorf("got %d race reports, want %d", cReport, wantReport)
+	}
+
+	// Regardless of when the stacks are printed, we expect each subtest to be
+	// marked as failed, and that failure should propagate up to the parents.
+	cFail := bytes.Count(out, []byte("--- FAIL:"))
+	wantFail := 0
+	if race.Enabled {
+		wantFail = 4
+	}
+	if cFail != wantFail {
+		t.Errorf(`got %d "--- FAIL:" lines, want %d`, cReport, wantReport)
+	}
+}
+
+func TestRaceInCleanup(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		t.Cleanup(doRace)
+		t.Parallel()
+		t.Run("sub", func(t *testing.T) {
+			t.Parallel()
+			// No race should be reported for sub.
+		})
+		return
+	}
+
+	out := runTest(t, "TestRaceInCleanup")
+
+	// There should be one race report, for the parent test only.
+	cReport := bytes.Count(out, []byte("race detected during execution of test"))
+	wantReport := 0
+	if race.Enabled {
+		wantReport = 1
+	}
+	if cReport != wantReport {
+		t.Errorf("got %d race reports, want %d", cReport, wantReport)
+	}
+
+	// Only the parent test should be marked as failed.
+	// (The subtest does not race, and should pass.)
+	cFail := bytes.Count(out, []byte("--- FAIL:"))
+	wantFail := 0
+	if race.Enabled {
+		wantFail = 1
+	}
+	if cFail != wantFail {
+		t.Errorf(`got %d "--- FAIL:" lines, want %d`, cReport, wantReport)
+	}
+}
+
+func TestDeepSubtestRace(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		t.Run("sub", func(t *testing.T) {
+			t.Run("subsub", func(t *testing.T) {
+				t.Run("subsubsub", func(t *testing.T) {
+					doRace()
+				})
+			})
+			doRace()
+		})
+		return
+	}
+
+	out := runTest(t, "TestDeepSubtestRace")
+
+	c := bytes.Count(out, []byte("race detected during execution of test"))
+	want := 0
+	// There should be two race reports.
+	if race.Enabled {
+		want = 2
+	}
+	if c != want {
+		t.Errorf("got %d race reports, want %d", c, want)
+	}
+}
+
+func TestRaceDuringParallelFailsAllSubtests(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		var ready sync.WaitGroup
+		ready.Add(2)
+		done := make(chan struct{})
+		go func() {
+			ready.Wait()
+			doRace() // This race happens while both subtests are running.
+			close(done)
+		}()
+
+		t.Run("sub", func(t *testing.T) {
+			t.Run("subsub1", func(t *testing.T) {
+				t.Parallel()
+				ready.Done()
+				<-done
+			})
+			t.Run("subsub2", func(t *testing.T) {
+				t.Parallel()
+				ready.Done()
+				<-done
+			})
+		})
+
+		return
+	}
+
+	out := runTest(t, "TestRaceDuringParallelFailsAllSubtests")
+
+	c := bytes.Count(out, []byte("race detected during execution of test"))
+	want := 0
+	// Each subtest should report the race independently.
+	if race.Enabled {
+		want = 2
+	}
+	if c != want {
+		t.Errorf("got %d race reports, want %d", c, want)
+	}
+}
+
+func TestRaceBeforeParallel(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		t.Run("sub", func(t *testing.T) {
+			doRace()
+			t.Parallel()
+		})
+		return
+	}
+
+	out := runTest(t, "TestRaceBeforeParallel")
+
+	c := bytes.Count(out, []byte("race detected during execution of test"))
+	want := 0
+	// We should see one race detector report.
+	if race.Enabled {
+		want = 1
+	}
+	if c != want {
+		t.Errorf("got %d race reports, want %d", c, want)
+	}
+}
+
+func TestRaceBeforeTests(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Skipf("can't find test executable: %v", err)
+	}
+
+	cmd := testenv.Command(t, exe, "-test.run=^$")
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GO_WANT_RACE_BEFORE_TESTS=1")
+	out, _ := cmd.CombinedOutput()
+	t.Logf("%s", out)
+
+	c := bytes.Count(out, []byte("race detected outside of test execution"))
+
+	want := 0
+	if race.Enabled {
+		want = 1
+	}
+	if c != want {
+		t.Errorf("got %d race reports; want %d", c, want)
+	}
+}
+
+func TestBenchmarkRace(t *testing.T) {
+	out := runTest(t, "BenchmarkRacy")
+	c := bytes.Count(out, []byte("race detected during execution of test"))
+
+	want := 0
+	// We should see one race detector report.
+	if race.Enabled {
+		want = 1
+	}
+	if c != want {
+		t.Errorf("got %d race reports; want %d", c, want)
+	}
+}
+
+func BenchmarkRacy(b *testing.B) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		b.Skipf("skipping intentionally-racy benchmark")
+	}
+	for i := 0; i < b.N; i++ {
+		doRace()
+	}
+}
+
+func TestBenchmarkSubRace(t *testing.T) {
+	out := runTest(t, "BenchmarkSubRacy")
+	c := bytes.Count(out, []byte("race detected during execution of test"))
+
+	want := 0
+	// We should see two race detector reports:
+	// one in the sub-bencmark, and one in the parent afterward.
+	if race.Enabled {
+		want = 2
+	}
+	if c != want {
+		t.Errorf("got %d race reports; want %d", c, want)
+	}
+}
+
+func BenchmarkSubRacy(b *testing.B) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		b.Skipf("skipping intentionally-racy benchmark")
+	}
+
+	b.Run("non-racy", func(b *testing.B) {
+		tot := 0
+		for i := 0; i < b.N; i++ {
+			tot++
+		}
+		_ = tot
+	})
+
+	b.Run("racy", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			doRace()
+		}
+	})
+
+	doRace() // should be reported separately
+}
+
+func TestRunningTests(t *testing.T) {
+	t.Parallel()
+
+	// Regression test for https://go.dev/issue/64404:
+	// on timeout, the "running tests" message should not include
+	// tests that are waiting on parked subtests.
+
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		for i := 0; i < 2; i++ {
+			t.Run(fmt.Sprintf("outer%d", i), func(t *testing.T) {
+				t.Parallel()
+				for j := 0; j < 2; j++ {
+					t.Run(fmt.Sprintf("inner%d", j), func(t *testing.T) {
+						t.Parallel()
+						for {
+							time.Sleep(1 * time.Millisecond)
+						}
+					})
+				}
+			})
+		}
+	}
+
+	timeout := 10 * time.Millisecond
+	for {
+		cmd := testenv.Command(t, os.Args[0], "-test.run=^"+t.Name()+"$", "-test.timeout="+timeout.String(), "-test.parallel=4")
+		cmd.Env = append(cmd.Environ(), "GO_WANT_HELPER_PROCESS=1")
+		out, err := cmd.CombinedOutput()
+		t.Logf("%v:\n%s", cmd, out)
+		if _, ok := err.(*exec.ExitError); !ok {
+			t.Fatal(err)
+		}
+
+		// Because the outer subtests (and TestRunningTests itself) are marked as
+		// parallel, their test functions return (and are no longer “running”)
+		// before the inner subtests are released to run and hang.
+		// Only those inner subtests should be reported as running.
+		want := []string{
+			"TestRunningTests/outer0/inner0",
+			"TestRunningTests/outer0/inner1",
+			"TestRunningTests/outer1/inner0",
+			"TestRunningTests/outer1/inner1",
+		}
+
+		got, ok := parseRunningTests(out)
+		if slices.Equal(got, want) {
+			break
+		}
+		if ok {
+			t.Logf("found running tests:\n%s\nwant:\n%s", strings.Join(got, "\n"), strings.Join(want, "\n"))
+		} else {
+			t.Logf("no running tests found")
+		}
+		t.Logf("retrying with longer timeout")
+		timeout *= 2
+	}
+}
+
+func TestRunningTestsInCleanup(t *testing.T) {
+	t.Parallel()
+
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		for i := 0; i < 2; i++ {
+			t.Run(fmt.Sprintf("outer%d", i), func(t *testing.T) {
+				// Not parallel: we expect to see only one outer test,
+				// stuck in cleanup after its subtest finishes.
+
+				t.Cleanup(func() {
+					for {
+						time.Sleep(1 * time.Millisecond)
+					}
+				})
+
+				for j := 0; j < 2; j++ {
+					t.Run(fmt.Sprintf("inner%d", j), func(t *testing.T) {
+						t.Parallel()
+					})
+				}
+			})
+		}
+	}
+
+	timeout := 10 * time.Millisecond
+	for {
+		cmd := testenv.Command(t, os.Args[0], "-test.run=^"+t.Name()+"$", "-test.timeout="+timeout.String())
+		cmd.Env = append(cmd.Environ(), "GO_WANT_HELPER_PROCESS=1")
+		out, err := cmd.CombinedOutput()
+		t.Logf("%v:\n%s", cmd, out)
+		if _, ok := err.(*exec.ExitError); !ok {
+			t.Fatal(err)
+		}
+
+		// TestRunningTestsInCleanup is blocked in the call to t.Run,
+		// but its test function has not yet returned so it should still
+		// be considered to be running.
+		// outer1 hasn't even started yet, so only outer0 and the top-level
+		// test function should be reported as running.
+		want := []string{
+			"TestRunningTestsInCleanup",
+			"TestRunningTestsInCleanup/outer0",
+		}
+
+		got, ok := parseRunningTests(out)
+		if slices.Equal(got, want) {
+			break
+		}
+		if ok {
+			t.Logf("found running tests:\n%s\nwant:\n%s", strings.Join(got, "\n"), strings.Join(want, "\n"))
+		} else {
+			t.Logf("no running tests found")
+		}
+		t.Logf("retrying with longer timeout")
+		timeout *= 2
+	}
+}
+
+func parseRunningTests(out []byte) (runningTests []string, ok bool) {
+	inRunningTests := false
+	for _, line := range strings.Split(string(out), "\n") {
+		if inRunningTests {
+			// Package testing adds one tab, the panic printer adds another.
+			if trimmed, ok := strings.CutPrefix(line, "\t\t"); ok {
+				if name, _, ok := strings.Cut(trimmed, " "); ok {
+					runningTests = append(runningTests, name)
+					continue
+				}
+			}
+
+			// This line is not the name of a running test.
+			return runningTests, true
+		}
+
+		if strings.TrimSpace(line) == "running tests:" {
+			inRunningTests = true
+		}
+	}
+
+	return nil, false
+}
+
+func TestConcurrentRun(t *testing.T) {
+	// Regression test for https://go.dev/issue/64402:
+	// this deadlocked after https://go.dev/cl/506755.
+
+	block := make(chan struct{})
+	var ready, done sync.WaitGroup
+	for i := 0; i < 2; i++ {
+		ready.Add(1)
+		done.Add(1)
+		go t.Run("", func(*testing.T) {
+			ready.Done()
+			<-block
+			done.Done()
+		})
+	}
+	ready.Wait()
+	close(block)
+	done.Wait()
+}
+
+func TestParentRun(t1 *testing.T) {
+	// Regression test for https://go.dev/issue/64402:
+	// this deadlocked after https://go.dev/cl/506755.
+
+	t1.Run("outer", func(t2 *testing.T) {
+		t2.Log("Hello outer!")
+		t1.Run("not_inner", func(t3 *testing.T) { // Note: this is t1.Run, not t2.Run.
+			t3.Log("Hello inner!")
+		})
+	})
+}

src/testing/testing_windows.go

@@ -0,0 +1,70 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package testing
+
+import (
+	"errors"
+	"internal/syscall/windows"
+	"math/bits"
+	"syscall"
+	"time"
+)
+
+// isWindowsRetryable reports whether err is a Windows error code
+// that may be fixed by retrying a failed filesystem operation.
+func isWindowsRetryable(err error) bool {
+	for {
+		unwrapped := errors.Unwrap(err)
+		if unwrapped == nil {
+			break
+		}
+		err = unwrapped
+	}
+	if err == syscall.ERROR_ACCESS_DENIED {
+		return true // Observed in https://go.dev/issue/50051.
+	}
+	if err == windows.ERROR_SHARING_VIOLATION {
+		return true // Observed in https://go.dev/issue/51442.
+	}
+	return false
+}
+
+// highPrecisionTime represents a single point in time with query performance counter.
+// time.Time on Windows has low system granularity, which is not suitable for
+// measuring short time intervals.
+//
+// TODO: If Windows runtime implements high resolution timing then highPrecisionTime
+// can be removed.
+type highPrecisionTime struct {
+	now int64
+}
+
+// highPrecisionTimeNow returns high precision time for benchmarking.
+func highPrecisionTimeNow() highPrecisionTime {
+	var t highPrecisionTime
+	// This should always succeed for Windows XP and above.
+	t.now = windows.QueryPerformanceCounter()
+	return t
+}
+
+func (a highPrecisionTime) sub(b highPrecisionTime) time.Duration {
+	delta := a.now - b.now
+
+	if queryPerformanceFrequency == 0 {
+		queryPerformanceFrequency = windows.QueryPerformanceFrequency()
+	}
+	hi, lo := bits.Mul64(uint64(delta), uint64(time.Second)/uint64(time.Nanosecond))
+	quo, _ := bits.Div64(hi, lo, uint64(queryPerformanceFrequency))
+	return time.Duration(quo)
+}
+
+var queryPerformanceFrequency int64
+
+// highPrecisionTimeSince returns duration since a.
+func highPrecisionTimeSince(a highPrecisionTime) time.Duration {
+	return highPrecisionTimeNow().sub(a)
+}

src/testing/testing_windows_test.go

@@ -0,0 +1,25 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"testing"
+	"time"
+)
+
+var sink time.Time
+var sinkHPT testing.HighPrecisionTime
+
+func BenchmarkTimeNow(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		sink = time.Now()
+	}
+}
+
+func BenchmarkHighPrecisionTimeNow(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		sinkHPT = testing.HighPrecisionTimeNow()
+	}
+}