Initial commit: Go 1.23 release state

test/recover4.go

@@ -0,0 +1,76 @@
+// run
+
+//go:build linux || darwin
+
+// 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.
+
+// Test that if a slice access causes a fault, a deferred func
+// sees the most recent value of the variables it accesses.
+// This is true today; the role of the test is to ensure it stays true.
+//
+// In the test, memcopy is the function that will fault, during dst[i] = src[i].
+// The deferred func recovers from the error and returns, making memcopy
+// return the current value of n. If n is not being flushed to memory
+// after each modification, the result will be a stale value of n.
+//
+// The test is set up by mmapping a 64 kB block of memory and then
+// unmapping a 16 kB hole in the middle of it. Running memcopy
+// on the resulting slice will fault when it reaches the hole.
+
+package main
+
+import (
+	"log"
+	"runtime/debug"
+	"syscall"
+)
+
+func memcopy(dst, src []byte) (n int, err error) {
+	defer func() {
+		if r, ok := recover().(error); ok {
+			err = r
+		}
+	}()
+
+	for i := 0; i < len(dst) && i < len(src); i++ {
+		dst[i] = src[i]
+		n++
+	}
+	return
+}
+
+func main() {
+	// Turn the eventual fault into a panic, not a program crash,
+	// so that memcopy can recover.
+	debug.SetPanicOnFault(true)
+
+	size := syscall.Getpagesize()
+
+	// Map 16 pages of data with a 4-page hole in the middle.
+	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
+	if err != nil {
+		log.Fatalf("mmap: %v", err)
+	}
+
+	// Create a hole in the mapping that's PROT_NONE.
+	// Note that we can't use munmap here because the Go runtime
+	// could create a mapping that ends up in this hole otherwise,
+	// invalidating the test.
+	hole := data[len(data)/2 : 3*(len(data)/4)]
+	if err := syscall.Mprotect(hole, syscall.PROT_NONE); err != nil {
+		log.Fatalf("mprotect: %v", err)
+	}
+
+	// Check that memcopy returns the actual amount copied
+	// before the fault.
+	const offset = 5
+	n, err := memcopy(data[offset:], make([]byte, len(data)))
+	if err == nil {
+		log.Fatal("no error from memcopy across memory hole")
+	}
+	if expect := len(data)/2 - offset; n != expect {
+		log.Fatalf("memcopy returned %d, want %d", n, expect)
+	}
+}