Feature: more robust updater DNS resolution

- Parallel resolver to resolve multiple hosts
- Repeat resolver to repeat resolution for a single host
- Additional parameters for fault toleration
- Do not update servers if e.g. > 10% DNS resolutions failed
- resolver package in updater package

internal/updater/resolver/repeat.go

@@ -0,0 +1,141 @@
+package resolver
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sort"
+	"time"
+)
+
+type Repeat interface {
+	Resolve(ctx context.Context, host string, settings RepeatSettings) (IPs []net.IP, err error)
+}
+
+type repeat struct {
+	resolver *net.Resolver
+}
+
+func NewRepeat(address string) Repeat {
+	return &repeat{
+		resolver: newResolver(address),
+	}
+}
+
+type RepeatSettings struct {
+	MaxDuration     time.Duration
+	BetweenDuration time.Duration
+	MaxNoNew        int
+	// Maximum consecutive DNS resolution failures
+	MaxFails int
+	SortIPs  bool
+}
+
+func (r *repeat) Resolve(ctx context.Context, host string, settings RepeatSettings) (ips []net.IP, err error) {
+	timedCtx, cancel := context.WithTimeout(ctx, settings.MaxDuration)
+	defer cancel()
+
+	noNewCounter := 0
+	failCounter := 0
+	uniqueIPs := make(map[string]struct{})
+
+	for err == nil {
+		// TODO
+		// - one resolving every 100ms for round robin DNS responses
+		// - one every second for time based DNS cycling responses
+		noNewCounter, failCounter, err = r.resolveOnce(ctx, timedCtx, host, settings, uniqueIPs, noNewCounter, failCounter)
+	}
+
+	if len(uniqueIPs) == 0 {
+		return nil, err
+	}
+
+	ips = uniqueIPsToSlice(uniqueIPs)
+
+	if settings.SortIPs {
+		sort.Slice(ips, func(i, j int) bool {
+			return bytes.Compare(ips[i], ips[j]) < 1
+		})
+	}
+
+	return ips, nil
+}
+
+var (
+	ErrMaxNoNew = errors.New("reached the maximum number of no new update")
+	ErrMaxFails = errors.New("reached the maximum number of consecutive failures")
+	ErrTimeout  = errors.New("reached the timeout")
+)
+
+func (r *repeat) resolveOnce(ctx, timedCtx context.Context, host string,
+	settings RepeatSettings, uniqueIPs map[string]struct{}, noNewCounter, failCounter int) (
+	newNoNewCounter, newFailCounter int, err error) {
+	IPs, err := r.lookupIPs(timedCtx, host)
+	if err != nil {
+		failCounter++
+		if settings.MaxFails > 0 && failCounter == settings.MaxFails {
+			return noNewCounter, failCounter, fmt.Errorf("%w: %d failed attempts resolving %s: %s",
+				ErrMaxFails, settings.MaxFails, host, err)
+		}
+		// it's fine to fail some of the resolutions
+		return noNewCounter, failCounter, nil
+	}
+	failCounter = 0 // reset the counter if we had no error
+
+	anyNew := false
+	for _, IP := range IPs {
+		key := IP.String()
+		if _, ok := uniqueIPs[key]; !ok {
+			anyNew = true
+			uniqueIPs[key] = struct{}{}
+		}
+	}
+
+	if !anyNew {
+		noNewCounter++
+	}
+
+	if settings.MaxNoNew > 0 && noNewCounter == settings.MaxNoNew {
+		// we reached the maximum number of resolutions without
+		// finding any new IP address to our unique IP addresses set.
+		return noNewCounter, failCounter,
+			fmt.Errorf("%w: %d times no updated for %d IP addresses found",
+				ErrMaxNoNew, noNewCounter, len(uniqueIPs))
+	}
+
+	timer := time.NewTimer(settings.BetweenDuration)
+	select {
+	case <-timer.C:
+		return noNewCounter, failCounter, nil
+	case <-ctx.Done():
+		if !timer.Stop() {
+			<-timer.C
+		}
+		return noNewCounter, failCounter, ctx.Err()
+	case <-timedCtx.Done():
+		if err := ctx.Err(); err != nil {
+			// timedCtx was canceled from its parent context
+			return noNewCounter, failCounter, err
+		}
+		return noNewCounter, failCounter,
+			fmt.Errorf("%w: %s", ErrTimeout, timedCtx.Err())
+	}
+}
+
+func (r *repeat) lookupIPs(ctx context.Context, host string) (ips []net.IP, err error) {
+	addresses, err := r.resolver.LookupIPAddr(ctx, host)
+	if err != nil {
+		return nil, err
+	}
+	ips = make([]net.IP, 0, len(addresses))
+	for i := range addresses {
+		ip := addresses[i].IP
+		if ip == nil {
+			continue
+		}
+		ips = append(ips, ip)
+	}
+	return ips, nil
+}