net/dns: add a simple test for resolv.conf inotify watcher

Updates #14699

Signed-off-by: Anton <anton@tailscale.com>

net/dns/direct_linux_test.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package dns
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"golang.org/x/sync/errgroup"
+)
+
+func TestWatchFile(t *testing.T) {
+	dir := t.TempDir()
+	filepath := dir + "/test.txt"
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	var callbackCalled atomic.Bool
+	callbackDone := make(chan bool)
+	callback := func() {
+		callbackDone <- true
+		callbackCalled.Store(true)
+	}
+
+	var eg errgroup.Group
+	eg.Go(func() error { return watchFile(ctx, dir, filepath, callback) })
+
+	// Keep writing until we get a callback.
+	func() {
+		for i := range 10000 {
+			if err := os.WriteFile(filepath, []byte(fmt.Sprintf("write%d", i)), 0644); err != nil {
+				t.Fatal(err)
+			}
+			select {
+			case <-callbackDone:
+				return
+			case <-time.After(10 * time.Millisecond):
+			}
+		}
+	}()
+
+	cancel()
+	if err := eg.Wait(); err != nil && !errors.Is(err, context.Canceled) {
+		t.Error(err)
+	}
+	if !callbackCalled.Load() {
+		t.Error("callback was not called")
+	}
+}