fix(promtail): Fix bug with Promtail config reloading getting stuck indefinitely

clients/pkg/promtail/targets/file/filetarget.go

@@ -25,6 +25,8 @@ const (
 	FilenameLabel = "filename"
 )
 
+var errFileTargetStopped = errors.New("File target is stopped")
+
 // Config describes behavior for Target
 type Config struct {
 	SyncPeriod time.Duration `mapstructure:"sync_period" yaml:"sync_period"`
@@ -223,6 +225,11 @@ func (t *FileTarget) run() {
 			}
 		case <-ticker.C:
 			err := t.sync()
+			if errors.Is(err, errFileTargetStopped) {
+				// This file target has been stopped.
+				// This is normal and there is no need to log an error.
+				return
+			}
 			if err != nil {
 				level.Error(t.logger).Log("msg", "error running sync function", "error", err)
 			}
@@ -291,14 +298,20 @@ func (t *FileTarget) sync() error {
 	t.watchesMutex.Lock()
 	toStartWatching := missing(t.watches, dirs)
 	t.watchesMutex.Unlock()
-	t.startWatching(toStartWatching)
+	err := t.startWatching(toStartWatching)
+	if errors.Is(err, errFileTargetStopped) {
+		return err
+	}
 
 	// Remove any directories which no longer need watching.
 	t.watchesMutex.Lock()
 	toStopWatching := missing(dirs, t.watches)
 	t.watchesMutex.Unlock()
 
-	t.stopWatching(toStopWatching)
+	err = t.stopWatching(toStopWatching)
+	if errors.Is(err, errFileTargetStopped) {
+		return err
+	}
 
 	// fsnotify.Watcher doesn't allow us to see what is currently being watched so we have to track it ourselves.
 	t.watchesMutex.Lock()
@@ -321,32 +334,42 @@ func (t *FileTarget) sync() error {
 	return nil
 }
 
-func (t *FileTarget) startWatching(dirs map[string]struct{}) {
+func (t *FileTarget) startWatching(dirs map[string]struct{}) error {
 	for dir := range dirs {
 		if _, ok := t.getWatch(dir); ok {
 			continue
 		}
 
 		level.Info(t.logger).Log("msg", "watching new directory", "directory", dir)
-		t.targetEventHandler <- fileTargetEvent{
+		select {
+		case <-t.quit:
+			return errFileTargetStopped
+		case t.targetEventHandler <- fileTargetEvent{
 			path:      dir,
 			eventType: fileTargetEventWatchStart,
+		}:
 		}
 	}
+	return nil
 }
 
-func (t *FileTarget) stopWatching(dirs map[string]struct{}) {
+func (t *FileTarget) stopWatching(dirs map[string]struct{}) error {
 	for dir := range dirs {
 		if _, ok := t.getWatch(dir); !ok {
 			continue
 		}
 
 		level.Info(t.logger).Log("msg", "removing directory from watcher", "directory", dir)
-		t.targetEventHandler <- fileTargetEvent{
+		select {
+		case <-t.quit:
+			return errFileTargetStopped
+		case t.targetEventHandler <- fileTargetEvent{
 			path:      dir,
 			eventType: fileTargetEventWatchStop,
+		}:
 		}
 	}
+	return nil
 }
 
 func (t *FileTarget) startTailing(ps []string) {

clients/pkg/promtail/targets/file/filetarget_test.go

@@ -336,6 +336,93 @@ func TestFileTarget_StopsTailersCleanly_Parallel(t *testing.T) {
 	ps.Stop()
 }
 
+// Make sure that Stop() doesn't hang if FileTarget is waiting on a channel send.
+func TestFileTarget_StopAbruptly(t *testing.T) {
+	w := log.NewSyncWriter(os.Stderr)
+	logger := log.NewLogfmtLogger(w)
+
+	dirName := newTestLogDirectories(t)
+	positionsFileName := filepath.Join(dirName, "positions.yml")
+	logDir1 := filepath.Join(dirName, "log1")
+	logDir2 := filepath.Join(dirName, "log2")
+	logDir3 := filepath.Join(dirName, "log3")
+
+	logfile1 := filepath.Join(logDir1, "test1.log")
+	logfile2 := filepath.Join(logDir2, "test1.log")
+	logfile3 := filepath.Join(logDir3, "test1.log")
+
+	ps, err := positions.New(logger, positions.Config{
+		SyncPeriod:    10 * time.Millisecond,
+		PositionsFile: positionsFileName,
+	})
+	require.NoError(t, err)
+
+	client := fake.New(func() {})
+	defer client.Stop()
+
+	// fakeHandler has to be a buffered channel so that we can call the len() function on it.
+	// We need to call len() to check if the channel is full.
+	fakeHandler := make(chan fileTargetEvent, 1)
+	pathToWatch := filepath.Join(dirName, "**", "*.log")
+	registry := prometheus.NewRegistry()
+	target, err := NewFileTarget(NewMetrics(registry), logger, client, ps, pathToWatch, "", nil, nil, &Config{
+		SyncPeriod: 10 * time.Millisecond,
+	}, DefaultWatchConig, nil, fakeHandler, "", nil)
+	assert.NoError(t, err)
+
+	// Create a directory, still nothing is watched.
+	err = os.MkdirAll(logDir1, 0750)
+	assert.NoError(t, err)
+	_, err = os.Create(logfile1)
+	assert.NoError(t, err)
+
+	// There should be only one WatchStart event in the channel so far.
+	ftEvent := <-fakeHandler
+	require.Equal(t, fileTargetEventWatchStart, ftEvent.eventType)
+
+	requireEventually(t, func() bool {
+		return target.getReadersLen() == 1
+	}, "expected 1 tailer to be created")
+
+	require.NoError(t, testutil.GatherAndCompare(registry, bytes.NewBufferString(`
+		# HELP promtail_files_active_total Number of active files.
+		# TYPE promtail_files_active_total gauge
+		promtail_files_active_total 1
+	`), "promtail_files_active_total"))
+
+	// Create two directories - one more than the buffer of fakeHandler,
+	// so that the file target hands until we call Stop().
+	err = os.MkdirAll(logDir2, 0750)
+	assert.NoError(t, err)
+	_, err = os.Create(logfile2)
+	assert.NoError(t, err)
+
+	err = os.MkdirAll(logDir3, 0750)
+	assert.NoError(t, err)
+	_, err = os.Create(logfile3)
+	assert.NoError(t, err)
+
+	// Wait until the file target is waiting on a channel send due to a full channel buffer.
+	requireEventually(t, func() bool {
+		return len(fakeHandler) == 1
+	}, "expected an event in the fakeHandler channel")
+
+	// If FileHandler works well, then it will stop waiting for
+	// the blocked fakeHandler and stop cleanly.
+	// This is why this time we don't drain fakeHandler.
+	requireEventually(t, func() bool {
+		target.Stop()
+		ps.Stop()
+		return true
+	}, "expected FileTarget not to hang")
+
+	require.NoError(t, testutil.GatherAndCompare(registry, bytes.NewBufferString(`
+		# HELP promtail_files_active_total Number of active files.
+		# TYPE promtail_files_active_total gauge
+		promtail_files_active_total 0
+	`), "promtail_files_active_total"))
+}
+
 func TestFileTargetPathExclusion(t *testing.T) {
 	w := log.NewSyncWriter(os.Stderr)
 	logger := log.NewLogfmtLogger(w)