Fixed race conditions in watch/inotify_tracker.go

watch/inotify_tracker.go

@@ -17,99 +17,137 @@ type InotifyTracker struct {
 	mux     sync.Mutex
 	watcher *fsnotify.Watcher
 	chans   map[string]chan *fsnotify.FileEvent
-	done    chan struct{}
+	watch   chan *watchInfo
+	remove  chan string
+	error   chan error
+}
+
+type watchInfo struct {
+	fname string
+	flags uint32
 }
 
 var (
+	// globally shared InotifyTracker; ensures only one fsnotify.Watcher is used
 	shared = &InotifyTracker{
 		mux:    sync.Mutex{},
 		chans:  make(map[string]chan *fsnotify.FileEvent),
+		watch:  make(chan *watchInfo),
+		remove: make(chan string),
+		error:  make(chan error),
 	}
-	logger = log.New(os.Stderr, "", log.LstdFlags)
-)
 
-// Watch calls fsnotify.Watch for the input filename, creating a new Watcher if the
-// previous Watcher was closed.
-func (shared *InotifyTracker) Watch(filename string) error {
-	return shared.WatchFlags(filename, fsnotify.FSN_ALL)
-}
-
-// WatchFlags calls fsnotify.WatchFlags for the input filename and flags, creating
-// a new Watcher if the previous Watcher was closed.
-func (shared *InotifyTracker) WatchFlags(filename string, flags uint32) error {
-	shared.mux.Lock()
-	defer shared.mux.Unlock()
-
-	// Start up shared struct if necessary
-	if len(shared.chans) == 0 {
-		watcher, err := fsnotify.NewWatcher()
-		if err != nil {
-			util.Fatal("Error creating Watcher")
-		}
-		shared.watcher = watcher
-		shared.done = make(chan struct{})
+	// these are used to ensure the shared InotifyTracker is run exactly once
+	once  = &sync.Once{}
+	goRun = func() {
 		go shared.run()
 	}
 
-	// Create a channel to which FileEvents for the input filename will be sent
-	ch := shared.chans[filename]
-	if ch == nil {
-		shared.chans[filename] = make(chan *fsnotify.FileEvent)
-	}
-	return shared.watcher.WatchFlags(filename, flags)
+	logger = log.New(os.Stderr, "", log.LstdFlags)
+)
+
+// WatchFlags signals the run goroutine to begin watching the input filename using
+// using all flags.
+func (shared *InotifyTracker) Watch(fname string) error {
+	return shared.WatchFlags(fname, fsnotify.FSN_ALL)
 }
 
-// RemoveWatch calls fsnotify.RemoveWatch for the input filename and closes the
-// corresponding events channel.
-func (shared *InotifyTracker) RemoveWatch(filename string) {
-	shared.mux.Lock()
-	defer shared.mux.Unlock()
+// WatchFlags signals the run goroutine to begin watching the input filename using
+// using the input flags.
+func (shared *InotifyTracker) WatchFlags(fname string, flags uint32) error {
+	// start running the shared InotifyTracker if not already running
+	once.Do(goRun)
 
-	_, found := shared.chans[filename]
-	if !found {
-		return
+	shared.watch <- &watchInfo{
+		fname: fname,
+		flags: flags,
 	}
+	return <-shared.error
+}
 
-	shared.watcher.RemoveWatch(filename)
-	delete(shared.chans, filename)
+// RemoveWatch signals the run goroutine to remove the watch for the input filename
+func (shared *InotifyTracker) RemoveWatch(fname string) {
+	// start running the shared InotifyTracker if not already running
+	once.Do(goRun)
 
-	// If this is the last target to be removed, close the shared Watcher
-	if len(shared.chans) == 0 {
-		shared.watcher.Close()
-		close(shared.done)
-	}
+	shared.remove <- fname
 }
 
 // Events returns a channel to which FileEvents corresponding to the input filename
 // will be sent. This channel will be closed when removeWatch is called on this
 // filename.
-func (shared *InotifyTracker) Events(filename string) <-chan *fsnotify.FileEvent {
+func (shared *InotifyTracker) Events(fname string) chan *fsnotify.FileEvent {
 	shared.mux.Lock()
 	defer shared.mux.Unlock()
 
-	return shared.chans[filename]
+	return shared.chans[fname]
 }
 
 // Cleanup removes the watch for the input filename and closes the shared Watcher
 // if there are no more targets.
-func Cleanup(filename string) {
-	shared.RemoveWatch(filename)
+func Cleanup(fname string) {
+	shared.RemoveWatch(fname)
+}
+
+// watchFlags calls fsnotify.WatchFlags for the input filename and flags, creating
+// a new Watcher if the previous Watcher was closed.
+func (shared *InotifyTracker) watchFlags(fname string, flags uint32) error {
+	shared.mux.Lock()
+	defer shared.mux.Unlock()
+
+	if shared.chans[fname] == nil {
+		shared.chans[fname] = make(chan *fsnotify.FileEvent)
+	}
+	return shared.watcher.WatchFlags(fname, flags)
+}
+
+// removeWatch calls fsnotify.RemoveWatch for the input filename and closes the
+// corresponding events channel.
+func (shared *InotifyTracker) removeWatch(fname string) {
+	shared.mux.Lock()
+	defer shared.mux.Unlock()
+
+	if ch := shared.chans[fname]; ch != nil {
+		shared.watcher.RemoveWatch(fname)
+
+		delete(shared.chans, fname)
+		close(ch)
+	}
+}
+
+// sendEvent sends the input event to the appropriate Tail.
+func (shared *InotifyTracker) sendEvent(event *fsnotify.FileEvent) {
+	ch := shared.Events(event.Name)
+	if ch != nil {
+		select {
+		case ch <- event:
+		default:
+		}
+	}
 }
 
 // run starts the goroutine in which the shared struct reads events from its
 // Watcher's Event channel and sends the events to the appropriate Tail.
 func (shared *InotifyTracker) run() {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		util.Fatal("failed to create Watcher")
+	}
+	shared.watcher = watcher
+
 	for {
 		select {
+		case winfo := <-shared.watch:
+			shared.error <- shared.watchFlags(winfo.fname, winfo.flags)
+
+		case fname := <-shared.remove:
+			shared.removeWatch(fname)
+
 		case event, open := <-shared.watcher.Event:
 			if !open {
 				return
 			}
-			// send the FileEvent to the appropriate Tail's channel
-			ch := shared.chans[event.Name]
-			if ch != nil {
-				ch <- event
-			}
+			shared.sendEvent(event)
 
 		case err, open := <-shared.watcher.Error:
 			if !open {
@@ -120,9 +158,6 @@ func (shared *InotifyTracker) run() {
 					logger.Printf("Error in Watcher Error channel: %s", err)
 				}
 			}
-
-		case <-shared.done:
-			return
 		}
 	}
 }