Fix data race on kevent buffer.

In the BSD implementation of fsnotify, the watcher's kbuf buffers a kevent
between syscall.SetKevent (which prepares the kevent) and syscall.Kevent
(which registers the kevent). The implementation intends to protect access to
kbuf, but fails to do so in addWatch and removeWatch.
This change fixes the data race by allocating a new kevent buffer for every
method invocation.

fsnotify_bsd.go

@@ -79,8 +79,6 @@ type Watcher struct {
 	Event           chan *FileEvent     // Events are returned on this channel
 	done            chan bool           // Channel for sending a "quit message" to the reader goroutine
 	isClosed        bool                // Set to true when Close() is first called
-	kbuf            [1]syscall.Kevent_t // An event buffer for Add/Remove watch
-	bufmut          sync.Mutex          // Protects access to kbuf.
 }
 
 // NewWatcher creates and returns a new kevent instance using kqueue(2)
@@ -207,15 +205,13 @@ func (w *Watcher) addWatch(path string, flags uint32) error {
 	w.enFlags[path] = flags
 	w.enmut.Unlock()
 
-	w.bufmut.Lock()
-	watchEntry := &w.kbuf[0]
+	var kbuf [1]syscall.Kevent_t
+	watchEntry := &kbuf[0]
 	watchEntry.Fflags = flags
 	syscall.SetKevent(watchEntry, watchfd, syscall.EVFILT_VNODE, syscall.EV_ADD|syscall.EV_CLEAR)
 	entryFlags := watchEntry.Flags
-	w.bufmut.Unlock()
-
-	wd, errno := syscall.Kevent(w.kq, w.kbuf[:], nil, nil)
-	if wd == -1 {
+	success, errno := syscall.Kevent(w.kq, kbuf[:], nil, nil)
+	if success == -1 {
 		return errno
 	} else if (entryFlags & syscall.EV_ERROR) == syscall.EV_ERROR {
 		return errors.New("kevent add error")
@@ -246,14 +242,14 @@ func (w *Watcher) removeWatch(path string) error {
 	if !ok {
 		return errors.New(fmt.Sprintf("can't remove non-existent kevent watch for: %s", path))
 	}
-	w.bufmut.Lock()
-	watchEntry := &w.kbuf[0]
+	var kbuf [1]syscall.Kevent_t
+	watchEntry := &kbuf[0]
 	syscall.SetKevent(watchEntry, watchfd, syscall.EVFILT_VNODE, syscall.EV_DELETE)
-	success, errno := syscall.Kevent(w.kq, w.kbuf[:], nil, nil)
-	w.bufmut.Unlock()
+	entryFlags := watchEntry.Flags
+	success, errno := syscall.Kevent(w.kq, kbuf[:], nil, nil)
 	if success == -1 {
 		return os.NewSyscallError("kevent_rm_watch", errno)
-	} else if (watchEntry.Flags & syscall.EV_ERROR) == syscall.EV_ERROR {
+	} else if (entryFlags & syscall.EV_ERROR) == syscall.EV_ERROR {
 		return errors.New("kevent rm error")
 	}
 	syscall.Close(watchfd)