windows: protect access to isClosed with mutex

windows.go

@@ -21,14 +21,16 @@ import (
 
 // Watcher watches a set of files, delivering events to a channel.
 type Watcher struct {
-	Events   chan Event
-	Errors   chan error
-	isClosed bool           // Set to true when Close() is first called
-	mu       sync.Mutex     // Map access
-	port     syscall.Handle // Handle to completion port
-	watches  watchMap       // Map of watches (key: i-number)
-	input    chan *input    // Inputs to the reader are sent on this channel
-	quit     chan chan<- error
+	Events chan Event
+	Errors chan error
+
+	port  syscall.Handle // Handle to completion port
+	input chan *input    // Inputs to the reader are sent on this channel
+	quit  chan chan<- error
+
+	mu       sync.Mutex // Protects access to watches, isClosed
+	watches  watchMap   // Map of watches (key: i-number)
+	isClosed bool       // Set to true when Close() is first called
 }
 
 // NewWatcher establishes a new watcher with the underlying OS and begins waiting for events.
@@ -51,6 +53,9 @@ func NewWatcher() (*Watcher, error) {
 
 // Close removes all watches and closes the events channel.
 func (w *Watcher) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
 	if w.isClosed {
 		return nil
 	}
@@ -67,9 +72,12 @@ func (w *Watcher) Close() error {
 
 // Add starts watching the named file or directory (non-recursively).
 func (w *Watcher) Add(name string) error {
+	w.mu.Lock()
 	if w.isClosed {
 		return errors.New("watcher already closed")
 	}
+	w.mu.Unlock()
+
 	in := &input{
 		op:    opAddWatch,
 		path:  filepath.Clean(name),