clientv3: close watcher stream once all watchers detach

Fixes #6134

clientv3/watch.go

@@ -311,7 +311,12 @@ func (w *watcher) Close() (err error) {
 }
 
 func (w *watchGrpcStream) Close() (err error) {
-	close(w.stopc)
+	w.mu.Lock()
+	if w.stopc != nil {
+		close(w.stopc)
+		w.stopc = nil
+	}
+	w.mu.Unlock()
 	<-w.donec
 	select {
 	case err = <-w.errc:
@@ -374,11 +379,17 @@ func (w *watchGrpcStream) addStream(resp *pb.WatchResponse, pendingReq *watchReq
 
 // closeStream closes the watcher resources and removes it
 func (w *watchGrpcStream) closeStream(ws *watcherStream) {
+	w.mu.Lock()
 	// cancels request stream; subscriber receives nil channel
 	close(ws.initReq.retc)
 	// close subscriber's channel
 	close(ws.outc)
 	delete(w.streams, ws.id)
+	if len(w.streams) == 0 && w.stopc != nil {
+		close(w.stopc)
+		w.stopc = nil
+	}
+	w.mu.Unlock()
 }
 
 // run is the root of the goroutines for managing a watcher client
@@ -404,6 +415,7 @@ func (w *watchGrpcStream) run() {
 
 	var pendingReq, failedReq *watchRequest
 	curReqC := w.reqc
+	stopc := w.stopc
 	cancelSet := make(map[int64]struct{})
 
 	for {
@@ -473,7 +485,7 @@ func (w *watchGrpcStream) run() {
 				failedReq = pendingReq
 			}
 			cancelSet = make(map[int64]struct{})
-		case <-w.stopc:
+		case <-stopc:
 			return
 		}
 
@@ -625,9 +637,7 @@ func (w *watchGrpcStream) serveStream(ws *watcherStream) {
 		}
 	}
 
-	w.mu.Lock()
 	w.closeStream(ws)
-	w.mu.Unlock()
 	// lazily send cancel message if events on missing id
 }
 
@@ -655,13 +665,14 @@ func (w *watchGrpcStream) resume() (ws pb.Watch_WatchClient, err error) {
 // openWatchClient retries opening a watchclient until retryConnection fails
 func (w *watchGrpcStream) openWatchClient() (ws pb.Watch_WatchClient, err error) {
 	for {
-		select {
-		case <-w.stopc:
+		w.mu.Lock()
+		stopc := w.stopc
+		w.mu.Unlock()
+		if stopc == nil {
 			if err == nil {
 				err = context.Canceled
 			}
 			return nil, err
-		default:
 		}
 		if ws, err = w.remote.Watch(w.ctx, grpc.FailFast(false)); ws != nil && err == nil {
 			break