v3rpc: don't close watcher if client closes send

grpc-gateway will CloseSend but still want to receive updates.

etcdserver/api/v3rpc/watch.go

@@ -131,10 +131,14 @@ func (ws *watchServer) Watch(stream pb.Watch_WatchServer) (err error) {
 	// but when stream.Context().Done() is closed, the stream's recv
 	// may continue to block since it uses a different context, leading to
 	// deadlock when calling sws.close().
-	go func() { errc <- sws.recvLoop() }()
-
+	go func() {
+		if rerr := sws.recvLoop(); rerr != nil {
+			errc <- rerr
+		}
+	}()
 	select {
 	case err = <-errc:
+		close(sws.ctrlStream)
 	case <-stream.Context().Done():
 		err = stream.Context().Err()
 		// the only server-side cancellation is noleader for now.
@@ -147,7 +151,6 @@ func (ws *watchServer) Watch(stream pb.Watch_WatchServer) (err error) {
 }
 
 func (sws *serverWatchStream) recvLoop() error {
-	defer close(sws.ctrlStream)
 	for {
 		req, err := sws.gRPCStream.Recv()
 		if err == io.EOF {