clientv3: backoff on reestablishing watches when Unavailable errors a…

…re encountered

clientv3/client.go

@@ -527,6 +527,20 @@ func isHaltErr(ctx context.Context, err error) bool {
 	return ev.Code() != codes.Unavailable && ev.Code() != codes.Internal
 }
 
+// isUnavailableErr returns true if the given error is an unavailable error
+func isUnavailableErr(ctx context.Context, err error) bool {
+	if ctx != nil && ctx.Err() != nil {
+		return false
+	}
+	if err == nil {
+		return false
+	}
+	ev, _ := status.FromError(err)
+	// Unavailable codes mean the system will be right back.
+	// (e.g., can't connect, lost leader)
+	return ev.Code() == codes.Unavailable
+}
+
 func toErr(ctx context.Context, err error) error {
 	if err == nil {
 		return nil

clientv3/watch.go

@@ -830,10 +830,13 @@ func (w *watchGrpcStream) joinSubstreams() {
 	}
 }
 
+var maxBackoff = 100 * time.Millisecond
+
 // openWatchClient retries opening a watch client until success or halt.
 // manually retry in case "ws==nil && err==nil"
 // TODO: remove FailFast=false
 func (w *watchGrpcStream) openWatchClient() (ws pb.Watch_WatchClient, err error) {
+	backoff := time.Millisecond
 	for {
 		select {
 		case <-w.ctx.Done():
@@ -849,6 +852,17 @@ func (w *watchGrpcStream) openWatchClient() (ws pb.Watch_WatchClient, err error)
 		if isHaltErr(w.ctx, err) {
 			return nil, v3rpc.Error(err)
 		}
+		if isUnavailableErr(w.ctx, err) {
+			// retry, but backoff
+			if backoff < maxBackoff {
+				// 25% backoff factor
+				backoff = backoff + backoff/4
+				if backoff > maxBackoff {
+					backoff = maxBackoff
+				}
+			}
+			time.Sleep(backoff)
+		}
 	}
 	return ws, nil
 }