[#798] morph/subscriber: Await for RPC initialization

Remote RPC node might be in the transition state and produce
events from the past. We should avoid listening such nodes.
To do that subscriber component can await minimal height of
remote node. If minimal height is not reached, then throw
error.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/morph/subscriber/subscriber.go

@@ -40,9 +40,11 @@ type (
 
 	// Params is a group of Subscriber constructor parameters.
 	Params struct {
-		Log         *zap.Logger
-		Endpoint    string
-		DialTimeout time.Duration
+		Log            *zap.Logger
+		Endpoint       string
+		DialTimeout    time.Duration
+		RPCInitTimeout time.Duration
+		StartFromBlock uint32
 	}
 )
 
@@ -192,6 +194,15 @@ func New(ctx context.Context, p *Params) (Subscriber, error) {
 		return nil, fmt.Errorf("could not init ws client: %w", err)
 	}
 
+	p.Log.Debug("event subscriber awaits RPC node",
+		zap.String("endpoint", p.Endpoint),
+		zap.Uint32("min_block_height", p.StartFromBlock))
+
+	err = awaitHeight(wsClient, p.StartFromBlock, p.RPCInitTimeout)
+	if err != nil {
+		return nil, err
+	}
+
 	sub := &subscriber{
 		RWMutex:    new(sync.RWMutex),
 		log:        p.Log,
@@ -209,3 +220,29 @@ func New(ctx context.Context, p *Params) (Subscriber, error) {
 
 	return sub, nil
 }
+
+// awaitHeight checks if remote client has least expected block height and
+// returns error if it is not reached that height after timeout duration.
+// This function is required to avoid connections to unsynced RPC nodes, because
+// they can produce events from the past that should not be processed by
+// NeoFS nodes.
+func awaitHeight(wsClient *client.WSClient, startFrom uint32, timeout time.Duration) error {
+	if startFrom == 0 {
+		return nil
+	}
+
+	for ch := time.After(timeout); ; {
+		select {
+		case <-ch:
+			return fmt.Errorf("could not init ws client: didn't reach expected height %d", startFrom)
+		default:
+		}
+		height, err := wsClient.GetBlockCount()
+		if err != nil {
+			return fmt.Errorf("could not get block height: %w", err)
+		} else if height >= startFrom {
+			return nil
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}