lntest: use test subscription to prevent SubscribeChannelGraph race

This race was possible due to us making a subscription request before
the ChannelRouter has started. We address it by creating a dummy
subscription before proceeding to the real one to ensure we can do so
successfully. We use a dummy one in order to not consume an update from
the real one. This addresses the common "timed out waiting for opened
channel" flake within the integration test suite since the subscription
was never properly created, so we'd never be notified of when new graph
updates were received.

lntest/node.go

@@ -494,6 +494,43 @@ func (hn *HarnessNode) initLightningClient(conn *grpc.ClientConn) error {
 		return err
 	}
 
+	// Due to a race condition between the ChannelRouter starting and us
+	// making the subscription request, it's possible for our graph
+	// subscription to fail. To ensure we don't start listening for updates
+	// until then, we'll create a dummy subscription to ensure we can do so
+	// successfully before proceeding. We use a dummy subscription in order
+	// to not consume an update from the real one.
+	err = WaitNoError(func() error {
+		req := &lnrpc.GraphTopologySubscription{}
+		ctx, cancelFunc := context.WithCancel(context.Background())
+		topologyClient, err := hn.SubscribeChannelGraph(ctx, req)
+		if err != nil {
+			return err
+		}
+
+		// We'll wait to receive an error back within a one second
+		// timeout. This is needed since creating the client's stream is
+		// independent of the graph subscription being created. The
+		// stream is closed from the server's side upon an error.
+		errChan := make(chan error, 1)
+		go func() {
+			if _, err := topologyClient.Recv(); err != nil {
+				errChan <- err
+			}
+		}()
+
+		select {
+		case err = <-errChan:
+		case <-time.After(time.Second):
+		}
+
+		cancelFunc()
+		return err
+	}, DefaultTimeout)
+	if err != nil {
+		return err
+	}
+
 	// Launch the watcher that will hook into graph related topology change
 	// from the PoV of this node.
 	hn.wg.Add(1)