lnd: rpcServer now asynchronously handles bi-di sendpayment streams

rpcserver.go

@@ -426,7 +426,12 @@ func (r *rpcServer) PendingChannels(ctx context.Context,
 // bi-directional stream allowing clients to rapidly send payments through the
 // Lightning Network with a single persistent connection.
 func (r *rpcServer) SendPayment(paymentStream lnrpc.Lightning_SendPaymentServer) error {
+	errChan := make(chan error, 1)
 	for {
+		select {
+		case err := <-errChan:
+			return err
+		default:
 			// Receive the next pending payment within the stream sent by
 			// the client. If we read the EOF sentinel, then the client has
 			// closed the stream, and we can exit normally.
@@ -453,18 +458,25 @@ func (r *rpcServer) SendPayment(paymentStream lnrpc.Lightning_SendPaymentServer)
 				msg:  htlcAdd,
 			}
 
+			// TODO(roasbeef): semaphore to limit num outstanding
+			// goroutines.
+			go func() {
 				// Finally, send this next packet to the routing layer in order
 				// to complete the next payment.
 				// TODO(roasbeef): this should go through the L3 router once
 				// multi-hop is in place.
 				if err := r.server.htlcSwitch.SendHTLC(htlcPkt); err != nil {
-			return err
+					errChan <- err
+					return
 				}
 
 				// TODO(roasbeef): proper responses
 				resp := &lnrpc.SendResponse{}
 				if err := paymentStream.Send(resp); err != nil {
-			return err
+					errChan <- err
+					return
+				}
+			}()
 		}
 	}