rpcServer: ensure we don't get blocked on bidirectional payment errors

This commit fixes a potential issue in the bidirectional sendPayment
case, where multiple goroutines could be sending on an errChan with
buffer 1. Instead we select on default as well, as it is enough to
handle the first error that was received.

rpcserver.go

@@ -3707,10 +3707,12 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 					errChan <- nil
 					return
 				} else if err != nil {
+					rpcsLog.Errorf("Failed receiving from "+
+						"stream: %v", err)
+
 					select {
 					case errChan <- err:
-					case <-reqQuit:
+					default:
-						return
 					}
 					return
 				}
@@ -3728,18 +3730,22 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 						PaymentError: err.Error(),
 						PaymentHash:  payIntent.rHash[:],
 					}); err != nil {
+						rpcsLog.Errorf("Failed "+
+							"sending on "+
+							"stream: %v", err)
+
 						select {
 						case errChan <- err:
-						case <-reqQuit:
+						default:
-							return
 						}
+						return
 					}
 					continue
 				}
 
 				// If the payment was well formed, then we'll
 				// send to the dispatch goroutine, or exit,
-				// which ever comes first
+				// which ever comes first.
 				select {
 				case payChan <- &payIntent:
 				case <-reqQuit:
@@ -3751,6 +3757,9 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 
 	for {
 		select {
+
+		// If we encounter and error either during sending or
+		// receiving, we return directly, closing the stream.
 		case err := <-errChan:
 			return err
 
@@ -3779,7 +3788,13 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 				// payment, then we'll return the error to the
 				// user, and terminate.
 				case saveErr != nil:
-					errChan <- saveErr
+					rpcsLog.Errorf("Failed dispatching "+
+						"payment intent: %v", saveErr)
+
+					select {
+					case errChan <- saveErr:
+					default:
+					}
 					return
 
 				// If we receive payment error than, instead of
@@ -3791,7 +3806,14 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 						PaymentHash:  payIntent.rHash[:],
 					})
 					if err != nil {
-						errChan <- err
+						rpcsLog.Errorf("Failed "+
+							"sending error "+
+							"response: %v", err)
+
+						select {
+						case errChan <- err:
+						default:
+						}
 					}
 					return
 				}
@@ -3811,7 +3833,13 @@ func (r *rpcServer) sendPayment(stream *paymentStream) error {
 					PaymentRoute:    marshalledRouted,
 				})
 				if err != nil {
-					errChan <- err
+					rpcsLog.Errorf("Failed sending "+
+						"response: %v", err)
+
+					select {
+					case errChan <- err:
+					default:
+					}
 					return
 				}
 			}()