multi: fix payment failure overwrite

channeldb/payment_control.go

@@ -582,14 +582,24 @@ func (p *PaymentControl) Fail(paymentHash lntypes.Hash,
 		// lets the last attempt to fail with a terminal write its
 		// failure to the PaymentControl without synchronizing with
 		// other attempts.
-		_, err = fetchPaymentStatus(bucket)
+		status, err := fetchPaymentStatus(bucket)
 		if errors.Is(err, ErrPaymentNotInitiated) {
 			updateErr = ErrPaymentNotInitiated
 			return nil
 		} else if err != nil {
 			return err
 		}
 
+		// We make sure that if the payment is already failed we do not
+		// overwrite the failure reason and remain consistent. We do
+		// not return the error in the batch function to avoid retrying
+		// the transaction.
+		if status == StatusFailed {
+			updateErr = ErrPaymentAlreadyFailed
+
+			return nil
+		}
+
 		// Put the failure reason in the bucket for record keeping.
 		v := []byte{byte(reason)}
 		err = bucket.Put(paymentFailInfoKey, v)

routing/router.go

@@ -1200,7 +1200,12 @@
 	// as failed if we don't skip temp error.
 	if !skipTempErr {
 		err := r.cfg.Control.FailPayment(paymentIdentifier, reason)
-		if err != nil {
+
+		// We might attempt to fail the payment twice here because the
+		// payment might already be failed when handling the switch
+		// error in the result collector.
+		if err != nil &&
+			!errors.Is(err, channeldb.ErrPaymentAlreadyFailed) {
 			return nil, err
 		}
 	}