multi: fix payment failure overwrite

We now make sure we do not overwrite the payment failure status
if it is already set and return an error. When using sendtoroute
we might fail a payment twice hence we do not error out if we
fail a payment twice.

Author: ziggie1984

Diff for: channeldb/payment_control.go

@@ -582,14 +582,23 @@ 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)

Diff for: routing/router.go

@@ -1200,7 +1200,12 @@ func (r *ChannelRouter) sendToRoute(htlcHash lntypes.Hash, rt *route.Route,
 	// as failed if we don't skip temp error.
 	if !skipTempErr {
 		err := r.cfg.Control.FailPayment(paymentIdentifier, reason)
-		if err != nil {
+
+		// We might to 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
 		}
 	}