htlcswitch: override amount check on custom records

Author: guggero

htlcswitch/link.go

@@ -3793,7 +3793,18 @@ func (l *channelLink) processExitHop(add lnwire.UpdateAddHTLC,
 	// As we're the exit hop, we'll double check the hop-payload included in
 	// the HTLC to ensure that it was crafted correctly by the sender and
 	// is compatible with the HTLC we were extended.
-	if add.Amount < fwdInfo.AmountToForward {
+	//
+	// For a special case, if the fwdInfo doesn't have any blinded path
+	// information, and the incoming HTLC had special extra data, then
+	// we'll skip this amount check. The invoice acceptor will make sure we
+	// reject the HTLC if it's not containing the correct amount after
+	// examining the custom data.
+	hasBlindedPath := fwdInfo.NextBlinding.IsSome()
+	customHTLC := len(add.CustomRecords) > 0 && !hasBlindedPath
+	log.Tracef("Exit hop has_blinded_path=%v custom_htlc_bypass=%v",
+		hasBlindedPath, customHTLC)
+
+	if !customHTLC && add.Amount < fwdInfo.AmountToForward {
 		l.log.Errorf("onion payload of incoming htlc(%x) has "+
 			"incompatible value: expected <=%v, got %v",
 			add.PaymentHash, add.Amount, fwdInfo.AmountToForward)