tapchannel: trigger noop HTLCs on ProduceHtlcExtraData

Whenever we use the default above-dust anchor amount to send an HTLC
with assets we also set the noop flag on the HTLC, which will produce
the according noop HTLC record and signal to LND that the satoshi amount
should be returned to the sender.

Author: GeorgeTsagk

tapchannel/aux_traffic_shaper.go

@@ -473,7 +473,14 @@ func (s *AuxTrafficShaper) ProduceHtlcExtraData(totalAmount lnwire.MilliSatoshi,
 	if htlc.Amounts.Val.Sum() > 0 {
 		log.Tracef("Already have asset amount (sum %d) in HTLC, not "+
 			"producing extra data", htlc.Amounts.Val.Sum())
-		return totalAmount, htlcCustomRecords, nil
+
+		htlc.SetNoopAdd(rfqmsg.UseNoOpHTLCs)
+		updatedRecords, err := htlc.ToCustomRecords()
+		if err != nil {
+			return 0, nil, err
+		}
+
+		return totalAmount, updatedRecords, nil
 	}
 
 	// Within the context of a payment we may negotiate multiple quotes. All
@@ -569,6 +576,12 @@ func (s *AuxTrafficShaper) ProduceHtlcExtraData(totalAmount lnwire.MilliSatoshi,
 	// amount that should be sent on-chain, which is a value in satoshi that
 	// is just above the dust limit.
 	htlcAmountMSat := rfqmath.DefaultOnChainHtlcMSat
+
+	// Now we set the flag that marks this HTLC as a noop_add, which means
+	// that the above dust will eventually return to us. This means that
+	// only the assets will be sent and not any btc balance.
+	htlc.SetNoopAdd(rfqmsg.UseNoOpHTLCs)
+
 	updatedRecords, err := htlc.ToCustomRecords()
 	if err != nil {
 		return 0, nil, fmt.Errorf("error encoding HTLC blob: %w", err)