rfq: update order handler for noop HTLCs

When routing HTLCs we also want to do 2 things:

a) Upon receiving an HTLC with no matching policy, we want to make sure
that this HTLC is not using the noop feature, as that would lead to loss
of funds.

b) Upon producing an outgoing HTLC over an asset channel (that carries
assets) we also want to set the noop flag, in order to not shift the
satoshi balance.

Author: GeorgeTsagk

rfq/order.go

@@ -17,6 +17,7 @@ import (
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
 	"github.com/lightningnetwork/lnd/lnutils"
+	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/tlv"
 )
@@ -284,6 +285,11 @@ func (c *AssetSalePolicy) GenerateInterceptorResponse(
 		fn.None[[]rfqmsg.ID](),
 	)
 
+	// We are about to create an outgoing HTLC that carries assets. Let's
+	// set the noop flag in order to eventually only settle the assets but
+	// never settle the sats anchor amount that will carry them.
+	htlcRecord.SetNoopAdd(rfqmsg.UseNoOpHTLCs)
+
 	customRecords, err := lnwire.ParseCustomRecords(htlcRecord.Bytes())
 	if err != nil {
 		return nil, fmt.Errorf("error parsing custom records: %w", err)
@@ -731,7 +737,21 @@ func (h *OrderHandler) handleIncomingHtlc(ctx context.Context,
 	}
 
 	if !ok {
+		noopTLV := uint64(lnwallet.NoOpHtlcTLVEntry.TypeVal())
+		_, noopActive := htlc.InWireCustomRecords[noopTLV]
+
+		// If we don't have a matching policy for this HTLC but the
+		// sender set the noop flag, that means we were about to push
+		// some outgoing sats amount that we would eventually never
+		// receive. The HTLC must be failed.
+		if noopActive {
+			return &lndclient.InterceptedHtlcResponse{
+				Action: lndclient.InterceptorActionFail,
+			}, nil
+		}
+
 		log.Debug("Failed to find a policy for the HTLC. Resuming.")
+
 		return &lndclient.InterceptedHtlcResponse{
 			Action: lndclient.InterceptorActionResume,
 		}, nil