rfqmsg: add noop option to HTLC

We add a new noop flag to the HTLC which, when set, will produce the
corresponding TLV record. We also hide this feature behind the dev flag,
as we ultimately want to be backwards compatible via some feature bit
related peer messages.

Author: GeorgeTsagk

go.sum

@@ -1144,8 +1144,6 @@ github.com/lightninglabs/lightning-node-connect/hashmailrpc v1.0.3 h1:NuDp6Z+QNM
 github.com/lightninglabs/lightning-node-connect/hashmailrpc v1.0.3/go.mod h1:bDnEKRN1u13NFBuy/C+bFLhxA5bfd3clT25y76QY0AM=
 github.com/lightninglabs/lightning-node-connect/mailbox v1.0.1 h1:RWmohykp3n/DTMWY8b18RNTEcLDf+KT/AZHKYdOObkM=
 github.com/lightninglabs/lightning-node-connect/mailbox v1.0.1/go.mod h1:NYtNexZE9gO1eOeegTxmIW9fqanl7eZ9cOrE9yewSAk=
-github.com/lightninglabs/lndclient v0.19.0-9 h1:ell27omDoks79upoAsO/7QY40O93ud4tAtBXXZilqok=
-github.com/lightninglabs/lndclient v0.19.0-9/go.mod h1:35d50tEMFxlJlKTZGYA6EdOllPsbxS4FUmEVbETUx+Q=
 github.com/lightninglabs/migrate/v4 v4.18.2-9023d66a-fork-pr-2 h1:eFjp1dIB2BhhQp/THKrjLdlYuPugO9UU4kDqu91OX/Q=
 github.com/lightninglabs/migrate/v4 v4.18.2-9023d66a-fork-pr-2/go.mod h1:99BKpIi6ruaaXRM1A77eqZ+FWPQ3cfRa+ZVy5bmWMaY=
 github.com/lightninglabs/neutrino v0.16.1 h1:5Kz4ToxncEVkpKC6fwUjXKtFKJhuxlG3sBB3MdJTJjs=

rfqmsg/aux_features.go

@@ -0,0 +1,9 @@
+//go:build !dev
+
+package rfqmsg
+
+var (
+	// UseNoOpHTLCs is set to false, as we don't want to enable it for
+	// production builds.
+	UseNoOpHTLCs = false
+)

rfqmsg/aux_features_dev.go

@@ -0,0 +1,8 @@
+//go:build dev
+
+package rfqmsg
+
+var (
+	// UseNoOpHTLCs is set to true, as we want to enable it for dev builds.
+	UseNoOpHTLCs = true
+)

rfqmsg/records.go

@@ -12,6 +12,7 @@ import (
 	"github.com/lightninglabs/taproot-assets/asset"
 	"github.com/lightninglabs/taproot-assets/fn"
 	"github.com/lightninglabs/taproot-assets/rfqmath"
+	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/tlv"
 )
@@ -169,6 +170,13 @@ type Htlc struct {
 	// picked when an HTLC is sent over to a peer. When one of these RFQ IDs
 	// gets picked it will be encoded as an Htlc.RfqID (see above field).
 	AvailableRfqIDs tlv.OptionalRecordT[AvailableRfqIDsType, HtlcRfqIDs]
+
+	// NoopAdd is a flag that indicates whether this HTLC should be marked
+	// as a noop_add for LND. A noop_add HTLC behaves identically to a
+	// normal HTLC except for the settlement step, where the satoshi amount
+	// is returned back to the sender, but the commitment blob is still
+	// updated to reflect the asset balance changes.
+	NoopAdd bool
 }
 
 // NewHtlc creates a new Htlc record with the given funded assets.
@@ -258,9 +266,19 @@ func (h *Htlc) Records() []tlv.Record {
 		},
 	)
 
+	if h.NoopAdd {
+		r := tlv.NewPrimitiveRecord[lnwallet.NoOpHtlcTLVType](true)
+		records = append(records, r.Record())
+	}
+
 	return records
 }
 
+// SetNoopAdd flags the HTLC as a noop_add.
+func (h *Htlc) SetNoopAdd(noopActive bool) {
+	h.NoopAdd = noopActive
+}
+
 // Encode serializes the Htlc to the given io.Writer.
 func (h *Htlc) Encode(w io.Writer) error {
 	tlvRecords := h.Records()