lnwallet: detect and handle noop HTLCs

We update the lightning channel state machine in some key areas. If the
noop TLV is set in the update_add_htlc custom records then we change the
entry type to noop. When settling the HTLC if the type is noop we credit
the satoshi amount back to the sender.

Author: GeorgeTsagk

lnwallet/aux_signer.go

@@ -10,9 +10,20 @@ import (
 	"github.com/lightningnetwork/lnd/tlv"
 )
 
-// htlcCustomSigType is the TLV type that is used to encode the custom HTLC
-// signatures within the custom data for an existing HTLC.
-var htlcCustomSigType tlv.TlvType65543
+var (
+	// htlcCustomSigType is the TLV type that is used to encode the custom
+	// HTLC signatures within the custom data for an existing HTLC.
+	htlcCustomSigType tlv.TlvType65543
+
+	// NoopHtlcType is the TLV that that's used in the update_add_htlc
+	// message to indicate the presence of a noop HTLC. This has no encoded
+	// value, but is used to indicate that the HTLC is a noop.
+	NoopHtlcType tlv.TlvType65544
+)
+
+// NoopAddHtlcType is the (golang) type of the TLV record that's used to signal
+// that an HTLC should be a noop HTLC.
+type NoopAddHtlcType = tlv.TlvType65544
 
 // AuxHtlcView is a struct that contains a safe copy of an HTLC view that can
 // be used by aux components.

lnwallet/channel.go

@@ -551,6 +551,13 @@ func (lc *LightningChannel) diskHtlcToPayDesc(feeRate chainfee.SatPerKWeight,
 		remoteOutputIndex = htlc.OutputIndex
 	}
 
+	customRecords := htlc.CustomRecords.Copy()
+	entryType := Add
+
+	if shouldSetNoop(customRecords, lc.channelState.ChanType) {
+		entryType = NoopAdd
+	}
+
 	// With the scripts reconstructed (depending on if this is our commit
 	// vs theirs or a pending commit for the remote party), we can now
 	// re-create the original payment descriptor.
@@ -559,7 +566,7 @@ func (lc *LightningChannel) diskHtlcToPayDesc(feeRate chainfee.SatPerKWeight,
 		RHash:              htlc.RHash,
 		Timeout:            htlc.RefundTimeout,
 		Amount:             htlc.Amt,
-		EntryType:          Add,
+		EntryType:          entryType,
 		HtlcIndex:          htlc.HtlcIndex,
 		LogIndex:           htlc.LogIndex,
 		OnionBlob:          htlc.OnionBlob,
@@ -570,7 +577,7 @@ func (lc *LightningChannel) diskHtlcToPayDesc(feeRate chainfee.SatPerKWeight,
 		theirPkScript:      theirP2WSH,
 		theirWitnessScript: theirWitnessScript,
 		BlindingPoint:      htlc.BlindingPoint,
-		CustomRecords:      htlc.CustomRecords.Copy(),
+		CustomRecords:      customRecords,
 	}, nil
 }
 
@@ -1100,6 +1107,10 @@ func (lc *LightningChannel) logUpdateToPayDesc(logUpdate *channeldb.LogUpdate,
 			},
 		}
 
+		if shouldSetNoop(pd.CustomRecords, lc.channelState.ChanType) {
+			pd.EntryType = NoopAdd
+		}
+
 		isDustRemote := HtlcIsDust(
 			lc.channelState.ChanType, false, lntypes.Remote,
 			feeRate, wireMsg.Amount.ToSatoshis(), remoteDustLimit,
@@ -1336,6 +1347,10 @@ func (lc *LightningChannel) remoteLogUpdateToPayDesc(logUpdate *channeldb.LogUpd
 			},
 		}
 
+		if shouldSetNoop(pd.CustomRecords, lc.channelState.ChanType) {
+			pd.EntryType = NoopAdd
+		}
+
 		// We don't need to generate an htlc script yet. This will be
 		// done once we sign our remote commitment.
 
@@ -1736,7 +1751,7 @@ func (lc *LightningChannel) restorePendingRemoteUpdates(
 		// but this Add restoration was a no-op as every single one of
 		// these Adds was already restored since they're all incoming
 		// htlcs on the local commitment.
-		if payDesc.EntryType == Add {
+		if payDesc.isAdd() {
 			continue
 		}
 
@@ -1881,7 +1896,7 @@ func (lc *LightningChannel) restorePendingLocalUpdates(
 		}
 
 		switch payDesc.EntryType {
-		case Add:
+		case Add, NoopAdd:
 			// The HtlcIndex of the added HTLC _must_ be equal to
 			// the log's htlcCounter at this point. If it is not we
 			// panic to catch this.
@@ -2993,6 +3008,50 @@ func (lc *LightningChannel) evaluateHTLCView(view *HtlcView,
 			)
 			if rmvHeight == 0 {
 				switch {
+				// If this a noop add, then when we settle the
+				// HTLC, we actually credit the sender with the
+				// amount again, thus making it a noop. Noop
+				// HTLCs are only triggered by external software
+				// using the AuxComponents and only for channels
+				// that use the custom tapscript root.
+				case entry.EntryType == Settle &&
+					addEntry.EntryType == NoopAdd:
+
+					channel := lc.channelState
+					party := party
+
+					delta := btcutil.Amount(
+						balanceDeltas.GetForParty(
+							party,
+						),
+					)
+
+					// If the receiver has existing balance
+					// above dust then we go ahead with
+					// crediting the amount back to the
+					// sender. Otherwise we give the amount
+					// to the receiver. We do this because
+					// the receiver needs some above-dust
+					// balance to anchor the AuxBlob. We
+					// also pass in the so-far calculated
+					// delta for the party, as that's
+					// effectively part of their balance
+					// within this view computation.
+					if channel.BalanceAboveReserve(
+						party, delta,
+					) {
+
+						party = party.CounterParty()
+					}
+
+					d := int64(entry.Amount)
+					balanceDeltas.ModifyForParty(
+						party,
+						func(acc int64) int64 {
+							return acc + d
+						},
+					)
+
 				// If an incoming HTLC is being settled, then
 				// this means that the preimage has been
 				// received by the settling party Therefore, we
@@ -3030,7 +3089,7 @@ func (lc *LightningChannel) evaluateHTLCView(view *HtlcView,
 		liveAdds := fn.Filter(
 			view.Updates.GetForParty(party),
 			func(pd *paymentDescriptor) bool {
-				isAdd := pd.EntryType == Add
+				isAdd := pd.isAdd()
 				shouldSkip := skip.GetForParty(party).
 					Contains(pd.HtlcIndex)
 
@@ -3069,7 +3128,7 @@ func (lc *LightningChannel) evaluateHTLCView(view *HtlcView,
 	// corresponding to whoseCommitmentChain.
 	isUncommitted := func(update *paymentDescriptor) bool {
 		switch update.EntryType {
-		case Add:
+		case Add, NoopAdd:
 			return update.addCommitHeights.GetForParty(
 				whoseCommitChain,
 			) == 0
@@ -3833,7 +3892,7 @@ func (lc *LightningChannel) validateCommitmentSanity(theirLogCounter,
 		// Go through all updates, checking that they don't violate the
 		// channel constraints.
 		for _, entry := range updates {
-			if entry.EntryType == Add {
+			if entry.isAdd() {
 				// An HTLC is being added, this will add to the
 				// number and amount in flight.
 				amtInFlight += entry.Amount
@@ -5712,7 +5771,7 @@ func (lc *LightningChannel) ReceiveRevocation(revMsg *lnwire.RevokeAndAck) (
 		// don't re-forward any already processed HTLC's after a
 		// restart.
 		switch {
-		case pd.EntryType == Add && committedAdd && shouldFwdAdd:
+		case pd.isAdd() && committedAdd && shouldFwdAdd:
 			// Construct a reference specifying the location that
 			// this forwarded Add will be written in the forwarding
 			// package constructed at this remote height.
@@ -5731,7 +5790,7 @@ func (lc *LightningChannel) ReceiveRevocation(revMsg *lnwire.RevokeAndAck) (
 				addUpdatesToForward, pd.toLogUpdate(),
 			)
 
-		case pd.EntryType != Add && committedRmv && shouldFwdRmv:
+		case !pd.isAdd() && committedRmv && shouldFwdRmv:
 			// Construct a reference specifying the location that
 			// this forwarded Settle/Fail will be written in the
 			// forwarding package constructed at this remote height.
@@ -5970,7 +6029,7 @@ func (lc *LightningChannel) GetDustSum(whoseCommit lntypes.ChannelParty,
 	// Grab all of our HTLCs and evaluate against the dust limit.
 	for e := lc.updateLogs.Local.Front(); e != nil; e = e.Next() {
 		pd := e.Value
-		if pd.EntryType != Add {
+		if !pd.isAdd() {
 			continue
 		}
 
@@ -5989,7 +6048,7 @@ func (lc *LightningChannel) GetDustSum(whoseCommit lntypes.ChannelParty,
 	// Grab all of their HTLCs and evaluate against the dust limit.
 	for e := lc.updateLogs.Remote.Front(); e != nil; e = e.Next() {
 		pd := e.Value
-		if pd.EntryType != Add {
+		if !pd.isAdd() {
 			continue
 		}
 
@@ -6062,9 +6121,15 @@ func (lc *LightningChannel) MayAddOutgoingHtlc(amt lnwire.MilliSatoshi) error {
 func (lc *LightningChannel) htlcAddDescriptor(htlc *lnwire.UpdateAddHTLC,
 	openKey *models.CircuitKey) *paymentDescriptor {
 
+	entryType := Add
+	customRecords := htlc.CustomRecords.Copy()
+	if shouldSetNoop(customRecords, lc.channelState.ChanType) {
+		entryType = NoopAdd
+	}
+
 	return &paymentDescriptor{
 		ChanID:         htlc.ChanID,
-		EntryType:      Add,
+		EntryType:      entryType,
 		RHash:          PaymentHash(htlc.PaymentHash),
 		Timeout:        htlc.Expiry,
 		Amount:         htlc.Amount,
@@ -6073,7 +6138,7 @@ func (lc *LightningChannel) htlcAddDescriptor(htlc *lnwire.UpdateAddHTLC,
 		OnionBlob:      htlc.OnionBlob,
 		OpenCircuitKey: openKey,
 		BlindingPoint:  htlc.BlindingPoint,
-		CustomRecords:  htlc.CustomRecords.Copy(),
+		CustomRecords:  customRecords,
 	}
 }
 
@@ -6126,17 +6191,23 @@ func (lc *LightningChannel) ReceiveHTLC(htlc *lnwire.UpdateAddHTLC) (uint64,
 			lc.updateLogs.Remote.htlcCounter)
 	}
 
+	entryType := Add
+	customRecords := htlc.CustomRecords.Copy()
+	if shouldSetNoop(customRecords, lc.channelState.ChanType) {
+		entryType = NoopAdd
+	}
+
 	pd := &paymentDescriptor{
 		ChanID:        htlc.ChanID,
-		EntryType:     Add,
+		EntryType:     entryType,
 		RHash:         PaymentHash(htlc.PaymentHash),
 		Timeout:       htlc.Expiry,
 		Amount:        htlc.Amount,
 		LogIndex:      lc.updateLogs.Remote.logIndex,
 		HtlcIndex:     lc.updateLogs.Remote.htlcCounter,
 		OnionBlob:     htlc.OnionBlob,
 		BlindingPoint: htlc.BlindingPoint,
-		CustomRecords: htlc.CustomRecords.Copy(),
+		CustomRecords: customRecords,
 	}
 
 	localACKedIndex := lc.commitChains.Remote.tail().messageIndices.Local
@@ -9825,7 +9896,7 @@ func (lc *LightningChannel) unsignedLocalUpdates(remoteMessageIndex,
 
 		// We don't save add updates as they are restored from the
 		// remote commitment in restoreStateLogs.
-		if pd.EntryType == Add {
+		if pd.isAdd() {
 			continue
 		}
 
@@ -9999,3 +10070,18 @@ func (lc *LightningChannel) ZeroConfRealScid() fn.Option[lnwire.ShortChannelID]
 
 	return fn.None[lnwire.ShortChannelID]()
 }
+
+// shouldSetNoop checks the custom records of the entry and the channel type and
+// returns a boolean indicating whether this add entry should be converted to a
+// noop add type. This will only return true if the TLV field signalling the use
+// of a noop HTLC is set, and the channel has a custom tapscript root.
+func shouldSetNoop(records lnwire.CustomRecords,
+	chanType channeldb.ChannelType) bool {
+
+	noopTLV := uint64(NoopHtlcType.TypeVal())
+	if _, ok := records[noopTLV]; ok && chanType.HasTapscriptRoot() {
+		return true
+	}
+
+	return false
+}

lnwallet/payment_descriptor.go

@@ -319,3 +319,8 @@ func (pd *paymentDescriptor) setCommitHeight(
 		)
 	}
 }
+
+// isAdd returns true if the paymentDescriptor is of type Add.
+func (pd *paymentDescriptor) isAdd() bool {
+	return pd.EntryType == Add || pd.EntryType == NoopAdd
+}