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 +551,20 @@ func (lc *LightningChannel) diskHtlcToPayDesc(feeRate chainfee.SatPerKWeight,
 		remoteOutputIndex = htlc.OutputIndex
 	}
 
+	customRecords := htlc.CustomRecords.Copy()
+	entryType := Add
+
+	// If the noop HTLC TLV is set then change this HTLC's type to noop.
+	noopTLV := uint64(NoopHtlcType.TypeVal())
+	if _, ok := customRecords[noopTLV]; ok {
+		entryType = NoopAdd
+	}
+
+	// The NoopAdd HTLC is an internal construct, and isn't meant to show up
+	// on the wire. So we'll remove the special element from the set of
+	// custom records.
+	delete(customRecords, noopTLV)
+
 	// 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 +573,7 @@ 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 +584,7 @@ func (lc *LightningChannel) diskHtlcToPayDesc(feeRate chainfee.SatPerKWeight,
 		theirPkScript:      theirP2WSH,
 		theirWitnessScript: theirWitnessScript,
 		BlindingPoint:      htlc.BlindingPoint,
-		CustomRecords:      htlc.CustomRecords.Copy(),
+		CustomRecords:      customRecords,
 	}, nil
 }
 
@@ -1100,6 +1114,11 @@ func (lc *LightningChannel) logUpdateToPayDesc(logUpdate *channeldb.LogUpdate,
 			},
 		}
 
+		noopTLV := uint64(NoopHtlcType.TypeVal())
+		if _, ok := pd.CustomRecords[noopTLV]; ok {
+			pd.EntryType = NoopAdd
+		}
+
 		isDustRemote := HtlcIsDust(
 			lc.channelState.ChanType, false, lntypes.Remote,
 			feeRate, wireMsg.Amount.ToSatoshis(), remoteDustLimit,
@@ -1336,6 +1355,11 @@ func (lc *LightningChannel) remoteLogUpdateToPayDesc(logUpdate *channeldb.LogUpd
 			},
 		}
 
+		noopTLV := uint64(NoopHtlcType.TypeVal())
+		if _, ok := pd.CustomRecords[noopTLV]; ok {
+			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 +1760,7 @@ 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 +1905,7 @@ 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.
@@ -2987,6 +3011,38 @@ 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.
+				case entry.EntryType == Settle &&
+					addEntry.EntryType == NoopAdd:
+					channel := lc.channelState
+
+					// 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.
+					if channel.BalanceAboveReserve(party) {
+						d := int64(entry.Amount)
+						balanceDeltas.ModifyForParty(
+							party.CounterParty(),
+							func(acc int64) int64 {
+								return acc + d
+							},
+						)
+					} else {
+						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
@@ -3024,7 +3080,7 @@ 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)
 
@@ -3063,7 +3119,7 @@ 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
@@ -3827,7 +3883,7 @@ 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
@@ -4483,6 +4539,14 @@ func (lc *LightningChannel) ProcessChanSyncMsg(ctx context.Context,
 		// Next, we'll need to send over any updates we sent as part of
 		// this new proposed commitment state.
 		for _, logUpdate := range commitDiff.LogUpdates {
+			//nolint:ll
+			if htlc, ok := logUpdate.UpdateMsg.(*lnwire.UpdateAddHTLC); ok {
+				delete(htlc.CustomRecords, uint64(NoopHtlcType.TypeVal()))
+
+				if len(htlc.CustomRecords) == 0 {
+					htlc.CustomRecords = nil
+				}
+			}
 			commitUpdates = append(
 				commitUpdates, logUpdate.UpdateMsg,
 			)
@@ -5706,7 +5770,7 @@ 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.
@@ -5725,7 +5789,7 @@ 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.
@@ -5964,7 +6028,7 @@ 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
 		}
 
@@ -5983,7 +6047,7 @@ 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
 		}
 
@@ -6056,9 +6120,16 @@ 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()
+	noopTLV := uint64(NoopHtlcType.TypeVal())
+	if _, ok := customRecords[noopTLV]; ok {
+		entryType = NoopAdd
+	}
+
 	return &paymentDescriptor{
 		ChanID:         htlc.ChanID,
-		EntryType:      Add,
+		EntryType:      entryType,
 		RHash:          PaymentHash(htlc.PaymentHash),
 		Timeout:        htlc.Expiry,
 		Amount:         htlc.Amount,
@@ -6067,7 +6138,7 @@ func (lc *LightningChannel) htlcAddDescriptor(htlc *lnwire.UpdateAddHTLC,
 		OnionBlob:      htlc.OnionBlob,
 		OpenCircuitKey: openKey,
 		BlindingPoint:  htlc.BlindingPoint,
-		CustomRecords:  htlc.CustomRecords.Copy(),
+		CustomRecords:  customRecords,
 	}
 }
 
@@ -6120,17 +6191,24 @@ func (lc *LightningChannel) ReceiveHTLC(htlc *lnwire.UpdateAddHTLC) (uint64,
 			lc.updateLogs.Remote.htlcCounter)
 	}
 
+	entryType := Add
+	customRecords := htlc.CustomRecords.Copy()
+	noopTLV := uint64(NoopHtlcType.TypeVal())
+	if _, ok := customRecords[noopTLV]; ok {
+		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
@@ -9819,7 +9897,7 @@ 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
 		}