lnwallet: add noop updateType to paymendDescriptor

We add a new update type to the payment descriptor to describe this new
type of htlc. This type of HTLC will only end up being set if explicitly
signalled by external software.

lnwallet/payment_descriptor.go

@@ -42,6 +42,13 @@ const (
 	// FeeUpdate is an update type sent by the channel initiator that
 	// updates the fee rate used when signing the commitment transaction.
 	FeeUpdate
+
+	// NoOpAdd is an update type that adds a new HTLC entry into the log.
+	// This differs from the normal Add type, in that when settled the
+	// balance may go back to the sender, rather than be credited for the
+	// receiver. The criteria about whether the balance will go back to the
+	// sender is whether the receiver is sitting above the channel reserve.
+	NoOpAdd
 )
 
 // String returns a human readable string that uniquely identifies the target
@@ -58,6 +65,8 @@ func (u updateType) String() string {
 		return "Settle"
 	case FeeUpdate:
 		return "FeeUpdate"
+	case NoOpAdd:
+		return "NoOpAdd"
 	default:
 		return "<unknown type>"
 	}
@@ -238,7 +247,7 @@ type paymentDescriptor struct {
 func (pd *paymentDescriptor) toLogUpdate() channeldb.LogUpdate {
 	var msg lnwire.Message
 	switch pd.EntryType {
-	case Add:
+	case Add, NoOpAdd:
 		msg = &lnwire.UpdateAddHTLC{
 			ChanID:        pd.ChanID,
 			ID:            pd.HtlcIndex,
@@ -290,7 +299,7 @@ func (pd *paymentDescriptor) setCommitHeight(
 	whoseCommitChain lntypes.ChannelParty, nextHeight uint64) {
 
 	switch pd.EntryType {
-	case Add:
+	case Add, NoOpAdd:
 		pd.addCommitHeights.SetForParty(
 			whoseCommitChain, nextHeight,
 		)