Add NoopAdd HTLCs

docs/release-notes/release-notes-0.20.0.md

@@ -40,6 +40,13 @@
 
 # New Features
 
+- Added [NoOp HTLCs](https://github.com/lightningnetwork/lnd/pull/9871). This
+allows sending HTLCs to the remote party without shifting the balances of the
+channel. This is currently only possible to use with custom channels, and only
+when the appropriate TLV flag is set. This allows for HTLCs carrying metadata to
+reflect their state on the channel commitment without having to send or receive
+a certain amount of msats.
+
 ## Functional Enhancements
 
 * RPCs `walletrpc.EstimateFee` and `walletrpc.FundPsbt` now

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
+
+	// NoOpHtlcTLVEntry 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, its presence is used to indicate that the HTLC is a noop.
+	NoOpHtlcTLVEntry tlv.TlvType65544
+)
+
+// NoOpHtlcTLVType is the (golang) type of the TLV record that's used to signal
+// that an HTLC should be a noop HTLC.
+type NoOpHtlcTLVType = tlv.TlvType65544
 
 // AuxHtlcView is a struct that contains a safe copy of an HTLC view that can
 // be used by aux components.
@@ -116,6 +127,18 @@ func (a *AuxHtlcDescriptor) AddHeight(
 	return a.addCommitHeightLocal
 }
 
+// IsAdd checks if the entry type of the Aux HTLC Descriptor is an add type.
+func (a *AuxHtlcDescriptor) IsAdd() bool {
+	switch a.EntryType {
+	case Add:
+		fallthrough
+	case NoOpAdd:
+		return true
+	default:
+		return false
+	}
+}
+
 // RemoveHeight returns the height at which the HTLC was removed from the
 // commitment chain. The height is returned based on the chain the HTLC is being
 // removed from (local or remote chain).