channeldb+lnwallet: define zero-fee channel type

channeldb/channel.go

@@ -244,6 +244,10 @@ const (
 	// that only the responder can decide to cooperatively close the
 	// channel.
 	FrozenBit ChannelType = 1 << 4
+
+	// ZeroHtlcTxFeeBit indicates that the channel should use zero-fee
+	// second-level HTLC transactions.
+	ZeroHtlcTxFeeBit ChannelType = 1 << 5
 )
 
 // IsSingleFunder returns true if the channel type if one of the known single
@@ -275,6 +279,12 @@ func (c ChannelType) HasAnchors() bool {
 	return c&AnchorOutputsBit == AnchorOutputsBit
 }
 
+// ZeroHtlcTxFee returns true if this channel type uses second-level HTLC
+// transactions signed with zero-fee.
+func (c ChannelType) ZeroHtlcTxFee() bool {
+	return c&ZeroHtlcTxFeeBit == ZeroHtlcTxFeeBit
+}
+
 // IsFrozen returns true if the channel is considered to be "frozen". A frozen
 // channel means that only the responder can initiate a cooperative channel
 // closure.

lnwallet/commitment.go

@@ -278,6 +278,12 @@ func CommitWeight(chanType channeldb.ChannelType) int64 {
 func HtlcTimeoutFee(chanType channeldb.ChannelType,
 	feePerKw chainfee.SatPerKWeight) btcutil.Amount {
 
+	// For zero-fee HTLC channels, this will always be zero, regardless of
+	// feerate.
+	if chanType.ZeroHtlcTxFee() {
+		return 0
+	}
+
 	if chanType.HasAnchors() {
 		return feePerKw.FeeForWeight(input.HtlcTimeoutWeightConfirmed)
 	}
@@ -290,6 +296,12 @@ func HtlcTimeoutFee(chanType channeldb.ChannelType,
 func HtlcSuccessFee(chanType channeldb.ChannelType,
 	feePerKw chainfee.SatPerKWeight) btcutil.Amount {
 
+	// For zero-fee HTLC channels, this will always be zero, regardless of
+	// feerate.
+	if chanType.ZeroHtlcTxFee() {
+		return 0
+	}
+
 	if chanType.HasAnchors() {
 		return feePerKw.FeeForWeight(input.HtlcSuccessWeightConfirmed)
 	}