Update handling of BumpTransactionEvent::ChannelClose for 0FC channels

Set an empty witness to spend the P2A anchor, and set the version of the
anchor transaction to 3.

We still check whether the commitment transaction has enough fees to be
broadcast on its own, as these transactions may be non-zero fee due to
trimmed / rounded outputs.

Author: tankyleo

lightning/src/events/bump_transaction/mod.rs

@@ -25,6 +25,7 @@ use crate::ln::chan_utils;
 use crate::ln::chan_utils::{
 	shared_anchor_script_pubkey, HTLCOutputInCommitment, ANCHOR_INPUT_WITNESS_WEIGHT,
 	HTLC_SUCCESS_INPUT_ANCHOR_WITNESS_WEIGHT, HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT,
+	P2A_ANCHOR_INPUT_WITNESS_WEIGHT,
 };
 use crate::ln::types::ChannelId;
 use crate::prelude::*;
@@ -669,6 +670,16 @@ where
 		commitment_tx: &Transaction, commitment_tx_fee_sat: u64,
 		anchor_descriptor: &AnchorDescriptor,
 	) -> Result<(), ()> {
+		let channel_type = &anchor_descriptor
+			.channel_derivation_parameters
+			.transaction_parameters
+			.channel_type_features;
+		let anchor_input_witness_weight = if channel_type.supports_anchor_zero_fee_commitments() {
+			P2A_ANCHOR_INPUT_WITNESS_WEIGHT
+		} else {
+			ANCHOR_INPUT_WITNESS_WEIGHT
+		};
+
 		// First, check if the commitment transaction has sufficient fees on its own.
 		let commitment_tx_feerate_sat_per_1000_weight = compute_feerate_sat_per_1000_weight(
 			commitment_tx_fee_sat,
@@ -689,7 +700,7 @@ where
 		let commitment_tx_fee_sat = Amount::from_sat(commitment_tx_fee_sat);
 		anchor_utxo.value += commitment_tx_fee_sat;
 		let starting_package_and_fixed_input_satisfaction_weight =
-			commitment_tx.weight().to_wu() + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT;
+			commitment_tx.weight().to_wu() + anchor_input_witness_weight + EMPTY_SCRIPT_SIG_WEIGHT;
 		let mut package_and_fixed_input_satisfaction_weight =
 			starting_package_and_fixed_input_satisfaction_weight;
 
@@ -714,8 +725,14 @@ where
 				)
 				.await?;
 
+			let version = if channel_type.supports_anchor_zero_fee_commitments() {
+				Version::non_standard(3)
+			} else {
+				Version::TWO
+			};
+
 			let mut anchor_tx = Transaction {
-				version: Version::TWO,
+				version,
 				lock_time: LockTime::ZERO, // TODO: Use next best height.
 				input: vec![anchor_descriptor.unsigned_tx_input()],
 				output: vec![],
@@ -724,7 +741,7 @@ where
 			let input_satisfaction_weight: u64 =
 				coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum();
 			let total_satisfaction_weight =
-				ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT + input_satisfaction_weight;
+				anchor_input_witness_weight + EMPTY_SCRIPT_SIG_WEIGHT + input_satisfaction_weight;
 			let total_input_amount = must_spend_amount
 				+ coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum();
 
@@ -780,18 +797,21 @@ where
 			log_debug!(self.logger, "Signing anchor transaction {}", anchor_txid);
 			anchor_tx = self.utxo_source.sign_psbt(anchor_psbt).await?;
 
-			let signer = self
-				.signer_provider
-				.derive_channel_signer(anchor_descriptor.channel_derivation_parameters.keys_id);
-			let channel_parameters =
-				&anchor_descriptor.channel_derivation_parameters.transaction_parameters;
-			let anchor_sig = signer.sign_holder_keyed_anchor_input(
-				channel_parameters,
-				&anchor_tx,
-				0,
-				&self.secp,
-			)?;
-			anchor_tx.input[0].witness = anchor_descriptor.tx_input_witness(&anchor_sig);
+			// No need to produce any witness to spend P2A anchors
+			if channel_type.supports_anchors_zero_fee_htlc_tx() {
+				let signer = self
+					.signer_provider
+					.derive_channel_signer(anchor_descriptor.channel_derivation_parameters.keys_id);
+				let channel_parameters =
+					&anchor_descriptor.channel_derivation_parameters.transaction_parameters;
+				let anchor_sig = signer.sign_holder_keyed_anchor_input(
+					channel_parameters,
+					&anchor_tx,
+					0,
+					&self.secp,
+				)?;
+				anchor_tx.input[0].witness = anchor_descriptor.tx_input_witness(&anchor_sig);
+			}
 
 			#[cfg(debug_assertions)]
 			{

lightning/src/ln/chan_utils.rs

@@ -89,7 +89,10 @@ pub const ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 114;
 #[cfg(not(feature = "grind_signatures"))]
 pub const ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 115;
 
-/// The maximum value of the P2A anchor
+/// The weight of a P2A anchor witness.
+pub const P2A_ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 1;
+
+/// The maximum value of a P2A anchor.
 pub const P2A_MAX_VALUE: u64 = 240;
 
 /// The upper bound weight of an HTLC timeout input from a commitment transaction with anchor