Create a single P2A anchor on commitment transactions in 0FC channels

Zero-fee commitment channels replace today's existing `to_local_anchor`
and `to_remote_anchor` outputs with a single `shared_anchor` output.

Co-authored-by: Matt Corallo <[email protected]>

Author: tankyleo

lightning/src/ln/chan_utils.rs

@@ -89,6 +89,12 @@ pub const ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 114;
 #[cfg(not(feature = "grind_signatures"))]
 pub const ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 115;
 
+/// The P2A scriptpubkey
+pub const P2A_SCRIPT: &[u8] = &[0x51, 0x02, 0x4e, 0x73];
+
+/// The maximum value of the P2A anchor
+pub const P2A_MAX_VALUE: u64 = 240;
+
 /// The upper bound weight of an HTLC timeout input from a commitment transaction with anchor
 /// outputs.
 pub const HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT: u64 = 288;
@@ -1232,6 +1238,11 @@ impl<'a> DirectedChannelTransactionParameters<'a> {
 	pub fn channel_type_features(&self) -> &'a ChannelTypeFeatures {
 		&self.inner.channel_type_features
 	}
+
+	/// The value locked in the channel, denominated in satoshis.
+	pub fn channel_value_satoshis(&self) -> u64 {
+		self.inner.channel_value_satoshis
+	}
 }
 
 /// Information needed to build and sign a holder's commitment transaction.
@@ -1637,7 +1648,7 @@ impl CommitmentTransaction {
 		let outputs = Self::build_outputs_and_htlcs(&keys, to_broadcaster_value_sat, to_countersignatory_value_sat, &mut nondust_htlcs, channel_parameters);
 
 		let (obscured_commitment_transaction_number, txins) = Self::build_inputs(commitment_number, channel_parameters);
-		let transaction = Self::make_transaction(obscured_commitment_transaction_number, txins, outputs);
+		let transaction = Self::make_transaction(obscured_commitment_transaction_number, txins, outputs, channel_parameters);
 		let txid = transaction.compute_txid();
 		CommitmentTransaction {
 			commitment_number,
@@ -1691,6 +1702,8 @@ impl CommitmentTransaction {
 		// First rebuild the htlc outputs, note that `outputs` is now the same length as `self.nondust_htlcs`
 		let mut outputs = Self::build_htlc_outputs(keys, &self.nondust_htlcs, channel_parameters.channel_type_features());
 
+		let nondust_htlcs_value_sum_sat = self.nondust_htlcs.iter().map(|htlc| htlc.to_bitcoin_amount()).sum();
+
 		// Check that the HTLC outputs are sorted by value, script pubkey, and cltv expiry.
 		// Note that this only iterates if the length of `outputs` and `self.nondust_htlcs` is >= 2.
 		if (1..outputs.len()).into_iter().any(|i| Self::is_left_greater(i, &outputs, &self.nondust_htlcs)) {
@@ -1713,11 +1726,11 @@ impl CommitmentTransaction {
 			self.to_broadcaster_value_sat,
 			self.to_countersignatory_value_sat,
 			channel_parameters,
-			!self.nondust_htlcs.is_empty(),
+			nondust_htlcs_value_sum_sat,
 			insert_non_htlc_output
 		);
 
-		let transaction = Self::make_transaction(obscured_commitment_transaction_number, txins, outputs);
+		let transaction = Self::make_transaction(obscured_commitment_transaction_number, txins, outputs, channel_parameters);
 		let txid = transaction.compute_txid();
 		let built_transaction = BuiltCommitmentTransaction {
 			transaction,
@@ -1727,9 +1740,14 @@ impl CommitmentTransaction {
 	}
 
 	#[rustfmt::skip]
-	fn make_transaction(obscured_commitment_transaction_number: u64, txins: Vec<TxIn>, outputs: Vec<TxOut>) -> Transaction {
+	fn make_transaction(obscured_commitment_transaction_number: u64, txins: Vec<TxIn>, outputs: Vec<TxOut>, channel_parameters: &DirectedChannelTransactionParameters) -> Transaction {
+		let version = if channel_parameters.channel_type_features().supports_anchor_zero_fee_commitments() {
+			Version::non_standard(3)
+		} else {
+			Version::TWO
+		};
 		Transaction {
-			version: Version::TWO,
+			version,
 			lock_time: LockTime::from_consensus(((0x20 as u32) << 8 * 3) | ((obscured_commitment_transaction_number & 0xffffffu64) as u32)),
 			input: txins,
 			output: outputs,
@@ -1747,7 +1765,8 @@ impl CommitmentTransaction {
 		// First build and sort the HTLC outputs.
 		// Also sort the HTLC output data in `nondust_htlcs` in the same order.
 		let mut outputs = Self::build_sorted_htlc_outputs(keys, nondust_htlcs, channel_parameters.channel_type_features());
-		let tx_has_htlc_outputs = !outputs.is_empty();
+
+		let nondust_htlcs_value_sum_sat = nondust_htlcs.iter().map(|htlc| htlc.to_bitcoin_amount()).sum();
 
 		// Initialize the transaction output indices; we will update them below when we
 		// add the non-htlc transaction outputs.
@@ -1784,7 +1803,7 @@ impl CommitmentTransaction {
 			to_broadcaster_value_sat,
 			to_countersignatory_value_sat,
 			channel_parameters,
-			tx_has_htlc_outputs,
+			nondust_htlcs_value_sum_sat,
 			insert_non_htlc_output
 		);
 
@@ -1797,7 +1816,7 @@ impl CommitmentTransaction {
 		to_broadcaster_value_sat: Amount,
 		to_countersignatory_value_sat: Amount,
 		channel_parameters: &DirectedChannelTransactionParameters,
-		tx_has_htlc_outputs: bool,
+		nondust_htlcs_value_sum_sat: Amount,
 		mut insert_non_htlc_output: F,
 	) where
 		F: FnMut(TxOut),
@@ -1807,6 +1826,7 @@ impl CommitmentTransaction {
 		let broadcaster_funding_key = &channel_parameters.broadcaster_pubkeys().funding_pubkey;
 		let channel_type = channel_parameters.channel_type_features();
 		let contest_delay = channel_parameters.contest_delay();
+		let tx_has_htlc_outputs = nondust_htlcs_value_sum_sat != Amount::ZERO;
 
 		if to_countersignatory_value_sat > Amount::ZERO {
 			let script = if channel_type.supports_anchors_zero_fee_htlc_tx() {
@@ -1849,6 +1869,16 @@ impl CommitmentTransaction {
 				});
 			}
 		}
+
+		if channel_type.supports_anchor_zero_fee_commitments() {
+				let channel_value_satoshis = Amount::from_sat(channel_parameters.channel_value_satoshis());
+				// These subtractions panic on underflow, but this should never happen
+				let trimmed_sum_sat = channel_value_satoshis - nondust_htlcs_value_sum_sat - to_broadcaster_value_sat - to_countersignatory_value_sat;
+				insert_non_htlc_output(TxOut {
+					script_pubkey: ScriptBuf::from_bytes(P2A_SCRIPT.to_vec()),
+					value: cmp::min(Amount::from_sat(P2A_MAX_VALUE), trimmed_sum_sat),
+				});
+		}
 	}
 
 	#[rustfmt::skip]