Expose HTLC transaction locktime in BumpTransactionEvent::HTLCResolution

While users could easily figure it out based on the set of HTLC
descriptors included within, we already track it within the
`OnchainTxHandler`, so we might as well expose it to users as a
nice-to-have. It's also yet another thing they must get right to ensure
their HTLC transaction broadcasts are valid.

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -2426,7 +2426,7 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 					}));
 				},
 				ClaimEvent::BumpHTLC {
-					target_feerate_sat_per_1000_weight, htlcs,
+					target_feerate_sat_per_1000_weight, htlcs, tx_lock_time,
 				} => {
 					let mut htlc_descriptors = Vec::with_capacity(htlcs.len());
 					for htlc in htlcs {
@@ -2444,6 +2444,7 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 					ret.push(Event::BumpTransaction(BumpTransactionEvent::HTLCResolution {
 						target_feerate_sat_per_1000_weight,
 						htlc_descriptors,
+						tx_lock_time,
 					}));
 				}
 			}

lightning/src/chain/onchaintx.rs

@@ -12,6 +12,8 @@
 //! OnchainTxHandler objects are fully-part of ChannelMonitor and encapsulates all
 //! building, tracking, bumping and notifications functions.
 
+#[cfg(anchors)]
+use bitcoin::PackedLockTime;
 use bitcoin::blockdata::transaction::Transaction;
 use bitcoin::blockdata::transaction::OutPoint as BitcoinOutPoint;
 use bitcoin::blockdata::script::Script;
@@ -201,6 +203,7 @@ pub(crate) enum ClaimEvent {
 	BumpHTLC {
 		target_feerate_sat_per_1000_weight: u32,
 		htlcs: Vec<ExternalHTLCClaim>,
+		tx_lock_time: PackedLockTime,
 	},
 }
 
@@ -544,6 +547,7 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner> OnchainTxHandler<ChannelSigner>
 							OnchainClaim::Event(ClaimEvent::BumpHTLC {
 								target_feerate_sat_per_1000_weight,
 								htlcs,
+								tx_lock_time: PackedLockTime(cached_request.package_timelock()),
 							}),
 						));
 					} else {

lightning/src/ln/monitor_tests.rs

@@ -1775,7 +1775,7 @@ fn test_yield_anchors_events() {
 	let mut htlc_txs = Vec::with_capacity(2);
 	for event in holder_events {
 		match event {
-			Event::BumpTransaction(BumpTransactionEvent::HTLCResolution { htlc_descriptors, .. }) => {
+			Event::BumpTransaction(BumpTransactionEvent::HTLCResolution { htlc_descriptors, tx_lock_time, .. }) => {
 				assert_eq!(htlc_descriptors.len(), 1);
 				let htlc_descriptor = &htlc_descriptors[0];
 				let signer = nodes[0].keys_manager.derive_channel_keys(
@@ -1784,11 +1784,7 @@ fn test_yield_anchors_events() {
 				let per_commitment_point = signer.get_per_commitment_point(htlc_descriptor.per_commitment_number, &secp);
 				let mut htlc_tx = Transaction {
 					version: 2,
-					lock_time: if htlc_descriptor.htlc.offered {
-						PackedLockTime(htlc_descriptor.htlc.cltv_expiry)
-					} else {
-						PackedLockTime::ZERO
-					},
+					lock_time: tx_lock_time,
 					input: vec![
 						htlc_descriptor.unsigned_tx_input(), // HTLC input
 						TxIn { ..Default::default() } // Fee input
@@ -2064,7 +2060,7 @@ fn test_anchors_aggregated_revoked_htlc_tx() {
 		};
 		let mut descriptors = Vec::with_capacity(4);
 		for event in events {
-			if let Event::BumpTransaction(BumpTransactionEvent::HTLCResolution { mut htlc_descriptors, .. }) = event {
+			if let Event::BumpTransaction(BumpTransactionEvent::HTLCResolution { mut htlc_descriptors, tx_lock_time, .. }) = event {
 				assert_eq!(htlc_descriptors.len(), 2);
 				for htlc_descriptor in &htlc_descriptors {
 					assert!(!htlc_descriptor.htlc.offered);
@@ -2076,6 +2072,7 @@ fn test_anchors_aggregated_revoked_htlc_tx() {
 					htlc_tx.output.push(htlc_descriptor.tx_output(&per_commitment_point, &secp));
 				}
 				descriptors.append(&mut htlc_descriptors);
+				htlc_tx.lock_time = tx_lock_time;
 			} else {
 				panic!("Unexpected event");
 			}

lightning/src/util/events.rs

@@ -467,6 +467,8 @@ pub enum BumpTransactionEvent {
 		/// The set of pending HTLCs on the confirmed commitment that need to be claimed, preferably
 		/// by the same transaction.
 		htlc_descriptors: Vec<HTLCDescriptor>,
+		/// The locktime required for the resulting HTLC transaction.
+		tx_lock_time: PackedLockTime,
 	},
 }