f! cache txid + debug assert + avoid clones + sighash_single note

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -1515,6 +1515,7 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 		let htlc_commitment_tx_output_idx =
 			if let Some(v) = htlc.transaction_output_index { v } else { return None; };
 
+		let mut htlc_spend_txid_opt = None;
 		let mut htlc_spend_tx_opt = None;
 		let mut holder_timeout_spend_pending = None;
 		let mut htlc_spend_pending = None;
@@ -1523,16 +1524,18 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 			match event.event {
 				OnchainEvent::HTLCUpdate { commitment_tx_output_idx, htlc_value_satoshis, .. }
 				if commitment_tx_output_idx == Some(htlc_commitment_tx_output_idx) => {
+					htlc_spend_txid_opt = Some(&event.txid);
 					debug_assert!(htlc_spend_tx_opt.is_none());
-					htlc_spend_tx_opt = event.transaction.clone();
+					htlc_spend_tx_opt = event.transaction.as_ref();
 					debug_assert!(holder_timeout_spend_pending.is_none());
 					debug_assert_eq!(htlc_value_satoshis.unwrap(), htlc.amount_msat / 1000);
 					holder_timeout_spend_pending = Some(event.confirmation_threshold());
 				},
 				OnchainEvent::HTLCSpendConfirmation { commitment_tx_output_idx, preimage, .. }
 				if commitment_tx_output_idx == htlc_commitment_tx_output_idx => {
+					htlc_spend_txid_opt = Some(&event.txid);
 					debug_assert!(htlc_spend_tx_opt.is_none());
-					htlc_spend_tx_opt = event.transaction.clone();
+					htlc_spend_tx_opt = event.transaction.as_ref();
 					debug_assert!(htlc_spend_pending.is_none());
 					htlc_spend_pending = Some((event.confirmation_threshold(), preimage.is_some()));
 				},
@@ -1547,20 +1550,23 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 		}
 		let htlc_resolved = self.htlcs_resolved_on_chain.iter()
 			.find(|v| if v.commitment_tx_output_idx == Some(htlc_commitment_tx_output_idx) {
+				htlc_spend_txid_opt = v.resolving_txid.as_ref();
 				debug_assert!(htlc_spend_tx_opt.is_none());
-				htlc_spend_tx_opt = v.resolving_tx.clone();
+				htlc_spend_tx_opt = v.resolving_tx.as_ref();
 				true
 			} else { false });
 		debug_assert!(holder_timeout_spend_pending.is_some() as u8 + htlc_spend_pending.is_some() as u8 + htlc_resolved.is_some() as u8 <= 1);
 
 		let htlc_commitment_outpoint = BitcoinOutPoint::new(confirmed_txid.unwrap(), htlc_commitment_tx_output_idx);
 		let htlc_output_to_spend =
 			if let Some(ref tx) = htlc_spend_tx_opt {
-				let htlc_input_idx = tx.input.iter().enumerate()
+				// Because HTLCs are signed with SIGHASH_SINGLE, we can locate the correct output
+				// by ensuring its adjacent input spends the HTLC output in the commitment.
+				let htlc_input_idx_opt = tx.input.iter().enumerate()
 					.find(|(_, input)| input.previous_output == htlc_commitment_outpoint)
-					.map(|(idx, _)| idx as u32)
-					.unwrap();
-				BitcoinOutPoint::new(tx.txid(), htlc_input_idx)
+					.map(|(idx, _)| idx as u32);
+				debug_assert!(htlc_input_idx_opt.is_some());
+				BitcoinOutPoint::new(*htlc_spend_txid_opt.unwrap(), htlc_input_idx_opt.unwrap_or(0))
 			} else {
 				htlc_commitment_outpoint
 			};
@@ -1585,9 +1591,8 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 					descriptor: SpendableOutputDescriptor::StaticOutput { .. }
 				} = &event.event {
 					if event.transaction.as_ref().map(|tx| tx.input.iter().any(|inp| {
-						if let Some(ref htlc_spend_tx) = htlc_spend_tx_opt {
-							let htlc_spend_txid = htlc_spend_tx.txid();
-							tx.txid() == htlc_spend_txid || inp.previous_output.txid == htlc_spend_txid
+						if let Some(htlc_spend_txid) = htlc_spend_txid_opt {
+							tx.txid() == *htlc_spend_txid || inp.previous_output.txid == *htlc_spend_txid
 						} else {
 							Some(inp.previous_output.txid) == confirmed_txid &&
 								inp.previous_output.vout == htlc_commitment_tx_output_idx