Track channel_keys_id in HolderHTLCOutput

The `ChannelMonitor` and `OnchainTxHandler` have historically been tied
together, often tracking some of the same state twice. As we introduce
support for splices in the `ChannelMonitor`, we'd like to avoid leaking
some of those details to the `OnchainTxHandler`. Ultimately, the
`OnchainTxHandler` should stand on its own and support claiming funds
from multiple `ChannelMonitor`s, allowing us to save on fees by batching
aggregatable claims across multiple in-flight closing channels.

This commit tracks the `channel_keys_id` for the channel the
`HolderHTLCOutput` claim originated from.

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -4004,7 +4004,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 			if let Some(transaction_output_index) = htlc.transaction_output_index {
 				let (htlc_output, counterparty_spendable_height) = if htlc.offered {
 					let htlc_output = HolderHTLCOutput::build_offered(
-						self.funding.channel_parameters.clone(),
+						self.funding.channel_parameters.clone(), self.channel_keys_id,
 						htlc.amount_msat, htlc.cltv_expiry,
 						self.funding.current_holder_commitment.tx.clone(),
 						self.funding.prev_holder_commitment.as_ref().map(|c| &c.tx).cloned(),
@@ -4018,7 +4018,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 						continue;
 					};
 					let htlc_output = HolderHTLCOutput::build_accepted(
-						self.funding.channel_parameters.clone(),
+						self.funding.channel_parameters.clone(), self.channel_keys_id,
 						payment_preimage, htlc.amount_msat,
 						self.funding.current_holder_commitment.tx.clone(),
 						self.funding.prev_holder_commitment.as_ref().map(|c| &c.tx).cloned(),
@@ -4186,15 +4186,17 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 			if let Some(vout) = htlc.transaction_output_index {
 				let (htlc_output, preimage) = if htlc.offered {
 					let htlc_output = HolderHTLCOutput::build_offered(
-						self.funding.channel_parameters.clone(), htlc.amount_msat, htlc.cltv_expiry,
+						self.funding.channel_parameters.clone(), self.channel_keys_id,
+						htlc.amount_msat, htlc.cltv_expiry,
 						self.funding.current_holder_commitment.tx.clone(),
 						self.funding.prev_holder_commitment.as_ref().map(|c| &c.tx).cloned(),
 					);
 					(htlc_output, None)
 				} else {
 					if let Some((preimage, _)) = self.payment_preimages.get(&htlc.payment_hash) {
 						let htlc_output = HolderHTLCOutput::build_accepted(
-							self.funding.channel_parameters.clone(), *preimage, htlc.amount_msat,
+							self.funding.channel_parameters.clone(), self.channel_keys_id,
+							*preimage, htlc.amount_msat,
 							self.funding.current_holder_commitment.tx.clone(),
 							self.funding.prev_holder_commitment.as_ref().map(|c| &c.tx).cloned(),
 						);

lightning/src/chain/onchaintx.rs

@@ -234,7 +234,7 @@ pub(crate) enum FeerateStrategy {
 #[derive(Clone)]
 pub struct OnchainTxHandler<ChannelSigner: EcdsaChannelSigner> {
 	channel_value_satoshis: u64,
-	pub(crate) channel_keys_id: [u8; 32],
+	channel_keys_id: [u8; 32], // Deprecated as of 0.2.
 	destination_script: ScriptBuf, // Deprecated as of 0.2.
 	holder_commitment: HolderCommitmentTransaction,
 	prev_holder_commitment: Option<HolderCommitmentTransaction>,
@@ -1207,6 +1207,11 @@ impl<ChannelSigner: EcdsaChannelSigner> OnchainTxHandler<ChannelSigner> {
 	pub(crate) fn channel_type_features(&self) -> &ChannelTypeFeatures {
 		&self.channel_transaction_parameters.channel_type_features
 	}
+
+	// Deprecated as of 0.2, only use in cases where it was not previously available.
+	pub(crate) fn channel_keys_id(&self) -> [u8; 32] {
+		self.channel_keys_id
+	}
 }
 
 #[cfg(test)]
@@ -1339,7 +1344,7 @@ mod tests {
 				holder_commit_txid,
 				htlc.transaction_output_index.unwrap(),
 				PackageSolvingData::HolderHTLCOutput(HolderHTLCOutput::build_offered(
-					tx_handler.channel_transaction_parameters.clone(),
+					tx_handler.channel_transaction_parameters.clone(), tx_handler.channel_keys_id,
 					htlc.amount_msat, htlc.cltv_expiry,
 					tx_handler.holder_commitment.clone(), tx_handler.prev_holder_commitment.clone(),
 				)),

lightning/src/chain/package.rs

@@ -376,12 +376,13 @@ pub(crate) struct HolderHTLCOutput {
 	current_holder_commitment: Option<HolderCommitmentTransaction>,
 	prev_holder_commitment: Option<HolderCommitmentTransaction>,
 	channel_parameters: Option<ChannelTransactionParameters>,
+	channel_keys_id: Option<[u8; 32]>,
 }
 
 impl HolderHTLCOutput {
 	pub(crate) fn build_offered(
-		channel_parameters: ChannelTransactionParameters, amount_msat: u64, cltv_expiry: u32,
-		current_holder_commitment: HolderCommitmentTransaction,
+		channel_parameters: ChannelTransactionParameters, channel_keys_id: [u8; 32],
+		amount_msat: u64, cltv_expiry: u32, current_holder_commitment: HolderCommitmentTransaction,
 		prev_holder_commitment: Option<HolderCommitmentTransaction>,
 	) -> Self {
 		let channel_type_features = channel_parameters.channel_type_features.clone();
@@ -393,11 +394,13 @@ impl HolderHTLCOutput {
 			current_holder_commitment: Some(current_holder_commitment),
 			prev_holder_commitment,
 			channel_parameters: Some(channel_parameters),
+			channel_keys_id: Some(channel_keys_id),
 		}
 	}
 
 	pub(crate) fn build_accepted(
-		channel_parameters: ChannelTransactionParameters, preimage: PaymentPreimage, amount_msat: u64,
+		channel_parameters: ChannelTransactionParameters, channel_keys_id: [u8; 32],
+		preimage: PaymentPreimage, amount_msat: u64,
 		current_holder_commitment: HolderCommitmentTransaction,
 		prev_holder_commitment: Option<HolderCommitmentTransaction>,
 	) -> Self {
@@ -410,6 +413,7 @@ impl HolderHTLCOutput {
 			current_holder_commitment: Some(current_holder_commitment),
 			prev_holder_commitment,
 			channel_parameters: Some(channel_parameters),
+			channel_keys_id: Some(channel_keys_id),
 		}
 	}
 
@@ -419,6 +423,8 @@ impl HolderHTLCOutput {
 	) -> Option<MaybeSignedTransaction> {
 		let channel_parameters = self.channel_parameters.as_ref()
 			.unwrap_or(&onchain_tx_handler.channel_transaction_parameters);
+		let channel_keys_id = self.channel_keys_id
+			.unwrap_or(onchain_tx_handler.channel_keys_id());
 		let get_signed_htlc_tx = |holder_commitment: &HolderCommitmentTransaction| {
 			let trusted_tx = holder_commitment.trust();
 			if trusted_tx.txid() != outp.txid {
@@ -435,7 +441,7 @@ impl HolderHTLCOutput {
 			let htlc_descriptor = HTLCDescriptor {
 				channel_derivation_parameters: ChannelDerivationParameters {
 					value_satoshis: channel_parameters.channel_value_satoshis,
-					keys_id: onchain_tx_handler.channel_keys_id,
+					keys_id: channel_keys_id,
 					transaction_parameters: channel_parameters.clone(),
 				},
 				commitment_txid: trusted_tx.txid(),
@@ -519,6 +525,7 @@ impl Writeable for HolderHTLCOutput {
 			(9, self.current_holder_commitment, option), // Added in 0.2.
 			(11, self.prev_holder_commitment, option), // Added in 0.2.
 			(13, self.channel_parameters, option), // Added in 0.2.
+			(15, self.channel_keys_id, option), // Added in 0.2.
 		});
 		Ok(())
 	}
@@ -534,6 +541,7 @@ impl Readable for HolderHTLCOutput {
 		let mut current_holder_commitment = None;
 		let mut prev_holder_commitment = None;
 		let mut channel_parameters = None;
+		let mut channel_keys_id = None;
 
 		read_tlv_fields!(reader, {
 			(0, amount_msat, required),
@@ -544,6 +552,7 @@ impl Readable for HolderHTLCOutput {
 			(9, current_holder_commitment, option), // Added in 0.2.
 			(11, prev_holder_commitment, option), // Added in 0.2.
 			(13, channel_parameters, (option: ReadableArgs, None)), // Added in 0.2.
+			(15, channel_keys_id, option), // Added in 0.2.
 		});
 
 		verify_channel_type_features(&channel_type_features, None)?;
@@ -558,6 +567,7 @@ impl Readable for HolderHTLCOutput {
 			current_holder_commitment,
 			prev_holder_commitment,
 			channel_parameters,
+			channel_keys_id,
 		})
 	}
 }
@@ -1660,7 +1670,7 @@ mod tests {
 				let commitment_tx = HolderCommitmentTransaction::dummy(0, &mut Vec::new());
 				let preimage = PaymentPreimage([2;32]);
 				PackageSolvingData::HolderHTLCOutput(HolderHTLCOutput::build_accepted(
-					channel_parameters, preimage, 0, commitment_tx, None,
+					channel_parameters, [0; 32], preimage, 0, commitment_tx, None,
 				))
 			}
 		}
@@ -1673,7 +1683,7 @@ mod tests {
 				channel_parameters.channel_type_features = $features;
 				let commitment_tx = HolderCommitmentTransaction::dummy(0, &mut Vec::new());
 				PackageSolvingData::HolderHTLCOutput(HolderHTLCOutput::build_offered(
-					channel_parameters, 0, $cltv_expiry, commitment_tx, None,
+					channel_parameters, [0; 32], 0, $cltv_expiry, commitment_tx, None,
 				))
 			}
 		}