Add change_script to FundingNegotiationContext

Instead of passing the optional change script as another parameter to
FundingNegotiationContext::into_interactive_tx_constructor, make it a
member of FundingNegotiationContext. Also, allow it to be passed to
ChannelManager::splice_channel.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -5954,17 +5954,19 @@ pub(super) struct FundingNegotiationContext {
 	/// The funding inputs we will be contributing to the channel.
 	#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
 	pub our_funding_inputs: Vec<(TxIn, TransactionU16LenLimited)>,
+	/// The change output script. This will be used if needed or -- if not set -- generated using
+	/// `SignerProvider::get_destination_script`.
+	#[allow(dead_code)] // TODO(splicing): Remove once splicing is enabled.
+	pub change_script: Option<ScriptBuf>,
 }
 
 impl FundingNegotiationContext {
 	/// Prepare and start interactive transaction negotiation.
-	/// `change_destination_opt` - Optional destination for optional change; if None,
-	///   default destination address is used.
 	/// If error occurs, it is caused by our side, not the counterparty.
 	#[cfg(splicing)]
 	fn into_interactive_tx_constructor<SP: Deref, ES: Deref>(
 		self, context: &ChannelContext<SP>, funding: &FundingScope, signer_provider: &SP,
-		entropy_source: &ES, holder_node_id: PublicKey, change_destination_opt: Option<ScriptBuf>,
+		entropy_source: &ES, holder_node_id: PublicKey,
 	) -> Result<InteractiveTxConstructor, AbortReason>
 	where
 		SP::Target: SignerProvider,
@@ -6001,12 +6003,12 @@ impl FundingNegotiationContext {
 				context.holder_dust_limit_satoshis,
 			)?;
 			if let Some(change_value) = change_value_opt {
-				let change_script = if let Some(script) = change_destination_opt {
+				let change_script = if let Some(script) = self.change_script {
 					script
 				} else {
-					signer_provider.get_destination_script(context.channel_keys_id).map_err(
-						|_err| AbortReason::InternalError("Error getting destination script"),
-					)?
+					signer_provider
+						.get_destination_script(context.channel_keys_id)
+						.map_err(|_err| AbortReason::InternalError("Error getting change script"))?
 				};
 				let mut change_output =
 					TxOut { value: Amount::from_sat(change_value), script_pubkey: change_script };
@@ -10573,11 +10575,13 @@ where
 	/// Initiate splicing.
 	/// - `our_funding_inputs`: the inputs we contribute to the new funding transaction.
 	///   Includes the witness weight for this input (e.g. P2WPKH_WITNESS_WEIGHT=109 for typical P2WPKH inputs).
+	/// - `change_script`: an option change output script. If `None` and needed, one will be
+	///   generated by `SignerProvider::get_destination_script`.
 	#[cfg(splicing)]
 	pub fn splice_channel(
 		&mut self, our_funding_contribution_satoshis: i64,
-		our_funding_inputs: Vec<(TxIn, Transaction, Weight)>, funding_feerate_per_kw: u32,
-		locktime: u32,
+		our_funding_inputs: Vec<(TxIn, Transaction, Weight)>, change_script: Option<ScriptBuf>,
+		funding_feerate_per_kw: u32, locktime: u32,
 	) -> Result<msgs::SpliceInit, APIError> {
 		// Check if a splice has been initiated already.
 		// Note: only a single outstanding splice is supported (per spec)
@@ -10648,6 +10652,7 @@ where
 			funding_feerate_sat_per_1000_weight: funding_feerate_per_kw,
 			shared_funding_input: Some(prev_funding_input),
 			our_funding_inputs: funding_inputs,
+			change_script,
 		};
 
 		self.pending_splice = Some(PendingSplice {
@@ -10755,6 +10760,7 @@ where
 			funding_feerate_sat_per_1000_weight: msg.funding_feerate_per_kw,
 			shared_funding_input: Some(prev_funding_input),
 			our_funding_inputs: Vec::new(),
+			change_script: None,
 		};
 
 		let mut interactive_tx_constructor = funding_negotiation_context
@@ -10764,7 +10770,6 @@ where
 				signer_provider,
 				entropy_source,
 				holder_node_id.clone(),
-				None,
 			)
 			.map_err(|err| {
 				ChannelError::Warn(format!(
@@ -10864,7 +10869,6 @@ where
 				signer_provider,
 				entropy_source,
 				holder_node_id.clone(),
-				None,
 			)
 			.map_err(|err| {
 				ChannelError::Warn(format!("V2 channel rejected due to sender error, {:?}", err))
@@ -12443,6 +12447,7 @@ where
 			funding_feerate_sat_per_1000_weight,
 			shared_funding_input: None,
 			our_funding_inputs: funding_inputs,
+			change_script: None,
 		};
 		let chan = Self {
 			funding,
@@ -12598,6 +12603,7 @@ where
 			funding_feerate_sat_per_1000_weight: msg.funding_feerate_sat_per_1000_weight,
 			shared_funding_input: None,
 			our_funding_inputs: our_funding_inputs.clone(),
+			change_script: None,
 		};
 		let shared_funding_output = TxOut {
 			value: Amount::from_sat(funding.get_value_satoshis()),

lightning/src/ln/channelmanager.rs

@@ -32,7 +32,7 @@ use bitcoin::secp256k1::Secp256k1;
 use bitcoin::secp256k1::{PublicKey, SecretKey};
 use bitcoin::{secp256k1, Sequence};
 #[cfg(splicing)]
-use bitcoin::{TxIn, Weight};
+use bitcoin::{ScriptBuf, TxIn, Weight};
 
 use crate::blinded_path::message::MessageForwardNode;
 use crate::blinded_path::message::{AsyncPaymentsContext, OffersContext};
@@ -4437,13 +4437,13 @@ where
 	#[rustfmt::skip]
 	pub fn splice_channel(
 		&self, channel_id: &ChannelId, counterparty_node_id: &PublicKey, our_funding_contribution_satoshis: i64,
-		our_funding_inputs: Vec<(TxIn, Transaction, Weight)>,
+		our_funding_inputs: Vec<(TxIn, Transaction, Weight)>, change_script: Option<ScriptBuf>,
 		funding_feerate_per_kw: u32, locktime: Option<u32>,
 	) -> Result<(), APIError> {
 		let mut res = Ok(());
 		PersistenceNotifierGuard::optionally_notify(self, || {
 			let result = self.internal_splice_channel(
-				channel_id, counterparty_node_id, our_funding_contribution_satoshis, our_funding_inputs, funding_feerate_per_kw, locktime
+				channel_id, counterparty_node_id, our_funding_contribution_satoshis, our_funding_inputs, change_script, funding_feerate_per_kw, locktime
 			);
 			res = result;
 			match res {
@@ -4459,8 +4459,8 @@ where
 	fn internal_splice_channel(
 		&self, channel_id: &ChannelId, counterparty_node_id: &PublicKey,
 		our_funding_contribution_satoshis: i64,
-		our_funding_inputs: Vec<(TxIn, Transaction, Weight)>, funding_feerate_per_kw: u32,
-		locktime: Option<u32>,
+		our_funding_inputs: Vec<(TxIn, Transaction, Weight)>, change_script: Option<ScriptBuf>,
+		funding_feerate_per_kw: u32, locktime: Option<u32>,
 	) -> Result<(), APIError> {
 		let per_peer_state = self.per_peer_state.read().unwrap();
 
@@ -4484,6 +4484,7 @@ where
 					let msg = chan.splice_channel(
 						our_funding_contribution_satoshis,
 						our_funding_inputs,
+						change_script,
 						funding_feerate_per_kw,
 						locktime,
 					)?;

lightning/src/ln/interactivetxs.rs

@@ -3003,6 +3003,7 @@ mod tests {
 			funding_feerate_sat_per_1000_weight,
 			shared_funding_input: None,
 			our_funding_inputs: inputs,
+			change_script: None,
 		};
 		assert_eq!(
 			calculate_change_output_value(&context, false, &ScriptBuf::new(), &outputs, 300),

lightning/src/ln/splicing_tests.rs

@@ -74,6 +74,7 @@ fn test_v1_splice_in() {
 			&acceptor_node.node.get_our_node_id(),
 			splice_in_sats as i64,
 			funding_inputs,
+			None, // change_script
 			funding_feerate_per_kw,
 			None, // locktime
 		)
@@ -322,6 +323,7 @@ fn test_v1_splice_in_negative_insufficient_inputs() {
 		&nodes[1].node.get_our_node_id(),
 		splice_in_sats as i64,
 		funding_inputs,
+		None, // change_script
 		1024, // funding_feerate_per_kw,
 		None, // locktime
 	);