Allow passing funding inputs when manually accepting a dual-funded channel

We introduce a `ChannelManager::accept_inbound_channel_with_contribution`
method allowing contributing to the overall channel capacity of an inbound
dual-funded channel by contributing inputs.

Author: dunxen

lightning/src/ln/channel.rs

@@ -5614,7 +5614,7 @@ pub(super) struct DualFundingChannelContext {
 	///
 	/// Note that this field may be emptied once the interactive negotiation has been started.
 	#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
-	pub our_funding_inputs: Vec<(TxIn, TransactionU16LenLimited)>,
+	pub our_funding_inputs: Vec<(TxIn, TransactionU16LenLimited, Weight)>,
 }
 
 // Holder designates channel data owned for the benefit of the user client.
@@ -10906,7 +10906,7 @@ where
 	pub fn new_outbound<ES: Deref, F: Deref, L: Deref>(
 		fee_estimator: &LowerBoundedFeeEstimator<F>, entropy_source: &ES, signer_provider: &SP,
 		counterparty_node_id: PublicKey, their_features: &InitFeatures, funding_satoshis: u64,
-		funding_inputs: Vec<(TxIn, TransactionU16LenLimited)>, user_id: u128, config: &UserConfig,
+		funding_inputs: Vec<(TxIn, TransactionU16LenLimited, Weight)>, user_id: u128, config: &UserConfig,
 		current_chain_height: u32, outbound_scid_alias: u64, funding_confirmation_target: ConfirmationTarget,
 		logger: L,
 	) -> Result<Self, APIError>
@@ -11048,23 +11048,19 @@ where
 
 	/// Creates a new dual-funded channel from a remote side's request for one.
 	/// Assumes chain_hash has already been checked and corresponds with what we expect!
-	/// TODO(dual_funding): Allow contributions, pass intended amount and inputs
 	#[allow(dead_code)] // TODO(dual_funding): Remove once V2 channels is enabled.
 	#[rustfmt::skip]
 	pub fn new_inbound<ES: Deref, F: Deref, L: Deref>(
 		fee_estimator: &LowerBoundedFeeEstimator<F>, entropy_source: &ES, signer_provider: &SP,
 		holder_node_id: PublicKey, counterparty_node_id: PublicKey, our_supported_features: &ChannelTypeFeatures,
-		their_features: &InitFeatures, msg: &msgs::OpenChannelV2,
-		user_id: u128, config: &UserConfig, current_chain_height: u32, logger: &L,
+		their_features: &InitFeatures, msg: &msgs::OpenChannelV2, user_id: u128, config: &UserConfig,
+		current_chain_height: u32, logger: &L, our_funding_satoshis: u64,
+		our_funding_inputs: Vec<(TxIn, TransactionU16LenLimited, Weight)>,
 	) -> Result<Self, ChannelError>
 		where ES::Target: EntropySource,
 			  F::Target: FeeEstimator,
 			  L::Target: Logger,
 	{
-		// TODO(dual_funding): Take these as input once supported
-		let our_funding_satoshis = 0u64;
-		let our_funding_inputs = Vec::new();
-
 		let channel_value_satoshis = our_funding_satoshis.saturating_add(msg.common_fields.funding_satoshis);
 		let counterparty_selected_channel_reserve_satoshis = get_v2_channel_reserve_satoshis(
 			channel_value_satoshis, msg.common_fields.dust_limit_satoshis);
@@ -11114,7 +11110,7 @@ where
 		context.channel_id = channel_id;
 
 		let dual_funding_context = DualFundingChannelContext {
-			our_funding_satoshis: our_funding_satoshis,
+			our_funding_satoshis,
 			their_funding_satoshis: Some(msg.common_fields.funding_satoshis),
 			funding_tx_locktime: LockTime::from_consensus(msg.locktime),
 			funding_feerate_sat_per_1000_weight: msg.funding_feerate_sat_per_1000_weight,

lightning/src/ln/channelmanager.rs

@@ -30,9 +30,7 @@ use bitcoin::hashes::{Hash, HashEngine, HmacEngine};
 
 use bitcoin::secp256k1::Secp256k1;
 use bitcoin::secp256k1::{PublicKey, SecretKey};
-use bitcoin::{secp256k1, Sequence};
-#[cfg(splicing)]
-use bitcoin::{TxIn, Weight};
+use bitcoin::{secp256k1, Sequence, TxIn, Weight};
 
 use crate::blinded_path::message::MessageForwardNode;
 use crate::blinded_path::message::{AsyncPaymentsContext, OffersContext};
@@ -122,8 +120,8 @@ use crate::util::errors::APIError;
 use crate::util::logger::{Level, Logger, WithContext};
 use crate::util::scid_utils::fake_scid;
 use crate::util::ser::{
-	BigSize, FixedLengthReader, LengthReadable, MaybeReadable, Readable, ReadableArgs, VecWriter,
-	Writeable, Writer,
+	BigSize, FixedLengthReader, LengthReadable, MaybeReadable, Readable, ReadableArgs,
+	TransactionU16LenLimited, VecWriter, Writeable, Writer,
 };
 use crate::util::string::UntrustedString;
 use crate::util::wakers::{Future, Notifier};
@@ -8290,6 +8288,8 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 			false,
 			user_channel_id,
 			config_overrides,
+			0,
+			vec![],
 		)
 	}
 
@@ -8321,14 +8321,82 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 			true,
 			user_channel_id,
 			config_overrides,
+			0,
+			vec![],
 		)
 	}
 
+	/// Accepts a request to open a dual-funded channel with a contribution provided by us after an
+	/// [`Event::OpenChannelRequest`].
+	///
+	/// The [`Event::OpenChannelRequest::channel_negotiation_type`] field will indicate the open channel
+	/// request is for a dual-funded channel when the variant is `InboundChannelFunds::DualFunded`.
+	///
+	/// The `temporary_channel_id` parameter indicates which inbound channel should be accepted,
+	/// and the `counterparty_node_id` parameter is the id of the peer which has requested to open
+	/// the channel.
+	///
+	/// The `user_channel_id` parameter will be provided back in
+	/// [`Event::ChannelClosed::user_channel_id`] to allow tracking of which events correspond
+	/// with which `accept_inbound_channel_*` call.
+	///
+	/// The `funding_inputs` parameter provides the `txin`s along with their previous transactions, and
+	/// a corresponding witness weight for each input that will be used to contribute towards our
+	/// portion of the channel value. Our contribution will be calculated as the total value of these
+	/// inputs minus the fees we need to cover for the interactive funding transaction. The witness
+	/// weights must correspond to the witnesses you will provide through [`ChannelManager::funding_transaction_signed`]
+	/// after receiving [`Event::FundingTransactionReadyForSigning`].
+	///
+	/// Note that this method will return an error and reject the channel if it requires support for
+	/// zero confirmations.
+	// TODO(dual_funding): Discussion on complications with 0conf dual-funded channels where "locking"
+	// of UTXOs used for funding would be required and other issues.
+	// See https://diyhpl.us/~bryan/irc/bitcoin/bitcoin-dev/linuxfoundation-pipermail/lightning-dev/2023-May/003922.txt
+	///
+	/// [`Event::OpenChannelRequest`]: events::Event::OpenChannelRequest
+	/// [`Event::OpenChannelRequest::channel_negotiation_type`]: events::Event::OpenChannelRequest::channel_negotiation_type
+	/// [`Event::ChannelClosed::user_channel_id`]: events::Event::ChannelClosed::user_channel_id
+	/// [`Event::FundingTransactionReadyForSigning`]: events::Event::FundingTransactionReadyForSigning
+	/// [`ChannelManager::funding_transaction_signed`]: ChannelManager::funding_transaction_signed
+	pub fn accept_inbound_channel_with_contribution(
+		&self, temporary_channel_id: &ChannelId, counterparty_node_id: &PublicKey,
+		user_channel_id: u128, config_overrides: Option<ChannelConfigOverrides>,
+		our_funding_satoshis: u64, funding_inputs: Vec<(TxIn, Transaction, Weight)>,
+	) -> Result<(), APIError> {
+		let funding_inputs = Self::length_limit_holder_input_prev_txs(funding_inputs)?;
+		self.do_accept_inbound_channel(
+			temporary_channel_id,
+			counterparty_node_id,
+			false,
+			user_channel_id,
+			config_overrides,
+			our_funding_satoshis,
+			funding_inputs,
+		)
+	}
+
+	fn length_limit_holder_input_prev_txs(
+		funding_inputs: Vec<(TxIn, Transaction, Weight)>,
+	) -> Result<Vec<(TxIn, TransactionU16LenLimited, Weight)>, APIError> {
+		funding_inputs
+			.into_iter()
+			.map(|(txin, tx, witness_weight)| match TransactionU16LenLimited::new(tx) {
+				Ok(tx) => Ok((txin, tx, witness_weight)),
+				Err(err) => Err(err),
+			})
+			.collect::<Result<Vec<(TxIn, TransactionU16LenLimited, Weight)>, ()>>()
+			.map_err(|_| APIError::APIMisuseError {
+				err: "One or more transactions had a serialized length exceeding 65535 bytes"
+					.into(),
+			})
+	}
+
 	/// TODO(dual_funding): Allow contributions, pass intended amount and inputs
 	#[rustfmt::skip]
 	fn do_accept_inbound_channel(
 		&self, temporary_channel_id: &ChannelId, counterparty_node_id: &PublicKey, accept_0conf: bool,
-		user_channel_id: u128, config_overrides: Option<ChannelConfigOverrides>
+		user_channel_id: u128, config_overrides: Option<ChannelConfigOverrides>, our_funding_satoshis: u64,
+		funding_inputs: Vec<(TxIn, TransactionU16LenLimited, Weight)>
 	) -> Result<(), APIError> {
 
 		let mut config = self.default_configuration.clone();
@@ -8387,7 +8455,7 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 							&self.channel_type_features(), &peer_state.latest_features,
 							&open_channel_msg,
 							user_channel_id, &config, best_block_height,
-							&self.logger,
+							&self.logger, our_funding_satoshis, funding_inputs,
 						).map_err(|_| MsgHandleErrInternal::from_chan_no_close(
 							ChannelError::Close(
 								(
@@ -8674,7 +8742,7 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 					&self.fee_estimator, &self.entropy_source, &self.signer_provider,
 					self.get_our_node_id(), *counterparty_node_id, &self.channel_type_features(),
 					&peer_state.latest_features, msg, user_channel_id,
-					&self.default_configuration, best_block_height, &self.logger,
+					&self.default_configuration, best_block_height, &self.logger, 0, vec![],
 				).map_err(|e| MsgHandleErrInternal::from_chan_no_close(e, msg.common_fields.temporary_channel_id))?;
 				let message_send_event = MessageSendEvent::SendAcceptChannelV2 {
 					node_id: *counterparty_node_id,

lightning/src/ln/dual_funding_tests.rs

@@ -51,7 +51,7 @@ fn do_test_v2_channel_establishment(session: V2ChannelEstablishmentTestSession)
 		&[session.initiator_input_value_satoshis],
 	)
 	.into_iter()
-	.map(|(txin, tx, _)| (txin, TransactionU16LenLimited::new(tx).unwrap()))
+	.map(|(txin, tx, weight)| (txin, TransactionU16LenLimited::new(tx).unwrap(), weight))
 	.collect();
 
 	// Alice creates a dual-funded channel as initiator.