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

@@ -13946,7 +13946,7 @@ where
 	pub funding: FundingScope,
 	pub context: ChannelContext<SP>,
 	pub unfunded_context: UnfundedChannelContext,
-	pub funding_negotiation_context: FundingNegotiationContext,
+	pub funding_negotiation_context: Option<FundingNegotiationContext>,
 	/// The current interactive transaction construction session under negotiation.
 	pub interactive_tx_constructor: Option<InteractiveTxConstructor>,
 }
@@ -14021,7 +14021,7 @@ where
 			funding,
 			context,
 			unfunded_context,
-			funding_negotiation_context,
+			funding_negotiation_context: Some(funding_negotiation_context),
 			interactive_tx_constructor: None,
 		};
 		Ok(chan)
@@ -14096,29 +14096,28 @@ where
 			},
 			funding_feerate_sat_per_1000_weight: self.context.feerate_per_kw,
 			second_per_commitment_point,
-			locktime: self.funding_negotiation_context.funding_tx_locktime.to_consensus_u32(),
+			locktime: self.funding_tx_locktime().to_consensus_u32(),
 			require_confirmed_inputs: None,
 		}
 	}
 
 	/// 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_contribution: Amount,
+		our_funding_inputs: Vec<FundingTxInput>,
 	) -> 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_contribution, our_funding_contribution_sats) = (SignedAmount::ZERO, 0u64);
-		let our_funding_inputs = Vec::new();
+		debug_assert!(our_funding_contribution <= Amount::MAX_MONEY);
+		let our_funding_contribution_sats = our_funding_contribution.to_sat();
 
 		let channel_value_satoshis =
 			our_funding_contribution_sats.saturating_add(msg.common_fields.funding_satoshis);
@@ -14163,37 +14162,30 @@ where
 
 		let funding_negotiation_context = FundingNegotiationContext {
 			is_initiator: false,
-			our_funding_contribution,
+			our_funding_contribution: our_funding_contribution
+				.to_signed()
+				.expect("our_funding_contribution should not be greater than Amount::MAX_MONEY"),
 			funding_tx_locktime: LockTime::from_consensus(msg.locktime),
 			funding_feerate_sat_per_1000_weight: msg.funding_feerate_sat_per_1000_weight,
 			shared_funding_input: None,
 			our_funding_inputs: our_funding_inputs.clone(),
 			our_funding_outputs: Vec::new(),
 			change_script: None,
 		};
-		let shared_funding_output = TxOut {
-			value: Amount::from_sat(funding.get_value_satoshis()),
-			script_pubkey: funding.get_funding_redeemscript().to_p2wsh(),
-		};
 
-		let interactive_tx_constructor = Some(InteractiveTxConstructor::new(
-			InteractiveTxConstructorArgs {
+		let mut interactive_tx_constructor = funding_negotiation_context
+			.into_interactive_tx_constructor(
+				&context,
+				&funding,
+				signer_provider,
 				entropy_source,
 				holder_node_id,
-				counterparty_node_id,
-				channel_id: context.channel_id,
-				feerate_sat_per_kw: funding_negotiation_context.funding_feerate_sat_per_1000_weight,
-				funding_tx_locktime: funding_negotiation_context.funding_tx_locktime,
-				is_initiator: false,
-				inputs_to_contribute: our_funding_inputs,
-				shared_funding_input: None,
-				shared_funding_output: SharedOwnedOutput::new(shared_funding_output, our_funding_contribution_sats),
-				outputs_to_contribute: funding_negotiation_context.our_funding_outputs.clone(),
-			}
-		).map_err(|err| {
-			let reason = ClosureReason::ProcessingError { err: err.reason.to_string() };
-			ChannelError::Close((err.reason.to_string(), reason))
-		})?);
+			)
+			.map_err(|err| {
+				let reason = ClosureReason::ProcessingError { err: err.reason.to_string() };
+				ChannelError::Close((err.reason.to_string(), reason))
+			})?;
+		debug_assert!(interactive_tx_constructor.take_initiator_first_message().is_none());
 
 		let unfunded_context = UnfundedChannelContext {
 			unfunded_channel_age_ticks: 0,
@@ -14202,8 +14194,8 @@ where
 		Ok(Self {
 			funding,
 			context,
-			funding_negotiation_context,
-			interactive_tx_constructor,
+			funding_negotiation_context: None,
+			interactive_tx_constructor: Some(interactive_tx_constructor),
 			unfunded_context,
 		})
 	}
@@ -14267,8 +14259,7 @@ where
 				}),
 				channel_type: Some(self.funding.get_channel_type().clone()),
 			},
-			funding_satoshis: self.funding_negotiation_context.our_funding_contribution.to_sat()
-				as u64,
+			funding_satoshis: self.our_funding_contribution().to_sat(),
 			second_per_commitment_point,
 			require_confirmed_inputs: None,
 		}
@@ -14283,6 +14274,24 @@ where
 	pub fn get_accept_channel_v2_message(&self) -> msgs::AcceptChannelV2 {
 		self.generate_accept_channel_v2_message()
 	}
+
+	pub fn our_funding_contribution(&self) -> Amount {
+		Amount::from_sat(self.funding.value_to_self_msat / 1000)
+	}
+
+	pub fn funding_tx_locktime(&self) -> LockTime {
+		self.funding_negotiation_context
+			.as_ref()
+			.map(|context| context.funding_tx_locktime)
+			.or_else(|| {
+				self.interactive_tx_constructor
+					.as_ref()
+					.map(|constructor| constructor.funding_tx_locktime())
+			})
+			.expect(
+				"either funding_negotiation_context or interactive_tx_constructor should be set",
+			)
+	}
 }
 
 // Unfunded channel utilities

lightning/src/ln/channelmanager.rs

@@ -30,7 +30,7 @@ use bitcoin::hashes::{Hash, HashEngine, HmacEngine};
 
 use bitcoin::secp256k1::Secp256k1;
 use bitcoin::secp256k1::{PublicKey, SecretKey};
-use bitcoin::{secp256k1, Sequence, SignedAmount};
+use bitcoin::{secp256k1, Amount, Sequence};
 
 use crate::blinded_path::message::{
 	AsyncPaymentsContext, BlindedMessagePath, MessageForwardNode, OffersContext,
@@ -65,7 +65,7 @@ use crate::ln::channel::{
 	WithChannelContext,
 };
 use crate::ln::channel_state::ChannelDetails;
-use crate::ln::funding::SpliceContribution;
+use crate::ln::funding::{FundingTxInput, SpliceContribution};
 use crate::ln::inbound_payment;
 use crate::ln::interactivetxs::InteractiveTxMessageSend;
 use crate::ln::msgs;
@@ -9814,6 +9814,8 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 			false,
 			user_channel_id,
 			config_overrides,
+			Amount::ZERO,
+			vec![],
 		)
 	}
 
@@ -9845,15 +9847,69 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 			true,
 			user_channel_id,
 			config_overrides,
+			Amount::ZERO,
+			vec![],
+		)
+	}
+
+	/// Accepts a request to open a dual-funded channel with a contribution provided 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 that 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 which UTXOs to use for `our_funding_contribution`
+	/// along with the corresponding satisfaction weight. They must be able to cover any fees needed
+	/// to pay for the contributed weight to the funding transaction. This includes the witnesses
+	/// provided through calling [`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_contribution: Amount, funding_inputs: Vec<FundingTxInput>,
+	) -> Result<(), APIError> {
+		self.do_accept_inbound_channel(
+			temporary_channel_id,
+			counterparty_node_id,
+			false,
+			user_channel_id,
+			config_overrides,
+			our_funding_contribution,
+			funding_inputs,
 		)
 	}
 
-	/// 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>
+		&self, temporary_channel_id: &ChannelId, counterparty_node_id: &PublicKey,
+		accept_0conf: bool, user_channel_id: u128, config_overrides: Option<ChannelConfigOverrides>,
+		our_funding_contribution: Amount, funding_inputs: Vec<FundingTxInput>
 	) -> Result<(), APIError> {
+		if our_funding_contribution > Amount::MAX_MONEY {
+			return Err(APIError::APIMisuseError {
+				err: format!("the funding contribution must be smaller than the total bitcoin supply, it was {}", our_funding_contribution)
+			});
+		}
 
 		let mut config = self.config.read().unwrap().clone();
 
@@ -9911,7 +9967,7 @@ 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_contribution, funding_inputs,
 						).map_err(|e| {
 							let channel_id = open_channel_msg.common_fields.temporary_channel_id;
 							MsgHandleErrInternal::from_chan_no_close(e, channel_id)
@@ -10055,7 +10111,7 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 
 					// Inbound V2 channels with contributed inputs are not considered unfunded.
 					if let Some(unfunded_chan) = chan.as_unfunded_v2() {
-						if unfunded_chan.funding_negotiation_context.our_funding_contribution > SignedAmount::ZERO {
+						if unfunded_chan.our_funding_contribution() > Amount::ZERO {
 							continue;
 						}
 					}
@@ -10197,7 +10253,7 @@ 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.config.read().unwrap(), best_block_height, &self.logger,
+					&self.config.read().unwrap(), best_block_height, &self.logger, Amount::ZERO, 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/interactivetxs.rs

@@ -1947,6 +1947,7 @@ pub(super) struct InteractiveTxConstructor {
 	is_initiator: bool,
 	initiator_first_message: Option<InteractiveTxMessageSend>,
 	channel_id: ChannelId,
+	funding_tx_locktime: AbsoluteLockTime,
 	inputs_to_contribute: Vec<(SerialId, InputOwned)>,
 	outputs_to_contribute: Vec<(SerialId, OutputOwned)>,
 	next_input_index: Option<usize>,
@@ -2112,6 +2113,7 @@ impl InteractiveTxConstructor {
 			is_initiator,
 			initiator_first_message: None,
 			channel_id,
+			funding_tx_locktime,
 			inputs_to_contribute,
 			outputs_to_contribute,
 			next_input_index,
@@ -2131,6 +2133,10 @@ impl InteractiveTxConstructor {
 		Ok(constructor)
 	}
 
+	pub(super) fn funding_tx_locktime(&self) -> AbsoluteLockTime {
+		self.funding_tx_locktime
+	}
+
 	fn into_negotiation_error(self, reason: AbortReason) -> NegotiationError {
 		let (contributed_inputs, contributed_outputs) = self.into_contributed_inputs_and_outputs();
 		NegotiationError { reason, contributed_inputs, contributed_outputs }
@@ -2320,6 +2326,11 @@ impl InteractiveTxConstructor {
 /// given inputs and outputs, and intended contribution. Takes into account the fees and the dust
 /// limit.
 ///
+/// Note that since the type of change output cannot be determined at this point, this calculation
+/// does not account for the weight contributed by the change output itself. The fees for the
+/// weight of this change output should be subtracted from the result of this function call to get
+/// the final amount for the change output (if above dust).
+///
 /// Three outcomes are possible:
 /// - Inputs are sufficient for intended contribution, fees, and a larger-than-dust change:
 ///   `Ok(Some(change_amount))`