Introduce FundingTransactionReadyForSignatures event

The `FundingTransactionReadyForSignatures` event requests witnesses
from the client for their contributed inputs to an interactively
constructed transaction.

The client calls `ChannelManager::funding_transaction_signed` to provide
the witnesses to LDK.

Author: dunxen

lightning/src/events/mod.rs

@@ -1541,7 +1541,56 @@ pub enum Event {
 		/// The node id of the peer we just connected to, who advertises support for
 		/// onion messages.
 		peer_node_id: PublicKey,
-	}
+	},
+	/// Indicates that a funding transaction constructed via interactive transaction construction for a
+	/// channel is ready to be signed by the client. This event will only be triggered
+	/// if at least one input was contributed by the holder and needs to be signed.
+	///
+	/// The transaction contains all inputs provided by both parties  along with the channel's funding
+	/// output and a change output if applicable.
+	///
+	/// No part of the transaction should be changed before signing as the content of the transaction
+	/// has already been negotiated with the counterparty.
+	///
+	/// Each signature MUST use the SIGHASH_ALL flag to avoid invalidation of the initial commitment and
+	/// hence possible loss of funds.
+	///
+	/// After signing, call [`ChannelManager::funding_transaction_signed`] with the (partially) signed
+	/// funding transaction.
+	///
+	/// Generated in [`ChannelManager`] message handling.
+	///
+	/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+	/// [`ChannelManager::funding_transaction_signed`]: crate::ln::channelmanager::ChannelManager::funding_transaction_signed
+	FundingTransactionReadyForSigning {
+		/// The channel_id of the channel which you'll need to pass back into
+		/// [`ChannelManager::funding_transaction_signed`].
+		///
+		/// [`ChannelManager::funding_transaction_signed`]: crate::ln::channelmanager::ChannelManager::funding_transaction_signed
+		channel_id: ChannelId,
+		/// The counterparty's node_id, which you'll need to pass back into
+		/// [`ChannelManager::funding_transaction_signed`].
+		///
+		/// [`ChannelManager::funding_transaction_signed`]: crate::ln::channelmanager::ChannelManager::funding_transaction_signed
+		counterparty_node_id: PublicKey,
+		// TODO(dual_funding): Enable links when methods are implemented
+		/// The `user_channel_id` value passed in to `ChannelManager::create_dual_funded_channel` for outbound
+		/// channels, or to [`ChannelManager::accept_inbound_channel`] or `ChannelManager::accept_inbound_channel_with_contribution`
+		/// for inbound channels if [`UserConfig::manually_accept_inbound_channels`] config flag is set to true.
+		/// Otherwise `user_channel_id` will be randomized for an inbound channel.
+		/// This may be zero for objects serialized with LDK versions prior to 0.0.113.
+		///
+		/// [`ChannelManager::accept_inbound_channel`]: crate::ln::channelmanager::ChannelManager::accept_inbound_channel
+		/// [`UserConfig::manually_accept_inbound_channels`]: crate::util::config::UserConfig::manually_accept_inbound_channels
+		// [`ChannelManager::create_dual_funded_channel`]: crate::ln::channelmanager::ChannelManager::create_dual_funded_channel
+		// [`ChannelManager::accept_inbound_channel_with_contribution`]: crate::ln::channelmanager::ChannelManager::accept_inbound_channel_with_contribution
+		user_channel_id: u128,
+		/// The unsigned transaction to be signed and passed back to
+		/// [`ChannelManager::funding_transaction_signed`].
+		///
+		/// [`ChannelManager::funding_transaction_signed`]: crate::ln::channelmanager::ChannelManager::funding_transaction_signed
+		unsigned_transaction: Transaction,
+	},
 }
 
 impl Writeable for Event {
@@ -1887,6 +1936,13 @@ impl Writeable for Event {
 					(8, former_temporary_channel_id, required),
 				});
 			},
+			&Event::FundingTransactionReadyForSigning { .. } => {
+				45u8.write(writer)?;
+				// We never write out FundingTransactionReadyForSigning events as, upon disconnection, peers
+				// drop any V2-established/spliced channels which have not yet exchanged the initial `commitment_signed`.
+				// We only exhange the initial `commitment_signed` after the client calls
+				// `ChannelManager::funding_transaction_signed` and ALWAYS before we send a `tx_signatures`
+			},
 			// Note that, going forward, all new events must only write data inside of
 			// `write_tlv_fields`. Versions 0.0.101+ will ignore odd-numbered events that write
 			// data via `write_tlv_fields`.

lightning/src/ln/channel.rs

@@ -14,7 +14,7 @@ use bitcoin::transaction::{Transaction, TxIn, TxOut};
 use bitcoin::sighash::EcdsaSighashType;
 use bitcoin::consensus::encode;
 use bitcoin::absolute::LockTime;
-use bitcoin::Weight;
+use bitcoin::{Weight, Witness};
 
 use bitcoin::hashes::Hash;
 use bitcoin::hashes::sha256::Hash as Sha256;
@@ -2630,7 +2630,7 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 			},
 		};
 
-		let funding_ready_for_sig_event = if signing_session.local_inputs_count() == 0 {
+		let funding_ready_for_sig_event_opt = if signing_session.local_inputs_count() == 0 {
 			debug_assert_eq!(our_funding_satoshis, 0);
 			if signing_session.provide_holder_witnesses(self.context.channel_id, Vec::new()).is_err() {
 				debug_assert!(
@@ -2644,28 +2644,12 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 			}
 			None
 		} else {
-			// TODO(dual_funding): Send event for signing if we've contributed funds.
-			// Inform the user that SIGHASH_ALL must be used for all signatures when contributing
-			// inputs/signatures.
-			// Also warn the user that we don't do anything to prevent the counterparty from
-			// providing non-standard witnesses which will prevent the funding transaction from
-			// confirming. This warning must appear in doc comments wherever the user is contributing
-			// funds, whether they are initiator or acceptor.
-			//
-			// The following warning can be used when the APIs allowing contributing inputs become available:
-			// <div class="warning">
-			// WARNING: LDK makes no attempt to prevent the counterparty from using non-standard inputs which
-			// will prevent the funding transaction from being relayed on the bitcoin network and hence being
-			// confirmed.
-			// </div>
-			debug_assert!(
-				false,
-				"We don't support users providing inputs but somehow we had more than zero inputs",
-			);
-			return Err(ChannelError::Close((
-				"V2 channel rejected due to sender error".into(),
-				ClosureReason::HolderForceClosed { broadcasted_latest_txn: Some(false) }
-			)));
+			Some(Event::FundingTransactionReadyForSigning {
+				channel_id: self.context.channel_id,
+				counterparty_node_id: self.context.counterparty_node_id,
+				user_channel_id: self.context.user_id,
+				unsigned_transaction: signing_session.unsigned_tx().build_unsigned_tx(),
+			})
 		};
 
 		let mut channel_state = ChannelState::FundingNegotiated(FundingNegotiatedFlags::new());
@@ -2676,7 +2660,7 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 		self.interactive_tx_constructor.take();
 		self.interactive_tx_signing_session = Some(signing_session);
 
-		Ok((commitment_signed, funding_ready_for_sig_event))
+		Ok((commitment_signed, funding_ready_for_sig_event_opt))
 	}
 }
 
@@ -6595,6 +6579,36 @@ impl<SP: Deref> FundedChannel<SP> where
 		}
 	}
 
+	fn verify_interactive_tx_signatures(&mut self, _witnesses: &Vec<Witness>) {
+		if let Some(ref mut _signing_session) = self.interactive_tx_signing_session {
+			// Check that sighash_all was used:
+			// TODO(dual_funding): Check sig for sighash
+		}
+	}
+
+	pub fn funding_transaction_signed<L: Deref>(&mut self, witnesses: Vec<Witness>, logger: &L) -> Result<Option<msgs::TxSignatures>, APIError>
+		where L::Target: Logger
+	{
+		self.verify_interactive_tx_signatures(&witnesses);
+		if let Some(ref mut signing_session) = self.interactive_tx_signing_session {
+			let logger = WithChannelContext::from(logger, &self.context, None);
+			if let Some(holder_tx_signatures) = signing_session
+				.provide_holder_witnesses(self.context.channel_id, witnesses)
+				.map_err(|err| APIError::APIMisuseError { err })?
+			{
+				if self.is_awaiting_initial_mon_persist() {
+					log_debug!(logger, "Not sending tx_signatures: a monitor update is in progress. Setting monitor_pending_tx_signatures.");
+					self.context.monitor_pending_tx_signatures = Some(holder_tx_signatures);
+					return Ok(None);
+				}
+				return Ok(Some(holder_tx_signatures));
+			} else { return Ok(None) }
+		} else {
+			return Err(APIError::APIMisuseError {
+				err: format!("Channel with id {} not expecting funding signatures", self.context.channel_id)});
+		}
+	}
+
 	pub fn tx_signatures<L: Deref>(&mut self, msg: &msgs::TxSignatures, logger: &L) -> Result<(Option<Transaction>, Option<msgs::TxSignatures>), ChannelError>
 		where L::Target: Logger
 	{

lightning/src/ln/channelmanager.rs

@@ -5514,6 +5514,59 @@ where
 		result
 	}
 
+	/// Handles a signed funding transaction generated by interactive transaction construction and
+	/// provided by the client.
+	///
+	/// Do NOT broadcast the funding transaction yourself. When we have safely received our
+	/// counterparty's signature(s) the funding transaction will automatically be broadcast via the
+	/// [`BroadcasterInterface`] provided when this `ChannelManager` was constructed.
+	///
+	/// SIGHASH_ALL MUST be used for all signatures when providing signatures.
+	///
+	/// <div class="warning">
+	/// WARNING: LDK makes no attempt to prevent the counterparty from using non-standard inputs which
+	///  will prevent the funding transaction from being relayed on the bitcoin network and hence being
+	///  confirmed.
+	/// </div>
+	pub fn funding_transaction_signed(
+		&self, channel_id: &ChannelId, counterparty_node_id: &PublicKey, transaction: Transaction
+	) -> Result<(), APIError> {
+		let witnesses: Vec<_> = transaction.input.into_iter().filter_map(|input| {
+			if input.witness.is_empty() { None } else { Some(input.witness) }
+		}).collect();
+
+		let per_peer_state = self.per_peer_state.read().unwrap();
+		let peer_state_mutex = per_peer_state.get(counterparty_node_id)
+			.ok_or_else(|| APIError::ChannelUnavailable {
+				err: format!("Can't find a peer matching the passed counterparty node_id {}",
+					counterparty_node_id) })?;
+
+		let mut peer_state_lock = peer_state_mutex.lock().unwrap();
+		let peer_state = &mut *peer_state_lock;
+
+		match peer_state.channel_by_id.get_mut(channel_id) {
+			Some(channel) => {
+				match channel.as_funded_mut() {
+					Some(chan) => {
+						if let Some(tx_signatures) = chan.funding_transaction_signed(witnesses, &self.logger)? {
+							peer_state.pending_msg_events.push(MessageSendEvent::SendTxSignatures {
+								node_id: *counterparty_node_id,
+								msg: tx_signatures,
+							});
+						}
+					},
+					None => return Err(APIError::APIMisuseError {
+						err: format!("Channel with id {} not expecting funding signatures", channel_id)}),
+				}
+			},
+			None => return Err(APIError::ChannelUnavailable{
+				err: format!("Channel with id {} not found for the passed counterparty node_id {}", channel_id,
+					counterparty_node_id) }),
+		}
+
+		Ok(())
+	}
+
 	/// Atomically applies partial updates to the [`ChannelConfig`] of the given channels.
 	///
 	/// Once the updates are applied, each eligible channel (advertised with a known short channel

lightning/src/ln/interactivetxs.rs

@@ -396,9 +396,13 @@ impl InteractiveTxSigningSession {
 	/// unsigned transaction.
 	pub fn provide_holder_witnesses(
 		&mut self, channel_id: ChannelId, witnesses: Vec<Witness>,
-	) -> Result<(), ()> {
-		if self.local_inputs_count() != witnesses.len() {
-			return Err(());
+	) -> Result<Option<TxSignatures>, String> {
+		let local_inputs_count = self.local_inputs_count();
+		if local_inputs_count != witnesses.len() {
+			return Err(format!(
+				"Provided witness count of {} does not match required count for {} inputs",
+				witnesses.len(), local_inputs_count
+			));
 		}
 
 		self.unsigned_tx.add_local_witnesses(witnesses.clone());
@@ -409,7 +413,11 @@ impl InteractiveTxSigningSession {
 			shared_input_signature: None,
 		});
 
-		Ok(())
+		if self.holder_sends_tx_signatures_first && self.has_received_commitment_signed {
+			Ok(self.holder_tx_signatures.clone())
+		} else {
+			Ok(None)
+		}
 	}
 
 	pub fn remote_inputs_count(&self) -> usize {