Support funding_transaction_signed for unfunded dual-funded channels

Now that we require users to first call
`ChannelManager::funding_transaction_signed` before releasing any
signatures, it's possible that it is called before we receive the
initial commitment signed from our counterparty, which would transition
the channel to funded. Because of this, we need to support the API call
while the channel is still in the unfunded phase.

Note that this commit is mostly a code move of
`FundedChannel::funding_transaction_signed` to
`Channel::funding_transaction_signed` that doesn't alter the signing
logic.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -1953,11 +1953,18 @@ where
 
 		let funding_tx_signed = if !has_local_contribution {
 			let funding_txid = signing_session.unsigned_tx().tx().compute_txid();
-			if let ChannelPhase::Funded(chan) = &mut self.phase {
-				chan.funding_transaction_signed(funding_txid, vec![], 0, logger).ok()
-			} else {
-				None
-			}
+			Some(self.funding_transaction_signed(funding_txid, vec![], 0, logger).map_err(
+				|err| {
+					log_error!(
+						logger,
+						"Failed signing funding transaction without local contribution: {err:?}"
+					);
+					self.fail_interactive_tx_negotiation(
+						AbortReason::InternalError("Signing failed"),
+						logger,
+					)
+				},
+			)?)
 		} else {
 			None
 		};
@@ -2143,6 +2150,146 @@ where
 		Ok(())
 	}
 
+	pub fn funding_transaction_signed<L: Deref>(
+		&mut self, funding_txid_signed: Txid, witnesses: Vec<Witness>, best_block_height: u32,
+		logger: &L,
+	) -> Result<FundingTxSigned, APIError>
+	where
+		L::Target: Logger,
+	{
+		let (context, funding, pending_splice) = match &mut self.phase {
+			ChannelPhase::Undefined => unreachable!(),
+			ChannelPhase::UnfundedV2(channel) => (&mut channel.context, &channel.funding, None),
+			ChannelPhase::Funded(channel) => {
+				(&mut channel.context, &channel.funding, channel.pending_splice.as_ref())
+			},
+			_ => {
+				return Err(APIError::APIMisuseError {
+					err: format!(
+						"Channel with id {} not expecting funding signatures",
+						self.context().channel_id
+					),
+				});
+			},
+		};
+
+		let signing_session = if let Some(signing_session) =
+			context.interactive_tx_signing_session.as_mut()
+		{
+			if let Some(pending_splice) = pending_splice.as_ref() {
+				debug_assert!(pending_splice
+					.funding_negotiation
+					.as_ref()
+					.map(|funding_negotiation| matches!(
+						funding_negotiation,
+						FundingNegotiation::AwaitingSignatures { .. }
+					))
+					.unwrap_or(false));
+			}
+
+			if signing_session.holder_tx_signatures().is_some() {
+				// Our `tx_signatures` either should've been the first time we processed them,
+				// or we're waiting for our counterparty to send theirs first.
+				return Ok(FundingTxSigned {
+					commitment_signed: None,
+
+					tx_signatures: None,
+					funding_tx: None,
+					splice_negotiated: None,
+					splice_locked: None,
+				});
+			}
+
+			signing_session
+		} else {
+			if Some(funding_txid_signed) == funding.get_funding_txid() {
+				// We may be handling a duplicate call and the funding was already locked so we
+				// no longer have the signing session present.
+				return Ok(FundingTxSigned {
+					commitment_signed: None,
+					tx_signatures: None,
+					funding_tx: None,
+					splice_negotiated: None,
+					splice_locked: None,
+				});
+			}
+			let err = format!("Channel {} not expecting funding signatures", context.channel_id);
+			return Err(APIError::APIMisuseError { err });
+		};
+
+		let tx = signing_session.unsigned_tx().tx();
+		if funding_txid_signed != tx.compute_txid() {
+			return Err(APIError::APIMisuseError {
+				err: "Transaction was malleated prior to signing".to_owned(),
+			});
+		}
+
+		let shared_input_signature =
+			if let Some(splice_input_index) = signing_session.unsigned_tx().shared_input_index() {
+				let sig = match &context.holder_signer {
+					ChannelSignerType::Ecdsa(signer) => signer.sign_splice_shared_input(
+						&funding.channel_transaction_parameters,
+						tx,
+						splice_input_index as usize,
+						&context.secp_ctx,
+					),
+					#[cfg(taproot)]
+					ChannelSignerType::Taproot(_) => todo!(),
+				};
+				Some(sig)
+			} else {
+				None
+			};
+		debug_assert_eq!(pending_splice.is_some(), shared_input_signature.is_some());
+
+		let tx_signatures = msgs::TxSignatures {
+			channel_id: context.channel_id,
+			tx_hash: funding_txid_signed,
+			witnesses,
+			shared_input_signature,
+		};
+		let (tx_signatures, funding_tx) = signing_session
+			.provide_holder_witnesses(tx_signatures, &context.secp_ctx)
+			.map_err(|err| APIError::APIMisuseError { err })?;
+
+		let logger = WithChannelContext::from(logger, &context, None);
+		if tx_signatures.is_some() {
+			log_info!(
+				logger,
+				"Sending tx_signatures for interactive funding transaction {funding_txid_signed}"
+			);
+		}
+
+		let funding = pending_splice
+			.as_ref()
+			.and_then(|pending_splice| pending_splice.funding_negotiation.as_ref())
+			.and_then(|funding_negotiation| funding_negotiation.as_funding())
+			.unwrap_or(funding);
+		let commitment_signed = context.get_initial_commitment_signed_v2(funding, &&logger);
+
+		// In the common case for zero conf channels, we don't expect the funding transaction to be
+		// ready for broadcast yet as our counterparty shouldn't have sent their `tx_signatures`
+		// without us having sent our initial commitment signed to them first. However, in the event
+		// they do, we choose to handle it anyway.
+		let (splice_negotiated, splice_locked) = if let Some(funding_tx) = funding_tx.clone() {
+			debug_assert!(tx_signatures.is_some());
+			let funded_channel = self.as_funded_mut().expect(
+				"Funding transactions ready for broadcast can only exist for funded channels",
+			);
+			funded_channel.on_tx_signatures_exchange(funding_tx, best_block_height, &logger)
+		} else {
+			(None, None)
+		};
+
+		Ok(FundingTxSigned {
+			commitment_signed,
+			tx_signatures,
+			funding_tx,
+			splice_negotiated,
+			splice_locked,
+		})
+	}
+
 	pub fn force_shutdown(&mut self, closure_reason: ClosureReason) -> ShutdownResult {
 		let (funding, context) = self.funding_and_context_mut();
 		context.force_shutdown(funding, closure_reason)
@@ -2212,7 +2359,7 @@ where
 						.unwrap_or(false);
 
 					// We delay processing this until the user manually approves the splice via
-					// [`FundedChannel::funding_transaction_signed`], as otherwise, there would be a
+					// [`Channel::funding_transaction_signed`], as otherwise, there would be a
 					// [`ChannelMonitorUpdateStep::RenegotiatedFunding`] committed that we would
 					// need to undo if they no longer wish to proceed.
 					if has_holder_tx_signatures {
@@ -2716,7 +2863,7 @@ enum FundingNegotiation {
 		is_initiator: bool,
 		/// The initial [`msgs::CommitmentSigned`] message received for the [`FundingScope`] above.
 		/// We delay processing this until the user manually approves the splice via
-		/// [`FundedChannel::funding_transaction_signed`], as otherwise, there would be a
+		/// [`Channel::funding_transaction_signed`], as otherwise, there would be a
 		/// [`ChannelMonitorUpdateStep::RenegotiatedFunding`] committed that we would need to undo
 		/// if they no longer wish to proceed.
 		///
@@ -9162,124 +9309,6 @@ where
 		}
 	}
 
-	pub fn funding_transaction_signed<L: Deref>(
-		&mut self, funding_txid_signed: Txid, witnesses: Vec<Witness>, best_block_height: u32,
-		logger: &L,
-	) -> Result<FundingTxSigned, APIError>
-	where
-		L::Target: Logger,
-	{
-		let signing_session =
-			if let Some(signing_session) = self.context.interactive_tx_signing_session.as_mut() {
-				if let Some(pending_splice) = self.pending_splice.as_ref() {
-					debug_assert!(pending_splice
-						.funding_negotiation
-						.as_ref()
-						.map(|funding_negotiation| matches!(
-							funding_negotiation,
-							FundingNegotiation::AwaitingSignatures { .. }
-						))
-						.unwrap_or(false));
-				}
-
-				if signing_session.holder_tx_signatures().is_some() {
-					// Our `tx_signatures` either should've been the first time we processed them,
-					// or we're waiting for our counterparty to send theirs first.
-					return Ok(FundingTxSigned {
-						commitment_signed: None,
-
-						tx_signatures: None,
-						funding_tx: None,
-						splice_negotiated: None,
-						splice_locked: None,
-					});
-				}
-
-				signing_session
-			} else {
-				if Some(funding_txid_signed) == self.funding.get_funding_txid() {
-					// We may be handling a duplicate call and the funding was already locked so we
-					// no longer have the signing session present.
-					return Ok(FundingTxSigned {
-						commitment_signed: None,
-						tx_signatures: None,
-						funding_tx: None,
-						splice_negotiated: None,
-						splice_locked: None,
-					});
-				}
-				let err =
-					format!("Channel {} not expecting funding signatures", self.context.channel_id);
-				return Err(APIError::APIMisuseError { err });
-			};
-
-		let tx = signing_session.unsigned_tx().tx();
-		if funding_txid_signed != tx.compute_txid() {
-			return Err(APIError::APIMisuseError {
-				err: "Transaction was malleated prior to signing".to_owned(),
-			});
-		}
-
-		let shared_input_signature =
-			if let Some(splice_input_index) = signing_session.unsigned_tx().shared_input_index() {
-				let sig = match &self.context.holder_signer {
-					ChannelSignerType::Ecdsa(signer) => signer.sign_splice_shared_input(
-						&self.funding.channel_transaction_parameters,
-						tx,
-						splice_input_index as usize,
-						&self.context.secp_ctx,
-					),
-					#[cfg(taproot)]
-					ChannelSignerType::Taproot(_) => todo!(),
-				};
-				Some(sig)
-			} else {
-				None
-			};
-		debug_assert_eq!(self.pending_splice.is_some(), shared_input_signature.is_some());
-
-		let tx_signatures = msgs::TxSignatures {
-			channel_id: self.context.channel_id,
-			tx_hash: funding_txid_signed,
-			witnesses,
-			shared_input_signature,
-		};
-		let (tx_signatures, funding_tx) = signing_session
-			.provide_holder_witnesses(tx_signatures, &self.context.secp_ctx)
-			.map_err(|err| APIError::APIMisuseError { err })?;
-
-		let logger = WithChannelContext::from(logger, &self.context, None);
-		if tx_signatures.is_some() {
-			log_info!(
-				logger,
-				"Sending tx_signatures for interactive funding transaction {funding_txid_signed}"
-			);
-		}
-
-		let (splice_negotiated, splice_locked) = if let Some(funding_tx) = funding_tx.clone() {
-			debug_assert!(tx_signatures.is_some());
-			self.on_tx_signatures_exchange(funding_tx, best_block_height, &logger)
-		} else {
-			(None, None)
-		};
-
-		let funding = self
-			.pending_splice
-			.as_ref()
-			.and_then(|pending_splice| pending_splice.funding_negotiation.as_ref())
-			.and_then(|funding_negotiation| funding_negotiation.as_funding())
-			.unwrap_or(&self.funding);
-		let commitment_signed = self.context.get_initial_commitment_signed_v2(funding, &&logger);
-
-		Ok(FundingTxSigned {
-			commitment_signed,
-			tx_signatures,
-			funding_tx,
-			splice_negotiated,
-			splice_locked,
-		})
-	}
-
 	pub fn tx_signatures<L: Deref>(
 		&mut self, msg: &msgs::TxSignatures, best_block_height: u32, logger: &L,
 	) -> Result<FundingTxSigned, ChannelError>