Handle receiving channel_reestablish with next_funding_txid

This follows the the specification closely in branching without being
too verbose, so that it should be easy to follow the logic.

See: https://github.com/lightning/bolts/blob/aa5207a/02-peer-protocol.md?plain=1#L2520-L2531

Author: dunxen

lightning/src/ln/channel.rs

@@ -2630,7 +2630,10 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 		self.context.assert_no_commitment_advancement(transaction_number, "initial commitment_signed");
 		let commitment_signed = self.context.get_initial_commitment_signed(&self.funding, logger);
 		let commitment_signed = match commitment_signed {
-			Ok(commitment_signed) => commitment_signed,
+			Ok(commitment_signed) => {
+				self.funding.funding_transaction = Some(signing_session.unsigned_tx().build_unsigned_tx());
+				commitment_signed
+			},
 			Err(err) => {
 				self.funding.channel_transaction_parameters.funding_outpoint = None;
 				return Err(ChannelError::Close((err.to_string(), ClosureReason::HolderForceClosed { broadcasted_latest_txn: Some(false) })));

lightning/src/ln/interactivetxs.rs

@@ -353,6 +353,14 @@ impl InteractiveTxSigningSession {
 		}
 	}
 
+	pub fn get_tx_signatures(&self) -> Option<TxSignatures> {
+		if self.has_received_commitment_signed {
+			self.holder_tx_signatures.clone()
+		} else {
+			None
+		}
+	}
+
 	/// Handles a `tx_signatures` message received from the counterparty.
 	///
 	/// If the holder is required to send their `tx_signatures` message and these signatures have