Enter FundingNegotiated state after constructing funding tx

The ChannelState::NegotiatingFunding assertion check in
ChannelContext::get_initial_commitment_signed will fail when
implementing splicing's channel_reestablish logic. In order to support
it and channel establishment, enter ChannelState::FundingNegotiated
prior to calling the method and update the assertion accordingly.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -5517,6 +5517,8 @@ where
 		funding
 			.channel_transaction_parameters.funding_outpoint = Some(outpoint);
 
+		self.channel_state = ChannelState::FundingNegotiated(FundingNegotiatedFlags::new());
+
 		if is_splice {
 			debug_assert_eq!(
 				holder_commitment_transaction_number,
@@ -5580,9 +5582,7 @@ where
 			});
 		};
 
-		let mut channel_state = ChannelState::FundingNegotiated(FundingNegotiatedFlags::new());
-		channel_state.set_interactive_signing();
-		self.channel_state = channel_state;
+		self.channel_state.set_interactive_signing();
 
 		Ok((commitment_signed, funding_ready_for_sig_event))
 	}
@@ -5636,16 +5636,7 @@ where
 		SP::Target: SignerProvider,
 		L::Target: Logger
 	{
-		if !matches!(
-			self.channel_state, ChannelState::NegotiatingFunding(flags)
-			if flags == (NegotiatingFundingFlags::OUR_INIT_SENT | NegotiatingFundingFlags::THEIR_INIT_SENT)
-		) {
-			debug_assert!(false);
-			let msg = "Tried to get an initial commitment_signed messsage at a time other than \
-				immediately after initial handshake completion (or tried to get funding_created twice)";
-			let reason = ClosureReason::ProcessingError { err: msg.to_owned() };
-			return Err(ChannelError::Close((msg.to_owned(), reason)));
-		}
+		assert!(matches!(self.channel_state, ChannelState::FundingNegotiated(_)));
 
 		let signature = match self.get_initial_counterparty_commitment_signature(funding, logger) {
 			Ok(res) => res,