Add an infallible no-sign version of send_commitment_no_status_check

In the coming commits we'll move to async `ChannelMonitorUpdate`
application, which means we'll want to generate a
`ChannelMonitorUpdate` (including a new counterparty commitment
transaction) before we actually send it to our counterparty. To do
that today we'd have to actually sign the commitment transaction
by calling the signer, then drop it, apply the
`ChannelMonitorUpdate`, then re-sign the commitment transaction to
send it to our peer.

In this commit we instead split `send_commitment_no_status_check`
and `send_commitment_no_state_update` into `build_` and `send_`
variants, allowing us to generate new counterparty commitment
transactions without actually signing, then build them for sending,
with signatures, later.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -5791,8 +5791,16 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		Ok(Some(res))
 	}
 
-	/// Only fails in case of bad keys
+	/// Only fails in case of signer rejection.
 	fn send_commitment_no_status_check<L: Deref>(&mut self, logger: &L) -> Result<(msgs::CommitmentSigned, ChannelMonitorUpdate), ChannelError> where L::Target: Logger {
+		let monitor_update = self.build_commitment_no_status_check(logger);
+		match self.send_commitment_no_state_update(logger) {
+			Ok((commitment_signed, _)) => Ok((commitment_signed, monitor_update)),
+			Err(e) => Err(e),
+		}
+	}
+
+	fn build_commitment_no_status_check<L: Deref>(&mut self, logger: &L) -> ChannelMonitorUpdate where L::Target: Logger {
 		log_trace!(logger, "Updating HTLC state for a newly-sent commitment_signed...");
 		// We can upgrade the status of some HTLCs that are waiting on a commitment, even if we
 		// fail to generate this, we still are at least at a position where upgrading their status
@@ -5825,15 +5833,9 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		}
 		self.resend_order = RAACommitmentOrder::RevokeAndACKFirst;
 
-		let (res, counterparty_commitment_txid, htlcs) = match self.send_commitment_no_state_update(logger) {
-			Ok((res, (counterparty_commitment_tx, mut htlcs))) => {
-				// Update state now that we've passed all the can-fail calls...
-				let htlcs_no_ref: Vec<(HTLCOutputInCommitment, Option<Box<HTLCSource>>)> =
-					htlcs.drain(..).map(|(htlc, htlc_source)| (htlc, htlc_source.map(|source_ref| Box::new(source_ref.clone())))).collect();
-				(res, counterparty_commitment_tx, htlcs_no_ref)
-			},
-			Err(e) => return Err(e),
-		};
+		let (counterparty_commitment_txid, mut htlcs_ref) = self.build_commitment_no_state_update(logger);
+		let htlcs: Vec<(HTLCOutputInCommitment, Option<Box<HTLCSource>>)> =
+			htlcs_ref.drain(..).map(|(htlc, htlc_source)| (htlc, htlc_source.map(|source_ref| Box::new(source_ref.clone())))).collect();
 
 		if self.announcement_sigs_state == AnnouncementSigsState::MessageSent {
 			self.announcement_sigs_state = AnnouncementSigsState::Committed;
@@ -5850,16 +5852,13 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 			}]
 		};
 		self.channel_state |= ChannelState::AwaitingRemoteRevoke as u32;
-		Ok((res, monitor_update))
+		monitor_update
 	}
 
-	/// Only fails in case of bad keys. Used for channel_reestablish commitment_signed generation
-	/// when we shouldn't change HTLC/channel state.
-	fn send_commitment_no_state_update<L: Deref>(&self, logger: &L) -> Result<(msgs::CommitmentSigned, (Txid, Vec<(HTLCOutputInCommitment, Option<&HTLCSource>)>)), ChannelError> where L::Target: Logger {
+	fn build_commitment_no_state_update<L: Deref>(&self, logger: &L) -> (Txid, Vec<(HTLCOutputInCommitment, Option<&HTLCSource>)>) where L::Target: Logger {
 		let counterparty_keys = self.build_remote_transaction_keys();
 		let commitment_stats = self.build_commitment_transaction(self.cur_counterparty_commitment_transaction_number, &counterparty_keys, false, true, logger);
 		let counterparty_commitment_txid = commitment_stats.tx.trust().txid();
-		let (signature, htlc_signatures);
 
 		#[cfg(any(test, fuzzing))]
 		{
@@ -5879,6 +5878,21 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 			}
 		}
 
+		(counterparty_commitment_txid, commitment_stats.htlcs_included)
+	}
+
+	/// Only fails in case of signer rejection. Used for channel_reestablish commitment_signed
+	/// generation when we shouldn't change HTLC/channel state.
+	fn send_commitment_no_state_update<L: Deref>(&self, logger: &L) -> Result<(msgs::CommitmentSigned, (Txid, Vec<(HTLCOutputInCommitment, Option<&HTLCSource>)>)), ChannelError> where L::Target: Logger {
+		// Get the fee tests from `build_commitment_no_state_update`
+		#[cfg(any(test, fuzzing))]
+		self.build_commitment_no_state_update(logger);
+
+		let counterparty_keys = self.build_remote_transaction_keys();
+		let commitment_stats = self.build_commitment_transaction(self.cur_counterparty_commitment_transaction_number, &counterparty_keys, false, true, logger);
+		let counterparty_commitment_txid = commitment_stats.tx.trust().txid();
+		let (signature, htlc_signatures);
+
 		{
 			let mut htlcs = Vec::with_capacity(commitment_stats.htlcs_included.len());
 			for &(ref htlc, _) in commitment_stats.htlcs_included.iter() {