f defer tx_signatures when mon update pending, remove unreachable check

Author: dunxen

lightning/src/ln/channel.rs

@@ -6807,6 +6807,9 @@ impl<SP: Deref> FundedChannel<SP> where
 
 		if msg.next_local_commitment_number >= INITIAL_COMMITMENT_NUMBER || msg.next_remote_commitment_number >= INITIAL_COMMITMENT_NUMBER ||
 			msg.next_local_commitment_number == 0 && msg.next_funding_txid.is_none() {
+			// Note: This also covers the following case in the V2 channel establishment specification:
+			//   if `next_funding_txid` is not set, and `next_commitment_number` is zero:
+			//     MUST immediately fail the channel and broadcast any relevant latest commitment transaction.
 			return Err(ChannelError::close("Peer sent an invalid channel_reestablish to force close in a non-standard way".to_owned()));
 		}
 
@@ -6948,14 +6951,30 @@ impl<SP: Deref> FundedChannel<SP> where
 							// if it has already received commitment_signed and it should sign first, as specified in the tx_signatures requirements:
 							if session.has_received_commitment_signed() && session.holder_sends_tx_signatures_first() {
 								// MUST send its tx_signatures for that funding transaction.
-								(commitment_update, session.holder_tx_signatures().clone(), None)
+								if self.context.channel_state.is_monitor_update_in_progress() {
+									log_debug!(logger, "Not sending tx_signatures: a monitor update is in progress. Setting monitor_pending_tx_signatures.");
+									self.context.monitor_pending_tx_signatures = session.holder_tx_signatures().clone();
+									// We can still send the initial commitment transaction if a monitor update is pending.
+									(commitment_update, None, None)
+								} else {
+									(commitment_update, session.holder_tx_signatures().clone(), None)
+								}
 							} else {
 								(commitment_update, None, None)
 							}
 						} else {
 							// if it has already received tx_signatures for that funding transaction:
 							// MUST send its tx_signatures for that funding transaction.
-							(None, session.holder_tx_signatures().clone(), None)
+							if self.context.channel_state.is_monitor_update_in_progress() {
+								log_debug!(logger, "Not sending tx_signatures: a monitor update is in progress. Setting monitor_pending_tx_signatures.");
+								self.context.monitor_pending_tx_signatures = session.holder_tx_signatures().clone();
+								(None, None, None)
+							} else {
+								// If `holder_tx_signatures` is `None` here, the `tx_signatures` message will be sent
+								// when the holder provides their witnesses as this will queue a `tx_signatures` if the
+								// holder must send one.
+								(None, session.holder_tx_signatures().clone(), None)
+							}
 						}
 					} else {
 						// MUST send tx_abort to let the sending node know that they can forget this funding transaction.
@@ -6965,15 +6984,6 @@ impl<SP: Deref> FundedChannel<SP> where
 					return Err(ChannelError::close("Counterparty set `next_funding_txid` at incorrect state".into()));
 				}
 			} else {
-				// if `next_funding_txid` is not set, and `next_commitment_number` is zero:
-				if msg.next_local_commitment_number == 0 {
-					// MUST immediately fail the channel and broadcast any relevant latest commitment transaction.
-					return Err(ChannelError::close(format!(
-						"Peer attempted to reestablish channel expecting a future local commitment transaction: {} (received) vs {} (expected)",
-						msg.next_remote_commitment_number,
-						our_commitment_transaction
-					)));
-				}
 				(None, None, None)
 			};