Check in-flight updates before completing events on closed chans

When we handle a `ChannelMonitorUpdate` completion we always
complete everything that was waiting on any updates to the same
channel all at once. Thus, we need to skip all updates if there's
pending updates besides the one that was just completed.

We handled this correctly for open channels, but the shortcut for
closed channels ignored any other pending updates entirely.

Here we fix this, which is ultimately required for tests which are
added in a few commits to pass.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -7638,27 +7638,34 @@ where
 		if peer_state_mutex_opt.is_none() { return }
 		peer_state_lock = peer_state_mutex_opt.unwrap().lock().unwrap();
 		let peer_state = &mut *peer_state_lock;
+
+		let remaining_in_flight =
+			if let Some(pending) = peer_state.in_flight_monitor_updates.get_mut(funding_txo) {
+				pending.retain(|upd| upd.update_id > highest_applied_update_id);
+				pending.len()
+			} else { 0 };
+
 		let channel =
 			if let Some(ChannelPhase::Funded(chan)) = peer_state.channel_by_id.get_mut(channel_id) {
 				chan
 			} else {
+				if remaining_in_flight != 0 {
+					return;
+				}
+
 				let update_actions = peer_state.monitor_update_blocked_actions
 					.remove(channel_id).unwrap_or(Vec::new());
 				mem::drop(peer_state_lock);
 				mem::drop(per_peer_state);
 				self.handle_monitor_update_completion_actions(update_actions);
 				return;
 			};
-		let remaining_in_flight =
-			if let Some(pending) = peer_state.in_flight_monitor_updates.get_mut(funding_txo) {
-				pending.retain(|upd| upd.update_id > highest_applied_update_id);
-				pending.len()
-			} else { 0 };
+
 		let logger = WithChannelContext::from(&self.logger, &channel.context, None);
 		log_trace!(logger, "ChannelMonitor updated to {}. Current highest is {}. {} pending in-flight updates.",
 			highest_applied_update_id, channel.context.get_latest_monitor_update_id(),
 			remaining_in_flight);
-		if !channel.is_awaiting_monitor_update() || remaining_in_flight != 0 {
+		if remaining_in_flight != 0 || !channel.is_awaiting_monitor_update() {
 			return;
 		}
 		handle_monitor_update_completion!(self, peer_state_lock, peer_state, per_peer_state, channel);