Handle monitor completion actions for closed channels

If a channel has been closed, there may still be some
`ChannelMonitorUpdate`(s) which are pending completion. These
in-flight updates may also be blocking another channel from letting
an update fly, e.g. for forwarded payments where the payment
preimage will be removed from the downstream channel after the
upstream channel has closed.

Luckily all the infrastructure to handle this case is already in
place - we just need to process the
`monitor_update_blocked_actions` for closed channels.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -5001,24 +5001,29 @@ 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 mut channel = {
-			match peer_state.channel_by_id.entry(funding_txo.to_channel_id()){
-				hash_map::Entry::Occupied(chan) => chan,
-				hash_map::Entry::Vacant(_) => return,
-			}
-		};
+		let channel =
+			if let Some(chan) = peer_state.channel_by_id.get_mut(&funding_txo.to_channel_id()) {
+				chan
+			} else {
+				let update_actions = peer_state.monitor_update_blocked_actions
+					.remove(&funding_txo.to_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 };
 		log_trace!(self.logger, "ChannelMonitor updated to {}. Current highest is {}. {} pending in-flight updates.",
-			highest_applied_update_id, channel.get().context.get_latest_monitor_update_id(),
+			highest_applied_update_id, channel.context.get_latest_monitor_update_id(),
 			remaining_in_flight);
-		if !channel.get().is_awaiting_monitor_update() || channel.get().context.get_latest_monitor_update_id() != highest_applied_update_id {
+		if !channel.is_awaiting_monitor_update() || channel.context.get_latest_monitor_update_id() != highest_applied_update_id {
 			return;
 		}
-		handle_monitor_update_completion!(self, peer_state_lock, peer_state, per_peer_state, channel.get_mut());
+		handle_monitor_update_completion!(self, peer_state_lock, peer_state, per_peer_state, channel);
 	}
 
 	/// Accepts a request to open a channel after a [`Event::OpenChannelRequest`].