Handle BackgroundEvents replaying non-closing monitor updates

`BackgroundEvent` was used to store `ChannelMonitorUpdate`s which
result in a channel force-close, avoiding relying on
`ChannelMonitor`s having been loaded while `ChannelManager`
block-connection methods are called during startup.

In the coming commit(s) we'll also generate non-channel-closing
`ChannelMonitorUpdate`s during startup, which will need to be
replayed prior to any other `ChannelMonitorUpdate`s generated from
normal operation.

In the next commit we'll handle that by handling `BackgroundEvent`s
immediately after locking the `total_consistency_lock`.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -509,7 +509,8 @@ enum BackgroundEvent {
 	/// Note that any such events are lost on shutdown, so in general they must be updates which
 	/// are regenerated on startup.
 	ClosingMonitorUpdateRegeneratedOnStartup((OutPoint, ChannelMonitorUpdate)),
-	/// Handle a ChannelMonitorUpdate.
+	/// Handle a ChannelMonitorUpdate. This may or may not close the channel and may unblock the
+	/// channel to continue normal operation.
 	///
 	/// Note that any such events are lost on shutdown, so in general they must be updates which
 	/// are regenerated on startup.
@@ -3790,10 +3791,33 @@ where
 					// monitor updating completing.
 					let _ = self.chain_monitor.update_channel(funding_txo, &update);
 				},
-				BackgroundEvent::MonitorUpdateRegeneratedOnStartup((_, funding_txo, update)) => {
-					// The channel has already been closed, so no use bothering to care about the
-					// monitor updating completing.
-					let _ = self.chain_monitor.update_channel(funding_txo, &update);
+				BackgroundEvent::MonitorUpdateRegeneratedOnStartup((counterparty_id, funding_txo, update)) => {
+					let update_res = self.chain_monitor.update_channel(funding_txo, &update);
+
+					let mut found_chan = false;
+					let res = {
+						let per_peer_state = self.per_peer_state.read().unwrap();
+						if let Some(peer_state_mutex) = per_peer_state.get(&counterparty_id) {
+							let mut peer_state_lock = peer_state_mutex.lock().unwrap();
+							let peer_state = &mut *peer_state_lock;
+							match peer_state.channel_by_id.entry(funding_txo.to_channel_id()) {
+								hash_map::Entry::Occupied(mut chan) => {
+									found_chan = true;
+									handle_new_monitor_update!(self, update_res, update.update_id, peer_state_lock, peer_state, per_peer_state, chan)
+								},
+								hash_map::Entry::Vacant(_) => {
+									Ok(())
+								},
+							}
+						} else { Ok(()) }
+					};
+					let _ = handle_error!(self, res, counterparty_id);
+					if !found_chan {
+						// TODO: If this channel has since closed, we're likely providing a payment
+						// preimage update, which we must ensure is durable! We currently don't,
+						// however, ensure that, and when we have a strategy therefor we should
+						// apply it here.
+					}
 				},
 			}
 		}