Do not always persist ChannelManager on channel_update messages

If we receive a `channel_update` message for a channel unrelated to
our own, we shouldn't trigger a persistence of our
`ChannelManager`. This avoids significant persistence traffic during
initial node startup.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -3346,14 +3346,15 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 		Ok(())
 	}
 
-	fn internal_channel_update(&self, counterparty_node_id: &PublicKey, msg: &msgs::ChannelUpdate) -> Result<(), MsgHandleErrInternal> {
+	/// Returns ShouldPersist if anything changed, otherwise either SkipPersist or an Err.
+	fn internal_channel_update(&self, counterparty_node_id: &PublicKey, msg: &msgs::ChannelUpdate) -> Result<NotifyOption, MsgHandleErrInternal> {
 		let mut channel_state_lock = self.channel_state.lock().unwrap();
 		let channel_state = &mut *channel_state_lock;
 		let chan_id = match channel_state.short_to_id.get(&msg.contents.short_channel_id) {
 			Some(chan_id) => chan_id.clone(),
 			None => {
 				// It's not a local channel
-				return Ok(())
+				return Ok(NotifyOption::SkipPersist)
 			}
 		};
 		match channel_state.by_id.entry(chan_id) {
@@ -3363,15 +3364,15 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 						// If the announcement is about a channel of ours which is public, some
 						// other peer may simply be forwarding all its gossip to us. Don't provide
 						// a scary-looking error message and return Ok instead.
-						return Ok(());
+						return Ok(NotifyOption::SkipPersist);
 					}
 					return Err(MsgHandleErrInternal::send_err_msg_no_close("Got a channel_update for a channel from the wrong node - it shouldn't know about our private channels!".to_owned(), chan_id));
 				}
 				try_chan_entry!(self, chan.get_mut().channel_update(&msg), channel_state, chan);
 			},
 			hash_map::Entry::Vacant(_) => unreachable!()
 		}
-		Ok(())
+		Ok(NotifyOption::DoPersist)
 	}
 
 	fn internal_channel_reestablish(&self, counterparty_node_id: &PublicKey, msg: &msgs::ChannelReestablish) -> Result<(), MsgHandleErrInternal> {
@@ -4116,8 +4117,13 @@ impl<Signer: Sign, M: Deref , T: Deref , K: Deref , F: Deref , L: Deref >
 	}
 
 	fn handle_channel_update(&self, counterparty_node_id: &PublicKey, msg: &msgs::ChannelUpdate) {
-		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(&self.total_consistency_lock, &self.persistence_notifier);
-		let _ = handle_error!(self, self.internal_channel_update(counterparty_node_id, msg), *counterparty_node_id);
+		PersistenceNotifierGuard::optionally_notify(&self.total_consistency_lock, &self.persistence_notifier, || {
+			if let Ok(persist) = handle_error!(self, self.internal_channel_update(counterparty_node_id, msg), *counterparty_node_id) {
+				persist
+			} else {
+				NotifyOption::SkipPersist
+			}
+		});
 	}
 
 	fn handle_channel_reestablish(&self, counterparty_node_id: &PublicKey, msg: &msgs::ChannelReestablish) {