Skip channel_update signature checks if we have a newer state

`channel_update` messages already have their signatures checked
with the network graph write lock held, so there's no reason to
check the signatures before doing other quicker checks first,
including checking if we're already aware of a newer update for the
channel.

This reduces common-case CPU usage as `channel_update`s are sent
rather liberally over the p2p network to gossip them.

Author: TheBlueMatt

lightning/src/routing/network_graph.rs

@@ -1376,8 +1376,8 @@ impl NetworkGraph {
 						}
 					}
 				}
-				macro_rules! maybe_update_channel_info {
-					( $target: expr, $src_node: expr) => {
+				macro_rules! check_update_latest {
+					($target: expr) => {
 						if let Some(existing_chan_info) = $target.as_ref() {
 							// The timestamp field is somewhat of a misnomer - the BOLTs use it to
 							// order updates to ensure you always have the latest one, only
@@ -1394,7 +1394,11 @@ impl NetworkGraph {
 						} else {
 							chan_was_enabled = false;
 						}
+					}
+				}
 
+				macro_rules! get_new_channel_info {
+					() => { {
 						let last_update_message = if msg.excess_data.len() <= MAX_EXCESS_BYTES_FOR_RELAY
 							{ full_msg.cloned() } else { None };
 
@@ -1410,29 +1414,31 @@ impl NetworkGraph {
 							},
 							last_update_message
 						};
-						$target = Some(updated_channel_update_info);
-					}
+						Some(updated_channel_update_info)
+					} }
 				}
 
 				let msg_hash = hash_to_message!(&Sha256dHash::hash(&msg.encode()[..])[..]);
 				if msg.flags & 1 == 1 {
 					dest_node_id = channel.node_one.clone();
+					check_update_latest!(channel.two_to_one);
 					if let Some((sig, ctx)) = sig_info {
 						secp_verify_sig!(ctx, &msg_hash, &sig, &PublicKey::from_slice(channel.node_two.as_slice()).map_err(|_| LightningError{
 							err: "Couldn't parse source node pubkey".to_owned(),
 							action: ErrorAction::IgnoreAndLog(Level::Debug)
 						})?, "channel_update");
 					}
-					maybe_update_channel_info!(channel.two_to_one, channel.node_two);
+					channel.two_to_one = get_new_channel_info!();
 				} else {
 					dest_node_id = channel.node_two.clone();
+					check_update_latest!(channel.one_to_two);
 					if let Some((sig, ctx)) = sig_info {
 						secp_verify_sig!(ctx, &msg_hash, &sig, &PublicKey::from_slice(channel.node_one.as_slice()).map_err(|_| LightningError{
 							err: "Couldn't parse destination node pubkey".to_owned(),
 							action: ErrorAction::IgnoreAndLog(Level::Debug)
 						})?, "channel_update");
 					}
-					maybe_update_channel_info!(channel.one_to_two, channel.node_one);
+					channel.one_to_two = get_new_channel_info!();
 				}
 			}
 		}