Allow to verify channel updates without applying them

We introduce a new `NetworkGraph::verify_channel_update` method that
allows to check whether an update would be applied by `update_channel`.

Author: tnull

lightning/src/routing/gossip.rs

@@ -1845,14 +1845,14 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 	/// For an already known (from announcement) channel, update info about one of the directions
 	/// of the channel.
 	///
-	/// You probably don't want to call this directly, instead relying on a P2PGossipSync's
-	/// RoutingMessageHandler implementation to call it indirectly. This may be useful to accept
+	/// You probably don't want to call this directly, instead relying on a [`P2PGossipSync`]'s
+	/// [`RoutingMessageHandler`] implementation to call it indirectly. This may be useful to accept
 	/// routing messages from a source using a protocol other than the lightning P2P protocol.
 	///
 	/// If built with `no-std`, any updates with a timestamp more than two weeks in the past or
 	/// materially in the future will be rejected.
 	pub fn update_channel(&self, msg: &msgs::ChannelUpdate) -> Result<(), LightningError> {
-		self.update_channel_intern(&msg.contents, Some(&msg), Some(&msg.signature))
+		self.update_channel_internal(&msg.contents, Some(&msg), Some(&msg.signature), false)
 	}
 
 	/// For an already known (from announcement) channel, update info about one of the directions
@@ -1862,10 +1862,23 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 	/// If built with `no-std`, any updates with a timestamp more than two weeks in the past or
 	/// materially in the future will be rejected.
 	pub fn update_channel_unsigned(&self, msg: &msgs::UnsignedChannelUpdate) -> Result<(), LightningError> {
-		self.update_channel_intern(msg, None, None)
+		self.update_channel_internal(msg, None, None, false)
 	}
 
-	fn update_channel_intern(&self, msg: &msgs::UnsignedChannelUpdate, full_msg: Option<&msgs::ChannelUpdate>, sig: Option<&secp256k1::ecdsa::Signature>) -> Result<(), LightningError> {
+	/// For an already known (from announcement) channel, verify the given [`ChannelUpdate`].
+	///
+	/// This checks whether the update currently is applicable by [`Self::update_channel`].
+	///
+	/// If built with `no-std`, any updates with a timestamp more than two weeks in the past or
+	/// materially in the future will be rejected.
+	pub fn verify_channel_update(&self, msg: &msgs::ChannelUpdate) -> Result<(), LightningError> {
+		self.update_channel_internal(&msg.contents, Some(&msg), Some(&msg.signature), true)
+	}
+
+	fn update_channel_internal(&self, msg: &msgs::UnsignedChannelUpdate,
+		full_msg: Option<&msgs::ChannelUpdate>, sig: Option<&secp256k1::ecdsa::Signature>,
+		only_verify: bool) -> Result<(), LightningError>
+	{
 		let chan_enabled = msg.flags & (1 << 1) != (1 << 1);
 
 		if msg.chain_hash != self.chain_hash {
@@ -1961,7 +1974,9 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 							action: ErrorAction::IgnoreAndLog(Level::Debug)
 						})?, "channel_update");
 					}
-					channel.two_to_one = get_new_channel_info!();
+					if !only_verify {
+						channel.two_to_one = get_new_channel_info!();
+					}
 				} else {
 					check_update_latest!(channel.one_to_two);
 					if let Some(sig) = sig {
@@ -1970,7 +1985,9 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 							action: ErrorAction::IgnoreAndLog(Level::Debug)
 						})?, "channel_update");
 					}
-					channel.one_to_two = get_new_channel_info!();
+					if !only_verify {
+						channel.one_to_two = get_new_channel_info!();
+					}
 				}
 			}
 		}
@@ -2411,6 +2428,7 @@ pub(crate) mod tests {
 		}
 
 		let valid_channel_update = get_signed_channel_update(|_| {}, node_1_privkey, &secp_ctx);
+		network_graph.verify_channel_update(&valid_channel_update).unwrap();
 		match gossip_sync.handle_channel_update(&valid_channel_update) {
 			Ok(res) => assert!(res),
 			_ => panic!(),