f comment and clearer debug assert string

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -2557,6 +2557,10 @@ where
 						channel.node_one.as_slice()
 					};
 					if sig.is_some() {
+						// PublicKey parsing isn't entirely trivial as it requires that we check
+						// that the provided point is on the curve. Thus, if we don't have a
+						// signature to verify, we want to skip the parsing step entirely.
+						// This represents a substantial speedup in applying RGS snapshots.
 						node_pubkey = Some(PublicKey::from_slice(node_id).map_err(|_| LightningError {
 							err: "Couldn't parse source node pubkey".to_owned(),
 							action: ErrorAction::IgnoreAndLog(Level::Debug),
@@ -2569,8 +2573,8 @@ where
 		if let Some(sig) = sig {
 			let msg_hash = hash_to_message!(&message_sha256d_hash(&msg)[..]);
 			if node_pubkey.is_none() {
-				debug_assert!(false);
-				let err = "Reached unreachable code".to_owned();
+				debug_assert!(false, "node_pubkey should have been decoded above");
+				let err = "node_pubkey wasn't decoded but we need it to check a sig".to_owned();
 				return Err(LightningError {
 					err, action: ErrorAction::IgnoreAndLog(Level::Error),
 				});