gossipd: check for existing channel announcement before sigcheck

Checking a signature is a CPU-intensive operation that should be performed only
if gossmap doesn't already have the channel announcement in question and we're
not already checking for the announcement's UTxO.

Changelog-Fixed: `gossipd` doesn't waste CPU cycles checking signatures on channel announcements that are already known
Issue: #7972

Author: whitslack

gossipd/gossmap_manage.c

@@ -588,6 +588,12 @@ const char *gossmap_manage_channel_announcement(const tal_t *ctx,
 		return NULL;
 	}
 
+	/* Already known? */
+	if (gossmap_find_chan(gossmap, &scid)
+	    || map_get(&gm->pending_ann_map, scid)
+	    || map_get(&gm->early_ann_map, scid))
+		return NULL;
+
 	warn = sigcheck_channel_announcement(ctx, &node_id_1, &node_id_2,
 					     &bitcoin_key_1, &bitcoin_key_2,
 					     &node_signature_1, &node_signature_2,
@@ -596,10 +602,6 @@ const char *gossmap_manage_channel_announcement(const tal_t *ctx,
 	if (warn)
 		return warn;
 
-	/* Already known? */
-	if (gossmap_find_chan(gossmap, &scid))
-		return NULL;
-
 	pca = tal(gm, struct pending_cannounce);
 	pca->scriptpubkey = scriptpubkey_p2wsh(pca,
 					       bitcoin_redeem_2of2(tmpctx,
@@ -613,9 +615,7 @@ const char *gossmap_manage_channel_announcement(const tal_t *ctx,
 	/* Are we supposed to add immediately without checking with lightningd?
 	 * Unless we already got it from a peer and we're processing now!
 	 */
-	if (known_amount
-	    && !map_get(&gm->pending_ann_map, scid)
-	    && !map_get(&gm->early_ann_map, scid)) {
+	if (known_amount) {
 		/* Set with timestamp 0 (we will update once we have a channel_update) */
 		gossip_store_add(gm->gs, announce, 0);
 		gossip_store_add(gm->gs,