gossipd: defer asking about txouts until we're synced or they're 6 deep.

The first one means we don't discard channels just because we're not
synced, and the second is implied by the spec: don't accept
channel_announcement if the channel isn't 6 deep.  Since LND defers in
such cases, we do too (unless it's newer than the current block, in
which case we simply discard).  Otherwise there's a risk that a slow
node might discard valid gossip.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/gossipd.c

@@ -547,12 +547,24 @@ static const u8 *handle_channel_announcement_msg(struct peer *peer,
 	/* If it's OK, tells us the short_channel_id to lookup; it notes
 	 * if this is the unknown channel the peer was looking for (in
 	 * which case, it frees and NULLs that ptr) */
-	err = handle_channel_announcement(peer->daemon->rstate, msg, &scid);
+	err = handle_channel_announcement(peer->daemon->rstate, msg,
+					  peer->daemon->current_blockheight,
+					  &scid);
 	if (err)
 		return err;
-	else if (scid)
+	else if (scid) {
-		daemon_conn_send(peer->daemon->master,
+		/* We give them some grace period, in case we don't know about
-				 take(towire_gossip_get_txout(NULL, scid)));
+		 * block yet. */
+		if (peer->daemon->current_blockheight == 0
+		    || !is_scid_depth_announceable(scid,
+						   peer->daemon->current_blockheight)) {
+			tal_arr_expand(&peer->daemon->deferred_txouts, *scid);
+		} else {
+			daemon_conn_send(peer->daemon->master,
+					 take(towire_gossip_get_txout(NULL,
+								      scid)));
+		}
+	}
 	return NULL;
 }
 
@@ -2350,6 +2362,24 @@ static struct io_plan *new_blockheight(struct io_conn *conn,
 {
 	if (!fromwire_gossip_new_blockheight(msg, &daemon->current_blockheight))
 		master_badmsg(WIRE_GOSSIP_NEW_BLOCKHEIGHT, msg);
+
+	/* Check if we can now send any deferred queries. */
+	for (size_t i = 0; i < tal_count(daemon->deferred_txouts); i++) {
+		const struct short_channel_id *scid
+			= &daemon->deferred_txouts[i];
+
+		if (!is_scid_depth_announceable(scid,
+						daemon->current_blockheight))
+			continue;
+
+		/* short_channel_id is deep enough, now ask about it. */
+		daemon_conn_send(daemon->master,
+				 take(towire_gossip_get_txout(NULL, scid)));
+
+		tal_arr_remove(&daemon->deferred_txouts, i);
+		i--;
+	}
+
 	return daemon_conn_read_next(conn, daemon->master);
 }
 
@@ -2885,6 +2915,7 @@ int main(int argc, char *argv[])
 	daemon = tal(NULL, struct daemon);
 	list_head_init(&daemon->peers);
 	daemon->unknown_scids = tal_arr(daemon, struct short_channel_id, 0);
+	daemon->deferred_txouts = tal_arr(daemon, struct short_channel_id, 0);
 	daemon->gossip_missing = NULL;
 	daemon->node_announce_timer = NULL;
 	daemon->current_blockheight = 0; /* i.e. unknown */

gossipd/gossipd.h

@@ -54,6 +54,9 @@ struct daemon {
 
 	/* Timer until we can send a new node_announcement */
 	struct oneshot *node_announce_timer;
+
+	/* Channels we have an announce for, but aren't deep enough. */
+	struct short_channel_id *deferred_txouts;
 };
 
 /* Search for a peer. */

gossipd/routing.c

@@ -1602,6 +1602,7 @@ bool routing_add_channel_announcement(struct routing_state *rstate,
 
 u8 *handle_channel_announcement(struct routing_state *rstate,
 				const u8 *announce TAKES,
+				u32 current_blockheight,
 				const struct short_channel_id **scid)
 {
 	struct pending_cannouncement *pending;
@@ -1636,6 +1637,19 @@ u8 *handle_channel_announcement(struct routing_state *rstate,
 		goto malformed;
 	}
 
+	/* If we know the blockheight, and it's in the future, reject
+	 * out-of-hand.  Remember, it should be 6 deep before they tell us
+	 * anyway. */
+	if (current_blockheight != 0
+	    && short_channel_id_blocknum(&pending->short_channel_id) > current_blockheight) {
+		status_debug("Ignoring future channel_announcment for %s"
+			     " (current block %u)",
+			     type_to_string(tmpctx, struct short_channel_id,
+					    &pending->short_channel_id),
+			     current_blockheight);
+		goto ignored;
+	}
+
 	/* If a prior txout lookup failed there is little point it trying
 	 * again. Just drop the announcement and walk away whistling. Any non-0
 	 * result means this failed before. */

gossipd/routing.h

@@ -346,6 +346,7 @@ struct chan *new_chan(struct routing_state *rstate,
  */
 u8 *handle_channel_announcement(struct routing_state *rstate,
 				const u8 *announce TAKES,
+				u32 current_blockheight,
 				const struct short_channel_id **scid);
 
 /**

gossipd/test/run-crc32_of_update.c

@@ -177,6 +177,7 @@ const char *got_pong(const u8 *pong UNNEEDED, size_t *num_pings_outstanding UNNE
 /* Generated stub for handle_channel_announcement */
 u8 *handle_channel_announcement(struct routing_state *rstate UNNEEDED,
 				const u8 *announce TAKES UNNEEDED,
+				u32 current_blockheight UNNEEDED,
 				const struct short_channel_id **scid UNNEEDED)
 { fprintf(stderr, "handle_channel_announcement called!\n"); abort(); }
 /* Generated stub for handle_channel_update */

gossipd/test/run-extended-info.c

@@ -184,6 +184,7 @@ const char *got_pong(const u8 *pong UNNEEDED, size_t *num_pings_outstanding UNNE
 /* Generated stub for handle_channel_announcement */
 u8 *handle_channel_announcement(struct routing_state *rstate UNNEEDED,
 				const u8 *announce TAKES UNNEEDED,
+				u32 current_blockheight UNNEEDED,
 				const struct short_channel_id **scid UNNEEDED)
 { fprintf(stderr, "handle_channel_announcement called!\n"); abort(); }
 /* Generated stub for handle_channel_update */