gossipd: exclude early blocks from random probes.

When probing, no point probing for before lightning became cool.  Current
logic means we often probe below block 500,000, and think things are OK
because there are no short_channel_ids.

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

gossipd/seeker.c

@@ -717,13 +717,23 @@ static void peer_gossip_probe_scids(struct seeker *seeker)
 
 static void probe_random_scids(struct seeker *seeker, size_t num_blocks)
 {
-	if (seeker->daemon->current_blockheight < num_blocks) {
+	u32 avail_blocks;
+
+	/* Ignore early blocks (unless we're before, which would be weird) */
+	if (seeker->daemon->current_blockheight
+	    < chainparams->when_lightning_became_cool)
+		avail_blocks = seeker->daemon->current_blockheight;
+	else
+		avail_blocks = seeker->daemon->current_blockheight
+			- chainparams->when_lightning_became_cool;
+
+	if (avail_blocks < num_blocks) {
 		seeker->scid_probe_start = 0;
 		seeker->scid_probe_end = seeker->daemon->current_blockheight;
 	} else {
 		seeker->scid_probe_start
-			= pseudorand(seeker->daemon->current_blockheight
-				     - num_blocks);
+			= chainparams->when_lightning_became_cool
+			+ pseudorand(avail_blocks - num_blocks);
 		seeker->scid_probe_end
 			= seeker->scid_probe_start + num_blocks - 1;
 	}