director: Track show long the last ring sync took.

src/director/director-connection.c

@@ -1466,6 +1466,10 @@ director_connection_sync_host(struct director_connection *conn,
 			dir_debug("Ring is synced (%s sent seq=%u, hosts_hash=%u)",
 				  conn->name, seq,
 				  mail_hosts_hash(dir->mail_hosts));
+			int sync_msecs =
+				timeval_diff_msecs(&ioloop_timeval, &dir->last_sync_start_time);
+			if (sync_msecs >= 0)
+				dir->last_sync_msecs = sync_msecs;
 			director_set_ring_synced(dir);
 		}
 	} else {

src/director/director.c

@@ -362,8 +362,10 @@ void director_sync_send(struct director *dir, struct director_host *host,
 {
 	string_t *str;
 
-	if (host == dir->self_host)
+	if (host == dir->self_host) {
 		dir->last_sync_sent_ring_change_counter = dir->ring_change_counter;
+		dir->last_sync_start_time = ioloop_timeval;
+	}
 
 	str = t_str_new(128);
 	str_printfa(str, "SYNC\t%s\t%u\t%u",

src/director/director.h

@@ -137,9 +137,15 @@ struct director {
 	unsigned int sync_seq;
 	unsigned int ring_change_counter;
 	unsigned int last_sync_sent_ring_change_counter;
+	/* Timestamp when the last SYNC was initiated by us */
+	struct timeval last_sync_start_time;
 	/* the lowest minor version supported by the ring */
 	unsigned int ring_min_version;
+	/* Timestamp when ring became synced or unsynced the last time */
 	time_t ring_last_sync_time;
+	/* How many milliseconds it took for the last SYNC to travel through
+	   the ring. */
+	unsigned int last_sync_msecs;
 
 	time_t ring_first_alone;