* @see WANObjectCache::getWithSetCallback()
*
* @param IDatabase $db1
- * @param IDatabase $db2 [optional]
+ * @param IDatabase|null $db2 [optional]
* @return array Map of values:
* - lag: highest lag of any of the DBs or false on error (e.g. replication stopped)
* - since: oldest UNIX timestamp of any of the DB lag estimates