public static function backup_includes_files($backupid) {
        // This function is called repeatedly in a backup with many files.
        // Loading the controller is a nontrivial operation (in a large test
        // backup it took 0.3 seconds), so we do a temporary cache of it within
        // this request.
        if (self::$includesfilescachebackupid === $backupid) {
            return self::$includesfilescache;
        }

        // Load controller, get value, then destroy controller and return result.
        self::$includesfilescachebackupid = $backupid;
        $bc = self::load_controller($backupid);
        self::$includesfilescache = $bc->get_include_files();
        $bc->destroy();
        return self::$includesfilescache;
    }