public function sleep()
    {
        $now = microtime(true);
        $min = null;
        foreach ($this->untils as $id => $until) {
            $diff = $until - $now;
            if ($diff < 0) {
                // @codeCoverageIgnoreStart
                return;
                // @codeCoverageIgnoreEnd
            }
            if ($min !== null && $diff >= $min) {
                continue;
            }
            $min = $diff;
        }
        $min && usleep($min * 1000000);
    }