void BackgroundSync::consume() {
    // this is just to get the op off the queue, it's been peeked at
    // and queued for application already
    BSONObj op = _buffer.blockingPop();
    bufferCountGauge.decrement(1);
    bufferSizeGauge.decrement(getSize(op));
}

    T blockingPop() {
        stdx::unique_lock<stdx::mutex> lk(_lock);
        _clearing = false;
        while (_queue.empty() && !_clearing)
            _cvNoLongerEmpty.wait(lk);
        if (_clearing) {
            return T{};
        }

        T t = _queue.front();
        _queue.pop();
        _currentSize -= _getSize(t);
        _cvNoLongerFull.notify_one();

        return t;
    }