Method volatile_size

src/bthread/work_stealing_queue.h:137–141

    }

    size_t volatile_size() const {
        const size_t b = _bottom.load(butil::memory_order_relaxed);
        const size_t t = _top.load(butil::memory_order_relaxed);
        return (b <= t ? 0 : (b - t));
    }

    size_t capacity() const { return _capacity; }