我正在尝试阅读LevelDB源代码。下面的一些代码：

void PosixEnv::Schedule( void (*background_work_function)(void* background_work_arg), void* background_work_arg) {
  background_work_mutex_.Lock();

  // Start the background thread, if we haven't done so already.
  if (!started_background_thread_) {
    started_background_thread_ = true;
    std::thread background_thread(PosixEnv::BackgroundThreadEntryPoint, this);  
    background_thread.detach();
  }

  // If the queue is empty, the background thread may be waiting for work.
  if (background_work_queue_.empty()) {
    background_work_cv_.Signal();
  }

  background_work_queue_.emplace(background_work_function, background_work_arg);
  background_work_mutex_.Unlock();
}

void PosixEnv::BackgroundThreadMain() {
  while (true) {
    background_work_mutex_.Lock();

    // Wait until there is work to be done.
    while (background_work_queue_.empty()) {
      background_work_cv_.Wait();
    }

    assert(!background_work_queue_.empty());
    auto background_work_function = background_work_queue_.front().function;
    void* background_work_arg = background_work_queue_.front().arg;
    background_work_queue_.pop();

    background_work_mutex_.Unlock();
    background_work_function(background_work_arg);
  }
}

关于Schedule，我想知道为什么在将新任务推入队列之前**当find queue为空时调用background_work_cv_.Signal()？
在我看来，Schedule可以写如下来做同样的事情：

void PosixEnv::Schedule( void (*background_work_function)(void* background_work_arg), void* background_work_arg) {
  background_work_mutex_.Lock();

  ...

  background_work_queue_.emplace(background_work_function, background_work_arg);
  background_work_mutex_.Unlock();
  background_work_cv_.Signal(); // cv.notify_one();
}

我想知道这两者有什么不同吗？为什么一定要第一次实施？第二次实施是否在某些方面有缺陷，没有考虑到某种情况？