boost.asio-不执行所有处理程序

我正在尝试为 boost::io_context ,这将始终在准备好执行的处理程序中选择优先级最高的处理程序。我的灵感来自 the official example 但在一个场景中很快遇到意外行为,其中一个处理程序在同一上下文中启动另一个异步操作。

这里是 MCVE . 我只修改了用户代码(如下 //--- )调用一个低优先级处理程序,在此之后,我希望调用高优先级和中优先级处理程序。只调用低优先级处理程序。

#include <boost/asio.hpp>
#include <boost/function.hpp>
#include <iostream>
#include <queue>

class handler_priority_queue
{
public:
  void add(int priority, boost::function<void()> function)
  {
    handlers_.push(queued_handler(priority, function));
  }

  void execute_all()
  {
    while (!handlers_.empty())
    {
      queued_handler handler = handlers_.top();
      handler.execute();
      handlers_.pop();
    }
  }

  // A generic wrapper class for handlers to allow the invocation to be hooked.
  template <typename Handler>
  class wrapped_handler
  {
  public:
    wrapped_handler(handler_priority_queue& q, int p, Handler h)
      : queue_(q), priority_(p), handler_(h)
    {
    }

    void operator()()
    {
      handler_();
    }

    template <typename Arg1>
    void operator()(Arg1 arg1)
    {
      handler_(arg1);
    }

    template <typename Arg1, typename Arg2>
    void operator()(Arg1 arg1, Arg2 arg2)
    {
      handler_(arg1, arg2);
    }

  //private:
    handler_priority_queue& queue_;
    int priority_;
    Handler handler_;
  };

  template <typename Handler>
  wrapped_handler<Handler> wrap(int priority, Handler handler)
  {
    return wrapped_handler<Handler>(*this, priority, handler);
  }

private:
  class queued_handler
  {
  public:
    queued_handler(int p, boost::function<void()> f)
      : priority_(p), function_(f)
    {
    }

    void execute()
    {
      function_();
    }

    friend bool operator<(const queued_handler& a,
        const queued_handler& b)
    {
      return a.priority_ < b.priority_;
    }

  private:
    int priority_;
    boost::function<void()> function_;
  };

  std::priority_queue<queued_handler> handlers_;
};

// Custom invocation hook for wrapped handlers.
template <typename Function, typename Handler>
void asio_handler_invoke(Function f,
    handler_priority_queue::wrapped_handler<Handler>* h)
{
  h->queue_.add(h->priority_, f);
}

//----------------------------------------------------------------------

void high_priority_handler()
{
  std::cout << "High priority handler\n";
}

void middle_priority_handler()
{
  std::cout << "Middle priority handler\n";
}

void low_priority_handler(
  boost::asio::io_service& io_service,
  handler_priority_queue& pri_queue)
{
  std::cout << "Low priority handler\n";

  io_service.post(pri_queue.wrap(1, middle_priority_handler));
  io_service.post(pri_queue.wrap(2, high_priority_handler));
}

int main()
{
  boost::asio::io_service io_service;
  handler_priority_queue pri_queue;

  // Post a completion handler to be run immediately.
  io_service.post(pri_queue.wrap(
      0, std::bind(low_priority_handler,
                   std::ref(io_service), std::ref(pri_queue))));

  while (io_service.run_one())
  {
    // The custom invocation hook adds the handlers to the priority queue
    // rather than executing them from within the poll_one() call.
    while (io_service.poll_one())
      ;

    pri_queue.execute_all();
  }

  return 0;
}

如果我打电话 io_service.restart() 循环后 main 然后复制粘贴相同的循环,然后按预期的顺序执行其余的处理程序。调试时,我可以看到一个处理程序已排队进入 asio_handler_invoke 只有一次。

为什么 boost::IO环境 停止在第一个处理程序之后运行?我要求的是可能的吗?