c – 为什么strand :: post()没有strand :: wrap()等价？

2023年8月7日 287次阅读

定义
strand::wrap()的行为,以便创建一个在调用时执行strand :: dispatch()的仿函数.我最近遇到了一个执行以下序列的应用程序中的错误：

my_great_function(..., s.wrap(a), s.wrap(b));

该应用程序保证s.wrap(a)创建的仿函数在s.wrap(b)之前被调用.但是,存在一种竞争条件,即第一个仿函数在strand之外被调用,因此推迟调用,而第二个仿函数在strand内调用并立即执行.这违反了应用程序对before b的排序假设,并导致了未定义的行为.

使用strand :: post()而不是strand :: dispatch()是解决这个问题的一种方法,但是使用strand.wrap()并没有简单的方法来实现这一点.我可以创建帮助函数来发布链,我想知道是否有更简单的方法？

最佳答案 纯粹猜测为什么strand.post的strand.wrap等价物不存在：

>我无法找到任何人在功能请求中需要strand.wrap()等效的情况.
>基于strand.wrap()最常见的用法,它可能用strand.dispatch()实现,以优化组合操作的中间处理程序.可以在满足完成条件后立即调用用户的完成处理程序,而不必为延迟调用发布完成处理程序.

最简单的解决方案可能是将链转移到my_great_function并与a和b处理程序一起.如果my_great_function需要特定的处理程序调用顺序,那么让my_great_function保证排序而不是将onus传递给调用者似乎是可以接受的,这可能会忽略必要的顺序.另一方面,如果my_great_function是相当通用的,并且处理程序需要它们之间的特定调用顺序,那么考虑将处理程序一起传递到直接或间接暗示排序的结构中,例如std :: tuple.

虽然这些解决方案都没有提供通用的可重用解决方案,但它可能是最简单的解决方案.官方支持的解决方案是使用自定义处理程序类型,此外还提供asio_handler_invoke中可用的asio_handler_invoke函数,以考虑操作的中间和完成处理程序.这是一个完全基于detail/wrapped_handler.hpp的示例：

#include <iostream>

#include <boost/asio.hpp>

/// @brief Custom handler wrapper type that will post into its dispatcher.
template <typename Dispatcher,
          typename Handler>
class post_handler
{
public:
  typedef void result_type;

  post_handler(Dispatcher dispatcher, Handler handler)
    : dispatcher_(dispatcher),
      handler_(handler)
  {}

  void operator()()
  {
    dispatcher_.post(handler_);
  }

  template <typename Arg1>
  void operator()(Arg1 arg1)
  {
    dispatcher_.post(boost::bind(handler_, arg1));
  }

  template <typename Arg1, typename Arg2>
  void operator()(Arg1 arg1, Arg2 arg2)
  {
    dispatcher_.post(boost::bind(handler_, arg1, arg2));
  }

  Dispatcher dispatcher_;
  Handler handler_;
};

// Custom invocation hooks for post_handler.  These must be declared in 
// post_handler's associated namespace for proper resolution.

template <typename Function, typename Dispatcher, typename Handler>
inline void asio_handler_invoke(Function& function,
    post_handler<Dispatcher, Handler>* this_handler)
{
  this_handler->dispatcher_.post(
      boost::asio::detail::rewrapped_handler<Function, Handler>(
        function, this_handler->handler_));
}

template <typename Function, typename Dispatcher, typename Handler>
inline void asio_handler_invoke(const Function& function,
    post_handler<Dispatcher, Handler>* this_handler)
{
  this_handler->dispatcher_.post(
      boost::asio::detail::rewrapped_handler<Function, Handler>(
        function, this_handler->handler_));
}

/// @brief Factory function used to create handlers that post through the
///        dispatcher.
template <typename Dispatcher, typename Handler>
post_handler<Dispatcher, Handler>
wrap_post(Dispatcher dispatcher, Handler handler)
{
  return post_handler<Dispatcher, Handler>(dispatcher, handler);
}

/// @brief Convenience factory function used to wrap handlers created from
///        strand.wrap.
template <typename Dispatcher, typename Handler>
post_handler<Dispatcher, 
             boost::asio::detail::wrapped_handler<Dispatcher, Handler> >
wrap_post(boost::asio::detail::wrapped_handler<Dispatcher, Handler> handler)
{
  return wrap_post(handler.dispatcher_, handler);
}

boost::asio::io_service io_service;
boost::asio::strand strand(io_service);
boost::asio::deadline_timer timer(io_service);

void a() { std::cout << "a" << std::endl; }
void b() { std::cout << "b" << std::endl; }
void c() { std::cout << "c" << std::endl; }
void d() { std::cout << "d" << std::endl; }
void noop() {}

void my_great_function()
{
  std::cout << "++my_great_function++" << std::endl;
  // Standard dispatch.
  strand.dispatch(&a);

  // Direct wrapping.
  wrap_post(strand, &b)();

  // Convenience wrapping.
  wrap_post(strand.wrap(&c))();

  // ADL hooks.
  timer.async_wait(wrap_post(strand.wrap(boost::bind(&d))));
  timer.cancel();
  std::cout << "--my_great_function--" << std::endl;
}

int main()
{
  // Execute my_great_function not within a strand.  The noop
  // is used to force handler invocation within strand.
  io_service.post(&my_great_function);
  strand.post(&noop);
  io_service.run();
  io_service.reset();

  // Execute my_great_function within a strand.
  std::cout << std::endl;
  io_service.post(strand.wrap(&my_great_function));
  strand.post(&noop);
  io_service.run();
}

其中产生以下输出：

++my_great_function++
--my_great_function--
a
b
c
d

++my_great_function++
a
--my_great_function--
b
c
d

一个稍微简单的解决方案,取决于实现细节,是适应detail :: wrapped_handler的Dispatcher类型参数.这种方法允许具有适应的Dispatcher类型的wrapped_handler在Boost.Asio的其余部分中透明地使用.

/// @brief Class used to adapter the wrapped_handler's Dispatcher type
///        requirement to post handlers instead of dispatching handlers.
template <typename Dispatcher>
struct post_adapter
{
  post_adapter(Dispatcher& dispatcher)
    : dispatcher_(dispatcher)
  {}

  template <typename Handler>
  void dispatch(const Handler& handler)
  {
    dispatcher_.post(handler);
  }

  Dispatcher dispatcher_;
};

/// @brief Factory function used to create handlers that post through an
///        adapted dispatcher.
template <typename Dispatcher, typename Handler>
boost::asio::detail::wrapped_handler<post_adapter<Dispatcher>, Handler>
wrap_post(Dispatcher& dispatcher, Handler handler)
{
  typedef post_adapter<Dispatcher> adapter_type;
  return boost::asio::detail::wrapped_handler<
    adapter_type, Handler>(adapter_type(dispatcher), handler);
}

/// @brief Convenience factory function used to wrap handlers created from
///        strand.wrap.
template <typename Dispatcher, typename Handler>
boost::asio::detail::wrapped_handler<
  post_adapter<Dispatcher>, 
  boost::asio::detail::wrapped_handler<Dispatcher, Handler> >
wrap_post(boost::asio::detail::wrapped_handler<Dispatcher, Handler> handler)
{
  return wrap_post(handler.dispatcher_, handler);
}

两个wrap_post解决方案可能引入关于定义的order of handler invocations的复杂程度.

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