Re: [Boost-users] boost::asio::deadline_timer destruction

Subject: Re: [Boost-users] boost::asio::deadline_timer destruction
From: Daniel (zin_corp_at_[hidden])
Date: 2013-04-19 14:01:12

Igor,

Why deadline_timer doesn't wait WaitHandler completion?

I don't understand why did you delete object from wait handler...

I meant a little bit different thing like this:

class graceful_deadline_timer : public boost::asio::deadline_timer
  {
    typedef boost::asio::deadline_timer super;
    boost::function1<void, const boost::system::error_code&> handler;
    boost::condition_variable_any wait_handler_done;
    boost::recursive_mutex wait_handler_lock;
    bool expired;

    void handler_wrapper(const boost::system::error_code &ec)
    {
      boost::lock_guard<boost::recursive_mutex> lock(wait_handler_lock);
      handler(ec);
      expired = true;
      wait_handler_done.notify_all();
    }

    public:

    explicit graceful_deadline_timer(boost::asio::io_service& io_service) : super(io_service), expired(true)
    {
      
    }

    ~graceful_deadline_timer()
    {
        boost::unique_lock<boost::recursive_mutex> lock(wait_handler_lock);

        if(!expired)
        {
          cancel();
          
          
          while(!expired)
          {
            wait_handler_done.wait(lock);
          }
        }
    
    }
    
    template <typename WaitHandler>
    void async_wait(WaitHandler _handler)
    {
      boost::lock_guard<boost::recursive_mutex> lock(wait_handler_lock);
      handler = _handler;
      expired = false;
      
      super::async_wait(boost::bind(&graceful_deadline_timer::handler_wrapper, this, boost::asio::placeholders::error));
      
    }
  };

Thank you,
Daniel

> >> > I've noticed that WaitHandler can be called after
> >> > boost::asio::deadline_timer destruction. It may cause using of uninitialized
> >> > data. Are there any other solutions of graceful destruction of
> >> > boost::asio::deadline_timer to be sure that after this destruction
> >> > WaitHandler will never be called?
> >>
> >>
> >> Yes, the solution is to use shared_from_this idiom.
> >
> <...>
> > But it requires of changing construction model. What if I want allocate objects at stack?
> > May be there are some technique which doesn't require from user to use shared_ptr technique?
> 
> 
> The bottom line is that the completion handler should outlive its
> async. operation. So you have 2 ways:
> 1) To create an "automatic linkage" between the operation and the
> handler, i.e. to cause the handler to be alive as long as the
> operation is in progress. Eg., you can pass the handler functor by
> value or by smart ptr.
> 2) To control the handler lifetime manually. No techniques here, just
> ad-hoc solution that matches your specific scenario.
> Of course the 1st approach is much more robust and preferable in most cases.
> (Note that it was not necessary to create the completion handler as a
> binder based on an object containing the i/o object that initiates the
> async. operation -- it was your deliberate choice... And by the way,
> it seems that you attempt to cancel the timer from a non-io-service
> thread, don't you? If yes, it's not thread-safe.)
> 
> 
> > For example why boost::asio::deadline_timer::~deadline_timer doesn't wait completion of WaitHandler?
> 
> It can't because it doesn't dispatch completion handlers. It's
> io_service who is in charge of this.
> But even if it could, it would be rather a problem than a solution.
> Consider the following trivial program:
> 
> #include <boost/asio.hpp>
> #include <boost/date_time/time_duration.hpp>
> #include <boost/bind.hpp>
> 
> using namespace boost;
> using namespace boost::asio;
> using namespace boost::posix_time;
> 
> void f(const system::error_code&, const deadline_timer *t)
> {
>   delete t; // to wait or not to wait?
> }
> 
> int main()
> {
>   io_service io;
>   deadline_timer *t = new deadline_timer(io);
>   t->expires_from_now(seconds(10));
>   t->async_wait(boost::bind(&f, _1, t));
>   io.run();
> }
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