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

Subject: Re: [Boost-users] boost::asio::deadline_timer destruction
From: Igor R (boost.lists_at_[hidden])
Date: 2013-04-19 16:09:19

<...>
>> 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();
>> }

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

Just to demonstrate a trivial use-case where your idea won't work.

> I meant a little bit different thing like this:
> class graceful_deadline_timer : public boost::asio::deadline_timer
>   {
<...>
>     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:
<...>
>     ~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));
>     }
>   };

Ok, now please apply your code to the case I mentioned before.
In particular, what the following code would do:

>           while(!expired)
>           {
>             wait_handler_done.wait(lock);
>           }

Seems like a deadlock, doesn't it?
(And again, all these locks in async_wait/d-tor/handler imply that you
intend to access deadline_timer from multiple threads. Note that it's
not safe as deadline_timer is not thread-safe:
http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/reference/basic_deadline_timer.html)