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Subject: Re: [boost] [thread] Do we can a non backward compatible version with non-blocking futures?
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-05-28 01:53:16

Le 27/05/15 18:55, Lee Clagett a écrit :
> On Wed, May 27, 2015 at 1:57 AM, Vicente J. Botet Escriba <
> vicente.botet_at_[hidden]> wrote:
>> Le 26/05/15 23:43, Lee Clagett a écrit :
>> On Sun, May 24, 2015 at 7:35 AM, Vicente J. Botet Escriba <
>>> vicente.botet_at_[hidden]> wrote:
>>> Le 23/05/15 19:05, Lee Clagett a écrit :
>>>> On Thu, May 21, 2015 at 5:13 PM, Vicente J. Botet Escriba <
>>>>> vicente.botet_at_[hidden]> wrote:
>>>>> Le 21/05/15 17:11, Lee Clagett a écrit :
>>>>>> A non-blocking future::then() is convenient for cases like the one
>>>>>>> Vicente
>>>>>>> described, but can the executor framework simulate the same behavior?
>>>>>>> Please , could you elaborate?
>>>>>> boost::async and boost::future::then seem error prone due to ownership
>>>>> of
>>>>> the execution handle.
>>>>> Are talking here of blocking or non-blocking futures?
>>>> I was referring to non-blocking futures.
>>> If the destructor of the boost::future automatically
>>>>> releases ownership, the execution handle cannot be managed.
>>>>> Why?
>>>> It was incorrect of me to say that the execution handle cannot be
>>> managed
>>> (see below). I thought the separate executor argument to boost::async
>>> could
>>> make it more obvious that the executor object would control the lifetime
>>> of
>>> the execution thread, and that it could prevent possible data races. I
>>> doubt it will make data races to stack references easier to identify, but
>>> it should help prevent threads after main, since the executor object could
>>> force all execution threads to stop on its destruction instead. It wasn't
>>> obvious previously, but I was wondering whether the detach on future
>>> destruction should only be enabled when boost::async takes an executor
>>> overload.
>>> It should
>>>>> prevent threads after main, which never seem like a good idea.
>>>>> Sorry, I don't understand, it should prevent what?
>>>> Static destruction could be complicated if there were multiple execution
>>> threads after main. That was another issue brought up about not blocking
>>> on
>>> destruction for the std async/future versions.
>> So, you are saying that async() should return a blocking future if there
>> is no executor parameters, and a non-blocking when the executor is given,
>> isn't it?
> Yes, that is what I am saying. My thinking was that the behavior for the
> corresponding std functions were chosen as the least worst alternative, and
> that boost should follow that behavior. When an executor is provided as an
> argument, the situation is slightly different, and the problems that led to
> the blocking decision _might_ go away. The behavior differences between
> overloads might provide more confusion though.
 From your and Gor comments I should preserve the async blocking future
behavior when no executor is given, deprecated the interface taking an
executor and let the the function taking the executor return a future
non-blocking at destruction. I would need a new name for this function,
what do you prefer, submit/spawn/post?

 From Gor comments, then() must return a non-blocking future, that will
have all the troubles you are mentioning, so there will be not coherency.

I believe that misunderstood the blocking future feature from the
begining. Currently only the destructor of the last future pointing to
the shared state block. Before this it was the destructor of the shared
state that blocked. The RAII you suggested will block on the destructor
of all the futures (which is much more simple). Is this what people
expect from a blocking future.

The current blocking implementation is too error prone. I would prefer
to not support it anymore. I will see if I can move to a blocking future
that blocks on destructor if valid as your RAII.

Should the shared futures coming from a blocking future block on
destruction as well?


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