Allow to set continuation future's destructor behavior to non-blocking

[Konrad Zemek <konrad.zemek@…>]

As ​discussed on the mailing list:

Currently, continuation futures created using future<>::then() with boost::launch::async or an executor block on destruction (until the value is ready).

I would like there to be a way to disable this behavior, whether through an additional argument in future<>::then() (much like boost::defer_lock in locks' constructors), a method explicitly setting the non-blocking behavior, or implicitly through a call to future<>::wait_for/until().

[viboes]

​https://github.com/boostorg/thread/commit/88ab663ac58a2a903ee26b47a4eedb57535c4d14

[viboes]

4321b59c1eedde71da58f592aa4c741a1feff046

[anonymous]

The blocking option seems to be broken. Future destructor doesn't block even if BOOST_THREAD_FUTURE_BLOCKING is defined and the futture was obtained by a call to boost::async() with boost::launch::async, or by calling other_future.then().

int main()
{
    {
        auto future = boost::async(boost::launch::async, []() {
            std::this_thread::sleep_for(1s);
            return 42;
        });
    } //future destructor doesn't block

    //do stuff...
    //-> will heppen before the future state is ready
}

[anonymous]

Replying to anonymous:

The blocking option seems to be broken. Future destructor doesn't block even if BOOST_THREAD_FUTURE_BLOCKING is defined and the futture was obtained by a call to boost::async() with boost::launch::async, or by calling other_future.then().

int main()
{
    {
        auto future = boost::async(boost::launch::async, []() {
            std::this_thread::sleep_for(1s);
            return 42;
        });
    } //future destructor doesn't block

    //do stuff...
    //-> will heppen before the future state is ready
}

The future destructor is never called even though the variable 'future' goes out of scope. See ticket #12220 for details.