Taskflow for async

[tjhei]

sitting on top of #15610

Thank you @bangerth

[bangerth]

Yes, good to go once the other PR is merged and this one is rebased!

[bangerth]

Can you rebase on master?

[tamiko]

@bangerth This PR is now rebased. I am not sure it is wise to merge this without a testsuite run.

[tjhei]

This does not work as mentioned during the hackathon. I see deadlocks inside the call to join.

[bangerth]

Yes, we're absolutely not going to merge this without testing on many platforms :-)

[bangerth]

For reference, the issue is this one: taskflow/taskflow#575 I must be fundamentally misunderstanding how to create asynchronous tasks.

[tjhei]

What a coincidence. I was just looking at this PR again a week or so ago. When updating to the development version of taskflow, the tests using async() no longer crash for me but they do hang in an infinite loop as you describe.

[bangerth]

The reason why this patch doesn't work in its current form is described in #16935 and tested in #16934. We need to also add a way to wait for tasks to finish that informs the scheduler that that's what we're currently doing.

[bangerth]

This seems to almost work. It hangs if a task ends with an exception (task_01_exception and derivative tests), but the other task-related tests seem to all work. I will have to debug this.

[bangerth]

This is a bug in TaskFlow 3.6:

template <typename R, typename F>
auto Executor::_make_promised_async(std::promise<R>&& p, F&& func) {
  return [p=make_moc(std::move(p)), func=std::forward<F>(func)]() mutable {
    if constexpr(std::is_same_v<R, void>) {
      func();
      p.object.set_value();
    }
    else {
      p.object.set_value(func());
    }
  };
}

The function wants to put the output of func into the std::promise object, but forgets that the function could exit via an exception. I will have to check whether that is perhaps fixed in 3.7.

[bangerth]

This seems to be fixed in the pre-release snapshot of TaskFlow that I'm importing in #16892. So now I will need to update that to the actual 3.7 release that came out a couple of days ago.

[bangerth]

Superseded by #16976.