If executor has been cancelled should not spin

[mauropasse]

This simple code would hang forever:

  ExecutorType executor; // Any executor
  auto executor_thread = std::thread([&](){ executor.spin();});
  executor.cancel();
  executor_thread.join();

If executor.cancel() gets called before executor.spin(), cancel won't have any effect and the executor will keep spinning.

In this PR I add an extra bool to check if the executor was cancelled before blocking in spin()

[alsora]

I'm not convinced by this approach.
The following code should result in the executor spinning

auto executor = std::make_shared<executor>();
executor->cancel();
.... do some stuff ...
executor->spin(); // This should spin, the cancel that we called before should not affect it

The problem that you describe is a race condition and I think that the already existing executor->is_spinning() method is the correct way to synchronize and avoid it.

[fujitatomoya]

@mauropasse thanks for opening PR. I am with @alsora on this. executor should be able to spin after cancel.

[mauropasse]

Makes sense, a cancel() call should only affect an spinning executor, so the user should make sure the executor is spinning before cancelling.