Using async on 2.0.0rc1 got ValueError: set_wakeup_fd only works in main thread

[greyli]

Use async on Windows on Python 3.8 will get this error:

ValueError: set_wakeup_fd only works in main thread

It seems a bug in asgiref/mod_wsgi, there is already an issue in asgiref (django/asgiref#143). We may need to fix it upstream before the 2.0 version is out.

Example:

@app.get('/')
async def say_hello():
    return {'message': 'Hello!'}

Traceback:

Traceback (most recent call last):
  File "c:\users\greyli\...\venv\lib\site-packages\flask\app.py", line 1953, in wsgi_app
    response = self.full_dispatch_request()
  File "c:\users\greyli\...\venv\lib\site-packages\flask\app.py", line 1454, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "c:\users\greyli\...\venv\lib\site-packages\flask\app.py", line 1452, in full_dispatch_request
    rv = self.dispatch_request()
  File "c:\users\greyli\...\app.py", line 318, in dispatch_request
    return self.view_functions[rule.endpoint](*req.view_args.values())
  File "c:\users\greyli\...\venv\lib\site-packages\flask\helpers.py", line 781, in outer
    return async_to_sync(inner)(*args, **kwargs)
  File "c:\users\...\venv\lib\site-packages\asgiref\sync.py", line 203, in __call__
    loop_future.result()
  File "C:\Users\greyli\.pyenv\pyenv-win\versions\3.8.0\lib\concurrent\futures\_base.py", line 432, in result
    return self.__get_result()
  File "C:\Users\greyli\.pyenv\pyenv-win\versions\3.8.0\lib\concurrent\futures\_base.py", line 388, in __get_result
    raise self._exception
  File "C:\Users\greyli\.pyenv\pyenv-win\versions\3.8.0\lib\concurrent\futures\thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "c:\users\greyli\...\venv\lib\site-packages\asgiref\sync.py", line 256, in _run_event_loop
    loop.close()
  File "C:\Users\greyli\.pyenv\pyenv-win\versions\3.8.0\lib\asyncio\proactor_events.py", line 679, in 
close
    signal.set_wakeup_fd(-1)
ValueError: set_wakeup_fd only works in main thread

Environment:

• OS: Window 10
• Python version: 3.8.0
• Flask version: 2.0.0rc1

[davidism]

Does it work on Windows on Python 3.9?

[greyli]

Does it work on Windows on Python 3.9?

Yes, it works fine on Python 3.9.

[davidism]

I think we leave this as a bug in Python 3.8 or asgiref then, which may be fixed later. We can make a note of it in the docs if you think that's needed.

[greyli]

Sounds good. I'll make a PR for it tomorrow. Besides, we may also need to mention it in the pre-release announcement.