asyncio is not compatible with gevent

[awoimbee]

See #5256

async-await: https://flask.palletsprojects.com/en/stable/async-await/

Using async with greenlet
When using gevent or eventlet to serve an application or patch the runtime, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is required.

Flask uses asgiref, which is not compatible with gevent: django/asgiref#443

BTW:
ASGI: https://flask.palletsprojects.com/en/stable/deploying/asgi/

The asgiref WsgiToAsgi adapter is recommended as it integrates with the event loop used for Flask’s Using async and await support.

This makes it seem like the event loop integration means a single worker could process multiple requests concurently, which it can't, the worker will block on await.

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I'm not the only one that found this the hard way: Flask, Gunicorn, Gevent and Asyncio Don't Mix

[awoimbee]

Oh no, it actually works with uwsgi.
It doesn't work either with uwsgi

RuntimeError: You cannot use AsyncToSync in the same thread as an async event loop - just await the async function directly.

[davidism]

It would be helpful if you showed a minimal example of what you are using using that causes issues.

[awoimbee]

Look at the linked flask-gunicorn-gevent-asyncio repo.
Also check the attached reproducer in benoitc/gunicorn#3070.

[davidism]

Please include the example and relevant details here. A whole Docker container is not a minimal example. I also can't tell if you're talking about gevent alone, or uwsgi, or gunicorn, or something else. Please be specific about the exact issue you are reporting.

[awoimbee]

Please include the example [...] A whole Docker container is not a minimal example.

The dockerfile is completely optional, you just need python and pip to run the example in flask-gunicorn-gevent-asyncio.

Here is the zip From the gunicorn issue: https://github.com/benoitc/gunicorn/files/12614245/reproduction.zip
It contains:

app.py
__init__.py
lots-of-curls.sh
requirements.txt

I also can't tell if you're talking about gevent alone, or uwsgi, or gunicorn, or something else.

I'm talking about gevent + asgiref (flask's async-await implementation).

asgiref does not support gevent (ref), so flask[async] cannot support gevent.
Which is fine, but then I have an issue with the docs having a "Using async with greenlet" section.

[davidism]

@pgjones could you take a look at this if you have a chance?