Async-capable middleware is not async-capable

[abetkin]

All the standard django middleware is marked as async-capable. As I understand that means that sync_to_async adapters will not be used if no other middleware is present.

However, if you call an endpoint that is handled by an async view, somehow the adapters are still used, and middleware is executed in a separate thread.

Moreover, if you add your custom middleware that is async_capable than it tries not to use adapters, and quickly fails:

https://code.djangoproject.com/attachment/ticket/33716/bug.png

AttributeError: 'coroutine' object has no attribute 'get'

In this example this was XFrameOptionsMiddleware that failed

Github project illustrating this: ​https://github.com/pwtail/django_bug

Is reproduced in thae main branch and in 4.0