AttributeError: 'coroutine' object has no attribute 'has_header' (Django 3.1)

Hi,

I cannot use this middleware with an ASGI server. I tried with Daphne and Uvicorn, they both give me the following error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.7/site-packages/django/utils/deprecation.py", line 116, in __call__
    response = self.process_response(request, response)
  File "/usr/local/lib/python3.7/site-packages/corsheaders/middleware.py", line 111, in process_response
    patch_vary_headers(response, ["Origin"])
  File "/usr/local/lib/python3.7/site-packages/django/utils/cache.py", line 286, in patch_vary_headers
    if response.has_header('Vary'):
AttributeError: 'coroutine' object has no attribute 'has_header'

Steps to reproduce:

1. pip install django==3.1 daphne
2. django-admin startproject example
3. Add corsheaders in INSTALLED_APPS
4. Add corsheaders.middleware.CorsMiddleware in MIDDLEWARE
5. daphne example.asgi:application
6. curl 127.0.0.1:8000 (in another terminal)

Best regards, Benoit

[kordian-kowalski]

After investigating the issue, I found out that Django seems to pass a coroutine instead of the response to the first middleware’s process_response on the list, which usually ends up being this corsheaders. All other middlewares receive a response object. I’m not sure if this is by design or a bug in Django. Creating a dummy middleware that does nothing and placing it in front of corsheaders seems to “solve” the issue.

[kordian-kowalski]

@xtof-osd This is more of a workaround: https://docs.djangoproject.com/en/3.1/topics/async/#async-views

You will only get the benefits of a fully-asynchronous request stack if you have no synchronous middleware loaded into your site. If there is a piece of synchronous middleware, then Django must use a thread per request to safely emulate a synchronous environment for it.

but if someone’s in a pinch this can be done in settings.py right now: (see my other comment below)

import corsheaders.middleware
corsheaders.middleware.CorsMiddleware.async_capable = False