Cache FetchMiddleware checks cache for auth despite UNAUTH_ONLY=True

[Yeago]

https://code.djangoproject.com/browser/django/trunk/django/middleware/cache.py#L133

The Fetch middleware makes no explicit check of its own to settings.CACHE_MIDDLEWARE_UNAUTHENTICATED_ONLY. This results in a check to the cache for the key. Currently, the only way it happens to work is because the key created by django.utils.cache.get_cache_key happens to cause a miss. If you use a simpler key which doesn't take into account the users session, the key will not miss and authenticated users will get a cached version despite the rather unambiguous setting.

Related to #17305 in the sense that this is another stumbling block for people wanting to customize the Cache Middlewares.

[Preston Holmes]

I believe the setting you are referring to is: CACHE_MIDDLEWARE_ANONYMOUS_ONLY

If a the content and associated key does not involve any user specific data, then it can be cached as if that user were an anonymous user. The logic for this is actually in UpdateCacheMiddleware._should_update_cache

see discussion and fix associated with: #13283

also note: #15201

[Yeago]

The problem I illustrate above is entirely on the Fetch side.