Decorator cache_page don't use CACHE_MIDDLEWARE_KEY_PREFIX properly

[Iuri de Silvio]

The cache_page decorator don't use the setting because it always pass key_prefix=None and CacheMiddleware consider it a valid config, so it overrides the default one.

​https://github.com/django/django/blob/854e9b066850b9b4eb1171966e996322b2c16d27/django/views/decorators/cache.py#L24
​https://github.com/django/django/blob/ca9872905559026af82000e46cde6f7dedc897b6/django/middleware/cache.py#L175

I discovered the issue because I was moving from Django 3.1 to Django 3.2 and a 3.1 HttpResponse is not compatible with 3.2 HttpResponse (which I think is ok, maybe could be better documented), so I have to clean the cache changing my key prefix with the CACHE_MIDDLEWARE_KEY_PREFIX.

[Iuri de Silvio]

I created a patch. Did some comments to it because I'm not totally happy with the result, but it works.

[Carlton Gibson]

Hi.

CACHE_MIDDLEWARE_KEY_PREFIX is for ​per-site caching not for cache_page, which allows you to set an additional prefix but is not intended to use the site-level settings.

[Iuri de Silvio]

I misread the docs, expected it to work with cache_page. Unfortunately, Django don't have an easy way for cleaning only cache_page cache, I'll make a cache page wrapper on my side. Thanks!

[Carlton Gibson]

No problem. See #5815 for a suggestion to add easier invalidation here. It's just looking for a hero to pick it up. 😉