An incorrect language is sometimes displayed - redirects adding a language code to a path can be inappropriate cached by HTTP caches

[Alex Hayward]

We have a multi-lingual site using LocaleMiddleware. With a CDN and caching reverse proxy in use, the following sequence of events can happen:

• Fetch ​https://example.com/some-page/ with Accept-Language header 'de-DE,de;q=0.9'. A redirect to ​https://example.com/de/some-page/ is returned.
• Fetch it again with Accept-Language header 'fr-FR,fr;q=0.9'. The cached redirect is returned by HTTP caches and the German page is displayed instead of the French one.

LocaleMiddleware is issuing a redirect based on the detected language from the Accept-Language header or from the cookie chosen by settings.LANGUAGE_COOKIE_NAME but is not telling caches that it has used these headers.

Adding a Vary header fixes this.

[Alex Hayward]

I've created a pull request, ​https://github.com/django/django/pull/14416

[Carlton Gibson]

OK, I think this looks right. Let's take it for review to get other eyes on it. Thanks.

[Carlton Gibson <carlton@…>]

In eeed488a:

Fixed #32768 -- Added Vary header when redirecting to prefixed i18n pattern.

get_language_from_request() uses Accept-Language and/or Cookie to determine the
correct redirect. Upstream caches need the matching Vary header to cache the
result.