When using i18n_patterns and prefix_default_language=False, 404 page redirects incorrectly

[Hovi]

Hello,

I just migrated to 1.10.2 using new cool localization with prefix_default_language=False, replacing solid-i18n-urls:
​https://github.com/st4lk/django-solid-i18n-urls
Only smaller issue is, that whenever I go to 404 page, I get redirected to /en/THE_OLD_URL/ (en is default language in my case).

My guess is bug in LocaleMiddleware, language_path variable is being constructed always using language code even in cases when there shouldn't be (default language).
I attached simple fix, that seems to work it out for me.

[Tim Graham]

Can you add a test? (Just to be sure, #27063 doesn't solve the issue?)

[Hovi]

It's definitely not same thing as #27063. I took a deeper look and it is a little more complicated.
When I just added test, it was passing anyway and it turned out problem is little more specific to my case (it is still bug though).
I have something like this in my url as last record without i18npatterns:

r'^{(?P<group1>.+)/(?P<group2>.+)/$}

So what exactly happens here:
1, language_path is constructed as /en/non-existent-page instead of /non-existent-page
2, language_path passes is_valid_path(language_path,... because that url matches my regex.
3, that leads to redirect

Code after that constructs url for redirect (variable language_url) also doesn't take into account possibility default language without prefix, so that part may also need reviewing but I am not even sure if that can ever happen.

Attaching simple test.

[Tim Graham]

I haven't looked into the details, but the new test isn't passing. Please send a pull request if you're able to improve the patch.

[Hovi]

Yeah, new merged tests actually broke it by coincidentally using similar url pattern.

​https://github.com/django/django/pull/7461

[François Freitag]

I've left a comment on the PR.

[Krzysztof Urbaniak]

I've submitted pull request ​https://github.com/django/django/pull/7483 with a improved version of the patch.

[François Freitag]

The ​PR looks good to me. Thanks !

[Tim Graham <timograham@…>]

In b8a815e9:

Fixed #27402 -- Fixed incorrect LocaleMiddleware redirects with prefix_default_language=False.

[Tim Graham <timograham@…>]

In 81b5971b:

[1.10.x] Fixed #27402 -- Fixed incorrect LocaleMiddleware redirects with prefix_default_language=False.

Backport of b8a815e9dfea89034ede7ff786551f89af84a31b from master