CachedFilesMixin url_converter doesn't handle absolute paths correctly

[lpetre]

This code works fine for an absolute url that is within one folder of static, ie: url(/static/foo/bar.png)

The bug I'm seeing happens with absolute paths and multiple subfolders: url(/static/foo/bar/yak.png)

All of the logic is within CachedFilesMixin -> url_converter -> converter

The result of the bug is: When you're using any staticfile storage that includes CachedFilesMixin, collecstatic fails (throws an exception) when it tries to update a css file that has an absolute reference to another asset that is in a subfolder.

I've created a small test application here:
​https://github.com/lpetre/django-cachedstorage-bug

I've also added regressiontests and a potential fix to a fork of the django project on github. I'll update this ticket shortly with that url.

[lpetre]

This diff adds some regression tests that demonstrate the bug

[lpetre]

This is a potential fix for this issue

[Jannis Leidel]

In [17622]:

Fixed #17806 -- Fixed off-by-n error in the CachedStaticFilesStorage that prevented it from finding files in nested directories. Many thanks to lpetre.