django.utils._os.safe_join breaks on Windows for paths that end in a slash/backslash

[gcbirzan]

On Linux:

os.path.abspath('/foo/bar/')
'/foo/bar'

On Windows:

os.path.abspath(r'c:/foo/bar/')
'c:\\foo\\bar\\'

At the beginning of the function, abspath is called on the base_path. This removes the trailing slash on Linux, but not on Windows. Further down, in order to compare whether the normalised path is under base_path, this happens:

normcase(final_path).startswith(normcase(base_path + sep))

On Windows, since base_path now has a trailing backslash, this will always fail.

There are two solutions I see here. The first is to use the fallback from ntpath, which is the same implementation as the one from posixpath.

os.path._abspath_fallback('c:/foo/bar/')
'c:\\foo\\bar'

The second is to just strip the trailing slash, as it'll be added anyway when needed. I'm, personally, leaning towards the first option...

[gcbirzan]

Ah. Actually, this only happens on Python 3.7.1. Hum. I'll just close it, I guess, since upgrading python will fix it.