manage.py compilemessages may skip locale folders if ignore is used.

[Andrew Cordery]

manage.py compilemessages uses os.walk and iterates through the same dirnames instance that it removes members from when the ignore option is used.

This will cause compilemessages to accidentally ignore a locale directory that directly follows a skipped directory in the list. Unfortunately it is not easy to predict when this will occur or which directories may be affected as os.walk provides the list of dirnames in 'arbitrary' order according to the python docs.

I've provided the miniscule patch to fix this as well as a much lengthier regression test, however I'm not positive that the test is capable of recreating the correct conditions on all operating systems. I've tested on OSX only. Regardless, the issue at hand is a basic language error (modifying a value that you are iterating through) and the fix is extremely simple (i.e. copy dirnames before iterating, in my patch I just wrap it in list()), so this should not be a controversial patch.

Note: this was originally mentioned as a comment on my other ticket https://code.djangoproject.com/ticket/34925, I'm separating it for clarity and I will attach the PR here.

[Andrew Cordery]

Here is the PR: ​https://github.com/django/django/pull/17452

[Natalia Bidart]

Thank you! Accepting.

[Mariusz Felisiak]

Fixed by ad41f1c53aa9f2c938df32e4386d8a80138923fc.