Directory creation race condition in FileSystemStorage

[Pi Delport]

My company recently encountered the following problem on a production server:

Traceback (most recent call last):

[...]

 File "[...]", line 191, in process_view
   storage.save(path, File([...]))

 File "[...]/python2.6/site-packages/django/core/files/storage.py", line 47, in save
   name = self._save(name, content)

 File "[...]/python2.6/site-packages/django/core/files/storage.py", line 146, in _save
   os.makedirs(directory)

 File "[...]/python2.6/os.py", line 157, in makedirs
   mkdir(name, mode)

OSError: [Errno 17] File exists: '[MEDIA_ROOT]/avatars/foo'

I tracked this down to a race condition in FileSystemStorage._save. When there is enough load, two concurrent processes can attempt to create the same intermediate directory, causing the second one to fail entirely.

To handle this robustly, FileSystemStorage should catch the EEXIST from os.makedirs and continue normally: I implemented this along with some tests, in the attached patch.

Note: For comparison, Python 3.2 added a keyword argument to os.makedirs to do this: see ​Issue 9299: os.makedirs(): Add a keyword argument to suppress "File exists" exception.

[Aymeric Augustin]

The patch looks good and contains extensive tests. It applies cleanly and the test suite passes. The implementation doesn't add much overhead. It's a fix for an edge case, so it does not deserve a mention in the release notes.

[Pi Delport]

Thank you. :)

[Jannis Leidel]

In [16280]:

Fixed #16082 -- Fixed race condition in the FileSystemStorage backend with regard to creating directories. Thanks, pjdelport.

[Aymeric Augustin]

#16108 describes a similar race condition in the file removal code.

[Jannis Leidel]

In [16287]:

Fixed #16108 -- Fixed another race condition in the FileSystemStorage backend with regard to deleting a file. Refs #16082, too. Thanks, Aymeric Augustin.