FileBasedCache has_key is susceptible to race conditions

[Marti Raudsepp]

I received the exception from Django's cache framework:

FileNotFoundError: [Errno 2] No such file or directory: '/app/var/cache/d729e4cf4ba88cba5a0f48e0396ec48a.djcache'
[...]
  File "django/core/cache/backends/base.py", line 229, in get_or_set
    self.add(key, default, timeout=timeout, version=version)
  File "django/core/cache/backends/filebased.py", line 26, in add
    if self.has_key(key, version):
  File "django/core/cache/backends/filebased.py", line 94, in has_key
    with open(fname, "rb") as f:

The code is:

    def has_key(self, key, version=None):
        fname = self._key_to_file(key, version)
        if os.path.exists(fname):
            with open(fname, "rb") as f:
                return not self._is_expired(f)
        return False

Between the exists() check and open(), it's possible for the file to be deleted. In fact, the _is_expired() method itself deletes the file if it finds it to be expired. So if many threads race to read an expired cache at once, it's not that unlikely to hit this window.