Opened 2 years ago

Last modified 2 years ago

#34209 closed Bug

FileBasedCache has_key is susceptible to race conditions — at Initial Version

Reported by: Marti Raudsepp Owned by: nobody
Component: Core (Cache system) Version: 4.1
Severity: Normal Keywords: race, race-condition, FileBasedCache
Cc: Triage Stage: Ready for checkin
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

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.

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