#34209: FileBasedCache has_key is susceptible to race conditions
-------------------------------------+-------------------------------------
Reporter: Marti Raudsepp | Owner: Marti
| Raudsepp
Type: Uncategorized | Status: assigned
Component: Core (Cache system) | Version: 4.1
Severity: Normal | Resolution:
Keywords: race, race- | Triage Stage:
condition, FileBasedCache | Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Description changed by Marti Raudsepp:
Old 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.
New 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:
{{{#!python
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.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/34209#comment:2>
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