id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
29403	Make PyLibMCCache backend handle TooBig exception from pylibmc	SKisContent	Hasan Ramezani	"I'm trying to use the memcached backend cache in a project, and I'm get a TooBig error on some requests. The error occurs in ''django/core/cache/backends/memcached.py'' on line 86:

{{{
        val = self._cache.get(key)
        if val is None:
            return default
        return val
    def set(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
        key = self.make_key(key, version=version)
**        if not self._cache.set(key, value, self.get_backend_timeout(timeout)):**
            # make sure the key doesn't keep its old value in case of failure to set (memcached's 1MB limit)
            self._cache.delete(key)
    def delete(self, key, version=None):
        key = self.make_key(key, version=version)
        self._cache.delete(key)
}}}


I can reproduce it using the Django shell:

{{{
$ ./manage.py shell
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type ""help"", ""copyright"", ""credits"" or ""license"" for more information.
(InteractiveConsole)
>>> from django.core.cache import cache
>>> cache.get('foo')
>>> cache.set('foo', 'bar')
>>> cache.get('foo')
'bar'
>>> big = [i for i in range(1024*1024)]
>>> cache.set('big_foo', big)
Traceback (most recent call last):
  File ""<console>"", line 1, in <module>
  File ""/local/lib/python2.7/site-packages/django/core/cache/backends/memcached.py"", line 86, in set
    if not self._cache.set(key, value, self.get_backend_timeout(timeout)):
TooBig

}}}

I can replicate the same behavior in pylibmc:

{{{
$ python
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type ""help"", ""copyright"", ""credits"" or ""license"" for more information.
>>> from pylibmc.test import make_test_client
>>> mc = make_test_client()
>>> mc.get('foo')
>>> mc.set('foo', 'bar')
True
>>> mc.get('foo')
'bar'
>>> big = [i for i in range(1024*1024)]
>>> mc.set('big_foo', big)
Traceback (most recent call last):
  File ""<console>"", line 1, in <module>
TooBig
}}}


The TooBig exception was added to pylibmc in 2016: [https://github.com/lericson/pylibmc/commit/736e21276ad04d557b68bd81b7f28be1a4b4e1ec]

The uncaught error breaks the application. Looking at the comment about the 1MB size limit on the next line (line 87), it seems like the original intent was to unset the value and fail silently. Therefore, the **if** block needs to be wrapped in **try...except**. "	Cleanup/optimization	closed	Core (Cache system)	1.11	Normal	needsinfo	memcached, TooBig		Accepted	1	0	0	0	0	0