Make PyLibMCCache backend handle TooBig exception from pylibmc

[SKisContent]

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.

[SKisContent]

To clarify, I'm using PyLibMCCache:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.PyLibMCCache',
        'LOCATION': '127.0.0.1:11211',
        'TIMEOUT': 1000,
        'BINARY': True,
        'OPTIONS': {
            'tcp_nodelay': True,
            'remove_failed': 4
        }
    }
}

[Tim Graham]

There's ​a related pull request.

[Flavio Curella]

I'm not convinced we should fail silently. I understand that some versions o memcached will, but I'd rather be warn the user that something went wrong instead of being consistent.

If we know something went wrong we should tell the user. I'd recommend raising a more descriptive exception, possibly using raise ... from ....

[Mariusz Felisiak]

Like Flavio Curella and ​Ed Morley, I'm not convinced that we should catch this exception, it was shortly discussed in #19914. See also arguments in #28342 (this is not a duplicate but is related). The main question is: should we force consistency between these two backends? IMO it's better to respect their different policies of handling errors.

I think we should go back to the DevelopersMailingList and reach a consensus to move it forward.