cached_db session with memcached forgets big values

[ccrisan]

Considering memcached's 1MB limit, putting a value larger than 1MB into a cached_db session will get written to DB but not to cache. However a previous value for the session key already exists in the cache, and thus the session gets restored with previous data from the cache, instead of the new information stored in django_session table, in the DB.
My guess is that the memcached backed should remove the key when not being able to set it to the new value.

[ccrisan]

patch for memcached.py that makes the backend clear a key if unable to set it

[ccrisan]

I've attached a small simple patch for memcached.py. I'm not sure if this is the right thing to to in this case. What should be a cache's behavior in case of failure to set a key, anyways?

[Aymeric Augustin]

This makes sense, could you:

• add a test case in tests/regressiontests/cache/tests.py?
• generate a single patch from the root of the Django repository (or make a pull request)?

[Michael Manfre]

Added a test and a pull request.

[Michael Manfre <mmanfre@…>]

In bc8abe36ba5508fe788f6d49a26749268d368583:

Fixed #16358 - Made memcache backend delete old value on a failure to set.

Default Memcached configuration allows for a maximum object of 1MB and
will fail to set the key if it is too large. The key will be deleted from
memcached if it fails to be set. This is needed to avoid an issue with
cache_db session backend using the old value stored in memcached, instead
of the newer value stored in the database.

[Michael Manfre <mmanfre@…>]

In a305695f28cb15165a19c3871aec878a230a5105:

Merge pull request #3481 from manfre/ticket-16358

Fixed #16358 - Made memcache backend delete old value on a failure to set.