Locmem cache.add() fails with infinitely cached objects

[Malcolm Box]

Simple test case:

from django.core.cache.backends.locmem import LocMemCache

c2 = LocMemCache("locmem", {})

In [18]: c2.set('test', 'locked', timeout=None)

In [19]: c2.add('test', 'new')
Out[19]: True

In [20]: c2.get('test')
Out[20]: 'new'

Since a timeout of None now means "cache forever", the cache.add() should not succeed and should return False.

Memcached behaviour for comparison:

In [24]: from django.core.cache.backends.memcached import PyLibMCCache

In [25]: memcache = PyLibMCCache(server="localhost:11211", params={})

In [26]: memcache.set('test', 'locked', timeout=None)

In [27]: memcache.add('test', 'new')
Out[27]: False

[Malcolm Box]

Testcase and fix available here: ​https://github.com/mbox/django/tree/ticket_22495

Pull request: ​https://github.com/django/django/pull/2603

[Claude Paroz <claude@…>]

In af5f688392b7fc110d06660ff91f79c742ae751d:

Fixed #22495 -- Locmem cache.add() failed with infinite timeouts

cache.add() incorrectly succeeded when there was an existing key
with an infinite (None) timeout.

[Claude Paroz <claude@…>]

In 07235aec9d0853b4fa58629c572912786b6270c9:

[1.7.x] Fixed #22495 -- Locmem cache.add() failed with infinite timeouts

cache.add() incorrectly succeeded when there was an existing key
with an infinite (None) timeout.
Backport of af5f688392 from master.