cache.get_or_set() works incorrectly with DummyCache backend

[Oleksiy Ivanenko]

There is some issue when you use django.core.cache.cache.get_or_set with DummyCache backend.

from django.core.cache import cache

cache.get_or_set('some_key', 'my_val')  # Always returns None. Not a default value

This thing brakes tests.
I think it will be better to return default value from dummy cache.

[Tim Graham]

A regression test is also required. If you can submit your patch as a pull request, that's ideal. Thanks!

[David Szotten]

one might argue that this is a bug in the BaseCache implementation of get_or_set:

def get_or_set(...)
    # [snip]
    val = self.add(key, default, timeout=timeout, version=version)
    if val:
        # unlikely, but we might have been added, and evicted again by the time we get here,
        # should we not call `get` with the default?
        return self.get(key, version=version)

[Oleksiy Ivanenko]

My solution provided as PR - ​https://github.com/django/django/pull/5751

[Emre Yılmaz]

Missed the above PR, but here is my approach. (different solution)

​https://github.com/django/django/pull/5756

[Tim Graham <timograham@…>]

In 8e838d9:

Fixed #25840 -- Fixed BaseCache.get_or_set() on the DummyCache backend.

This also fixes a possible data eviction race condition between
setting and getting a key. Another thread could remove the key
before get_and_set() accesses it again. In this case, now the
default value will be returned instead of None.

[Tim Graham <timograham@…>]

In 9733ff5:

[1.9.x] Fixed #25840 -- Fixed BaseCache.get_or_set() on the DummyCache backend.

This also fixes a possible data eviction race condition between
setting and getting a key. Another thread could remove the key
before get_and_set() accesses it again. In this case, now the
default value will be returned instead of None.

Backport of 8e838d9c869083597dc9e161ae2fe37acaa90de9 from master