Sites cache is not multi-process safe

[Bertrand Bordage]

The site cache, consisting on a dictionary, is thread safe.
However, changing the object within a process has no effect on other processes, unless you restart all processes.
Here is a minimal example.

In a first django shell:

>>> from django.contrib.sites.models import Site
>>> Site.objects.get_current()
<Site: mysite.com>

In a second one:

>>> from django.contrib.sites.models import Site
>>> site = Site.objects.get_current()
>>> site.domain = 'monsite.fr'
>>> site.save()
>>> Site.objects.get_current()
<Site: monsite.fr>

Then back in the first shell:

>>> Site.objects.get_current()
<Site: mysite.com>

The object is not changed.

I don’t think we can fix this without using cache backends. However, this caching must be disabled with the default LocMem cache, since it’s not multi-process safe.

[Tim Graham]

Duplicate of #15894.