sessions.backend.cache.SessionStore.clear_expired doesn't clean

[Efe Öge]

Hello,

We've been using Redis as cache backend for session storage. To clean up old sessions, we run the "python manage.py clearsessions" management command. This command internally calls the clear_expired method on the associated SessionStore. If clear_expired raises a NotImplementedError, the command catches it and raises a CommandError, indicating that the session backend doesn't support session cleanup. This behavior makes sense. ​https://github.com/django/django/blob/main/django/contrib/sessions/management/commands/clearsessions.py

However, in the case of the cache backend, the clear_expired method is currently just an empty function. ​https://github.com/django/django/blob/main/django/contrib/sessions/backends/cache.py#L140

I would therefore recommend either explicitly raising a NotImplementedError here, or implementing a (potentially slow) cleanup mechanism that actually removes expired sessions.
What are your thoughts on this? If this looks like a bug to you, I’d be happy to help by sending a patch.