Redis cache.clear() should respect KEY_PREFIX

[Peter Baumgartner]

The Redis cache backend clear method calls the Redis command flushdb. This makes sense when KEY_PREFIX is not defined and the user has access to the full keyspace.

Current Behavior

If a user has defined a KEY_PREFIX, it is likely they don't have access to the full keyspace or don't want the Redis cache changing keys outside the KEY_PREFIX. In these scenarios, the user either gets a permissions error calling cache.clear() or deletes keys that are outside what the Django cache manages.

Expected Behavior

When KEY_PREFIX is defined, cache.clear() only deletes keys with that prefix.

Possible solutions

​This StackOverflow answer benchmarks a few different ways to delete all keys with a specific prefix.

django-redis has a delete_pattern method ​https://github.com/jazzband/django-redis#scan--delete-keys-in-bulk

[Mariusz Felisiak]

This behavior is consistent with other backends and is ​documented so it's not a bug:

"Be careful with this; clear() will remove everything from the cache, not just the keys set by your application"

We agreed in #33012 to keep the builtin Redis backend minimal and consistent with other backends. It looks like you should use django-redis for your use case.