Background

Recently, MentorPageMentorManager::loadMentorUser started to use an in-process cache for the mentor, with indefinite expiry. However, this cache is never purged, while it should be invalidated when the MentorPageMentorManager::setMentorForUser method gets called, to ensure the proper mentor is loaded.

Impact

This is IMO doomed to cause troubles, if it is not a part of T275429: Homepage mentor is not stored persistently at Romanian Wikipedia already. Basically a request which first loads a mentor, then changes it, and then loads it again will have incorrect mentor loaded. Unsure whether that happens frequently in our codebase.