Fixes#13504.
This commit is purely an improvement in error handling.
We used to not do any validation on keys before passing them to
memcached, which meant for invalid keys, memcached's own key
validation would throw an exception. Unfortunately, the resulting
error messages are super hard to read; the traceback structure doesn't
even show where the call into memcached happened.
In this commit we add validation to all the basic cache_* functions, and
appropriate handling in their callers.
We also add a lot of tests for the new behavior, which has the nice
effect of giving us decent coverage of all these core caching
functions which previously had been primarily tested manually.
In the unlikely event that someone edited the properties of a system
bot and then saved the result, we were still caching the old version
indefinitely in the get_system_bot cache.
This led to a confusing case where a newly installed Zulip server
didn't have is_api_super_user properly set on its EMAIL_GATEWAY_BOT in
memcached.
Co-authored-by: Mateusz Mandera <mateusz.mandera@protonmail.com>
Mateusz Mandera
Tim Abbott
Darshan Markandaiah