I was trying to create a unit test for this situation but could not emulate different sessions.

There is a session cache
$cache = cache::make_from_params(cache_store::MODE_SESSION, ...);

I have two users simultaneously working with a website. One of them updates information and purges the cache. The second one still sees the same data it was before purging. If the second user logs out and in, he can see the updated data.

I would expect that when cache is purged, it is purged for all users.

Also I wrote several unit tests that fail:
https://github.com/marinaglancy/moodle/compare/master...wip-MDL-38165-master