Clearing the sessions inside the transaction makes Zulip vulnerable to
a narrow window where the deleted session has not yet been committed,
but has been removed from the memcached cache. During this window, a
request with the session-id which has just been deleted can
successfully re-fill the memcached cache, as the in-database delete is
not yet committed, and thus not yet visible. After the delete
transaction commits, the cache will be left with a cached session,
which allows further site access until it expires (after
SESSION_COOKIE_AGE seconds), is ejected from the cache due to memory
pressure, or the server is upgraded.
Move the session deletion outside of the transaction.
Because the testsuite runs inside of a transaction, it is impossible
to test this is CI; the testsuite uses the non-caching
`django.contrib.sessions.backends.db` backend, regardless. The test
added in this commit thus does not fail before this commit; it is
merely a base expression that the session should be deleted somehow,
and does not exercise the assert added in the previous commit.
e6eace307e