The main limitation of this version is that it's controlled entirely
from settings, with nothing in the database and no web UI or even
management command to control it. That makes it a bit more of a
burden for the server admins than it'd ideally be, but that's fine
for now.
Relatedly, the web flow for realm creation still requires choosing a
subdomain even if the realm is destined to live at an alias domain.
Specific to the dev environment, there is an annoying quirk: the
special dev login flow doesn't work on a REALM_HOSTS realm. Also,
in this version the `add_new_realm` and `add_new_user` management
commands, which are intended for use in development environments only,
don't support this feature.
In manual testing, I've confirmed that a REALM_HOSTS realm works for
signup and login, with email/password, Google SSO, or GitHub SSO.
Most of that was in dev; I used zulipstaging.com to also test
* logging in with email and password;
* logging in with Google SSO... far enough to correctly determine
that my email address is associated with some other realm.
If a Zulip install at example.org got a request at an HTTP `Host`
like foo.example.org.evil.com (or even foo.example.orgevil.com),
we would accept it as subdomain foo. This isn't likely to happen
in practice because it shouldn't pass ALLOWED_HOSTS, and it's not
obvious to me that anything untoward could be done with it even
if ALLOWED_HOSTS were set wide open, but if nothing else it
multiplies the cases in analyzing this logic.
The reason we had a loose match like this, I assume, is to allow
the user to come from arbitrary ports -- especially in development.
So tighten the pattern to allow just that, and add some tests for
that behavior and a comment explaining why this complication is
needed.
Greg Price