Fall back to checking power levels when sourcing local restricted join users

[nexy7574]

Fix #19120 by always falling back to checking power levels for local users if a local creator cannot be found in a v12 room.

Complement tests: matrix-org/complement#836

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[MadLittleMods]

This kind of change deserves a test of some sort to ensure we don't regress into the future. It's probably easiest to write a Complement test to setup this kind of scenario. Is this something you'd be comfortable with tackling?

Probably something in tests/restricted_rooms_test.go

As an example of running Complement tests for the Synapse project, here is how to run a single test:

COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh ./tests -run TestRestrictedRoomsLocalJoin

(checkout the complement repo as a sibling to the synapse repo)

[nexy7574]

Sure, I'll give it a shot

[nexy7574]

Opened matrix-org/complement#836, let me know if I missed a step or anything.

[MadLittleMods]

The Complement tests look good 👍. Thank you for creating a PR there as well. We will just wait to merge this PR until the Complement side is close to merge as well ⏩

[MadLittleMods]

The fix itself looks good 👍

In terms of the surrounding context, it's weird that we use both get_users_which_can_issue_invite(...) and get_user_which_could_invite(...) as the disparity between them is what's causing the problem in the first place. As a follow-up PR, it would probably be good to settle one direction or the other.

synapse/synapse/handlers/room_member.py

Lines 1308 to 1341 in 7a24faf

	# If the local host has a user who can issue invites, then a local
	# join can be done.
	#
	# If not, generate a new list of remote hosts based on which
	# can issue invites.
	event_map = await self.store.get_events(state_before_join.values())
	current_state = {
	state_key: event_map[event_id]
	for state_key, event_id in state_before_join.items()
	}
	allowed_servers = get_servers_from_users(
	get_users_which_can_issue_invite(current_state)
	)
	# If the local server is not one of allowed servers, then a remote
	# join must be done. Return the list of prospective servers based on
	# which can issue invites.
	if self.hs.hostname not in allowed_servers:
	return True, list(allowed_servers)
	# Ensure the member should be allowed access via membership in a room.
	await self.event_auth_handler.check_restricted_join_rules(
	state_before_join, room_version, user_id, previous_membership
	)
	# If this is going to be a local join, additional information must
	# be included in the event content in order to efficiently validate
	# the event.
	content[
	EventContentFields.AUTHORISING_USER
	] = await self.event_auth_handler.get_user_which_could_invite(
	room_id,
	state_before_join,
	)

[MadLittleMods]

Tracked by #19374