Sliding Sync: Rooms with new requested `required_state` aren't sent until other activity occurs

[stefanceriu]

Description

Changing the required_state but preserving the current pos offset will result in no new data being passed down.
This seems to be a bug as the spec states that "if new entries are added to required_state then the server must send down matching current state events".

Originally reported here, can provide network logs on request.

Steps to reproduce

• Have a client that preserves the sliding sync pos e.g. Element X iOS
• Update said client so it requests more state the next time it run e.g. ["m.space.parent", "*"], ["m.space.child", "*"]
• Observe how no new state gets passed down in subsequent requests.

Homeserver

matrix.org

Synapse Version

1.136.0 (b=matrix-org-hotfixes-priv,a4db588e87)

Installation Method

I don't know

Database

Workers

Single process

Platform

Configuration

No response

Relevant log output

-

Anything else that would be useful to know?

No response

[MadLittleMods]

Currently, we do send down the new requested required_state that should show up but only when other activity occurs that would cause the room to be sent down. We need to update compute_interested_rooms(...) in Synapse to account for required_state changes. We even have a relevant test for this scenario. i.e. we should have a new test just like that but without the # Send a message so the room comes down sync.

Separately, we should also test that using the same pos token while expanding required_state also works.