Function state_at_event 

pub(crate) async fn state_at_event(
    services: &Services,
    origin: &ServerName,
    room_id: &RoomId,
    event_id: &EventId,
) -> Result<(RoomVersionId, Vec<OwnedEventId>)>

Reads the state of a room at an event for a federation response.

The origin must pass the room’s access check and the event must belong to the room. The state comes back as event IDs, beside the room’s version.