Function fallback_shortstatehash 

async fn fallback_shortstatehash(
    services: &Services,
    room_id: &RoomId,
    sender_user: &UserId,
    bypass_visibility: bool,
) -> Result<Option<ShortStateHash>>

Selects the state a requester reads in place of an event’s missing state.

Admitted requesters reach the selection, which checks access again; departed members read their departure snapshot, admins included. Only an admin may read current state when the selection admits none.