Function set_fully_read 

async fn set_fully_read(
    services: &Services,
    room_id: &RoomId,
    user_id: &UserId,
    event: &EventId,
    allow_backward: bool,
) -> Result

Stores event as the user’s fully-read marker unless that would move the marker backwards.

The marker moves only when event sits strictly after the stored one or either position does not resolve, so writes that arrive one after another never move it back; two in flight at once are not ordered against each other. MSC4446’s allow_backward skips the comparison. Positions compare on the global counter, whichever room the event belongs to.