Function state_pdu 

pub(crate) async fn state_pdu(
    services: &Services,
    event_id: &EventId,
    room_version: &RoomVersionId,
) -> Option<Box<RawValue>>

Reads a state event for a federation response, or omits it.

Every event the state names is expected in storage, so any read error, a missing event included, is logged before the event is omitted. The rest of the state still goes out, so one unreadable event never fails the response.