Function auth_chain_pdu 

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

Reads an auth chain event for a federation response, or omits it.

An ancestor missing from storage is a normal backfill gap and is omitted quietly, while any other read error is logged first. The rest of the chain still goes out, so one unreadable event never fails the response.