Function record_offline_ack 

fn record_offline_ack(
    result: Result<PacketId, ClientError>,
    queued_tx: Option<Sender<Result<(), Error>>>,
    completion_tx: Sender<Result<Vec<ReasonCode>, Error>>,
    client: &Client<JournaledSession>,
    offline_acks: &mut HashMap<PacketId, PendingAck>,
    queued_waiting: &mut Vec<Sender<Result<(), Error>>>,
)

Parks the completion future for an offline SUBSCRIBE/UNSUBSCRIBE keyed by its allocated packet id, or fails it if allocation failed. After parking, any earlier request the new one superseded (same filter, now orphaned) is failed.