basket_service.rs

    // A map of keeping count of pending order per customer,
    // owned by the "basket" service.
    let mut pending_orders = HashMap::new();

    for _ in 0..4 {
        let customer_id = CustomerId(Uuid::new_v4());
        match pending_orders.entry(customer_id) {
            Entry::Vacant(entry) => {
                entry.insert(1);
            }
            Entry::Occupied(mut entry) => {
                *entry.get_mut() += 1;
            }
        };
        let _ = order_request_sender.send(OrderRequest::NewOrder(customer_id));
    }

    // Have the main thread of the test double-up as the "basket" service.
    loop {
        match order_result_receiver.recv() {
            Ok(OrderResult(customer_id, _succeeded)) => {
                match pending_orders.entry(customer_id) {
                    Entry::Vacant(_) => {
                        panic!("Got an order confirmation for an unknonw customer.");
                    }
                    Entry::Occupied(mut entry) => {
                        if *entry.get() == 1 {
                            entry.remove_entry();
                        } else {
                            *entry.get_mut() -= 1;
                        }
                    }
                };
                if pending_orders.is_empty() {
                    let _ = order_request_sender.send(OrderRequest::ShutDown);
                    break;
                }
            }
            _ => panic!("Error receiving an order result."),
        }
    }