main_2.rs

for msg in gen_port {
    // Receive generated numbers from the "generate" stage.
    let generated_num = match msg {
        PipelineMsg::Generated(num) => num,
        _ => panic!("unexpected message receiving from gen stage"),
    };
    // Cycle through the workers and distribute work.
    let worker = square_workers.pop_front().unwrap();
    let _ = worker.send(msg);
    square_workers.push_back(worker);
    if generated_num == 3 {
        // Once we get the "last number" we need,
        // we just stop iterating, 
        // dropping the gen_port, and stopping the generator thread.
        break;
    }
}