rustlings/exercises/20_threads/threads2.rs

// threads2.rs
//
// Building on the last exercise, we want all of the threads to complete their
// work but this time the spawned threads need to be in charge of updating a
// shared value: JobStatus.jobs_completed
//
// Execute `rustlings hint threads2` or use the `hint` watch subcommand for a
// hint.

use std::sync::{ Arc, Mutex };
use std::thread;
use std::time::Duration;

struct JobStatus {
    jobs_completed: u32,
}

fn main() {
    let status = Arc::new(Mutex::new(JobStatus { jobs_completed: 0 }));
    let mut handles = vec![];
    for _ in 0..10 {
        let status_shared = Arc::clone(&status);
        let handle = thread::spawn(move || {
            thread::sleep(Duration::from_millis(250));
            // TODO: You must take an action before you update a shared value
            // status_shared.jobs_completed += 1;

            // * The action taken was to lock and unrwap the status_shared var
            // * When multiple threads want to access or modify shared data that's
            // * protected by a Mutex, they must first lock the mutex
            status_shared.lock().unwrap().jobs_completed += 1;
        });
        handles.push(handle);
    }
    for handle in handles {
        handle.join().unwrap();
        // TODO: Print the value of the JobStatus.jobs_completed. Did you notice
        // anything interesting in the output? Do you have to 'join' on all the
        // handles?

        // * If you simply try to print status.jobs_completed, you'll get an
        // * unknown field error.
        // * You also have to lock and unwrap here:

        // println!("jobs completed {}", status.lock().unwrap().jobs_completed)

        // * After the change above, you get multiple printouts with the same
        // * value for the status

        // * And here's an example where we properly deal with errors instead
        // * of using unwrap()
        match status.lock() {
            Ok(status_locked) => {
                println!("jobs completed: {}", status_locked.jobs_completed);
            }
            Err(e) => {
                eprintln!("Error while locking the Mutex: {:?}", e);
            }
        }
    }
}