thread_pool.rs

use std::{
    sync::{mpsc, Arc, Mutex}, 
    thread,
};

// ==================== Types ====================

type Job = Box<dyn FnOnce() + Send + 'static>;

// ==================== Structs ====================

pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: Option<mpsc::Sender<Job>>,
}

struct Worker {
    id: usize,
    work: Option<thread::JoinHandle<()>>,
}

// ==================== Impl(ementations) ====================

impl ThreadPool {
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

        let (sender, receiver) = mpsc::channel();
        let receiver: Arc<Mutex<mpsc::Receiver<Job>>> = Arc::new(Mutex::new(receiver));

        let mut workers: Vec<Worker> = Vec::with_capacity(size);
        for id in 0..size {
            workers.push(Worker::new(id, Arc::clone(&receiver)));
        }

        ThreadPool { 
            workers, 
            sender: Some(sender), 
        }
    }

    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job: Box<F> = Box::new(f);
        self.sender
            .as_ref()
            .unwrap()
            .send(job)
            .unwrap();
    }
}

impl Drop for ThreadPool {
    fn drop(&mut self) {
        drop(self.sender.take());

        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);
            
            if let Some(thread) = worker.work.take() {
                thread.join().unwrap();
            }
        }
    }
}

impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let work: thread::JoinHandle<()> = thread::spawn(move || loop {
            let message: Result<Box<dyn FnOnce() + Send>, mpsc::RecvError> = 
                receiver.lock().unwrap().recv();

            match message {
                Ok(job) => job(),
                Err(_) => {
                    println!("Worker {id} disconnected");
                    break;
                }
            }
        });

        Worker { id,  work: Some(work), }
    }
}