appsec/src/bin/thread_pool.rs

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


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

// Box: pointer for heap allocation (smart pointer); dyn: unknown type to compile time
type Job = Box<dyn FnOnce() + Send + 'static>;


impl ThreadPool {
    /// Create a new ThreadPool.
    ///
    /// The size is the number of threads in the pool.
    ///
    /// # Panics
    ///
    /// The `new` function will panic if the size is zero.
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

        // create asynchronous channel with sender/receiver pair (for jobs!)
        let (sender, receiver) = mpsc::channel();

        // protect receiver with thread-safe shared (Arc) mutex to avoid multiple access
        let receiver = Arc::new(Mutex::new(receiver));

        // pre(!)allocate space for workers
        let mut workers = Vec::with_capacity(size);

        // initialize all workers...
        for id in 0..size {
            // create worker with unique id and pointer to the shared mutex
            workers.push(Worker::new(id, Arc::clone(&receiver)));
        }

        // set thread pool variables
        ThreadPool {
            workers,
            // Some --> enum variant (Option): indicates that sender may not exist (=be null)
            sender: Some(sender),
        }
    }

    pub fn execute<F>(&self, f: F)
        where
            // FnOnce() --> only callable once; Send --> transfer to another thread; 'static --> lifetime
            F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);

        // commit job to sender object --> pass to threads for execution
        self.sender.as_ref().unwrap().send(job).unwrap();
    }
}

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

        // then drop workers
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);

            // join worker thread
            if let Some(thread) = worker.thread.take() {
                thread.join().unwrap();
            }
        }
    }
}


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

impl Worker {
    /// Create a new Worker..
    ///
    /// The id is a unique identifier.
    /// The receiver is a shared pointer to a receiver protected by Mutex.
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        // spawn thread and enter infinite loop
        let thread = thread::spawn(move || loop {
            // lock mutex and wait for job
            let message = receiver.lock().unwrap().recv();

            match message {
                // normal operation --> execute job
                Ok(job) => {
                    println!("Worker {id} got a job; executing.");

                    job();
                }
                // exit gracefully when pool is closed --> recv will return error then
                Err(_) => {
                    println!("Worker {id} disconnected; shutting down.");
                    break;
                }
            }
        });

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