Graceful Shutdown and Cleanup

Problem: Resource Cleanup

Current thread pool implementation doesn’t clean up resources when dropping. Worker threads continue running indefinitely, preventing graceful shutdown.

Solution: Implement `Drop` Trait

1. Basic Drop Implementation

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

Problem: Workers are blocked on recv() calls - join() waits indefinitely.

2. Signaling Shutdown

Close channel to signal workers:

impl Drop for ThreadPool {
    fn drop(&mut self) {
        // Drop sender to close channel
        drop(self.sender.take());
        
        // Wait for workers to finish
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);
            if let Some(thread) = worker.thread.take() {
                thread.join().unwrap();
            }
        }
    }
}

Update Worker to handle channel closure:

impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let message = receiver.lock().unwrap().recv();
            
            match message {
                Ok(job) => {
                    println!("Worker {id} got a job; executing.");
                    job();
                }
                Err(_) => {
                    println!("Worker {id} disconnected; shutting down.");
                    break;
                }
            }
        });
        
        Worker { 
            id, 
            thread: Some(thread) 
        }
    }
}

3. Updated Data Structures

ThreadPool with Optional sender:

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

Worker with Optional thread:

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

Complete Implementation

ThreadPool:

impl ThreadPool {
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);
        
        let (sender, receiver) = mpsc::channel();
        let receiver = Arc::new(Mutex::new(receiver));
        let mut workers = 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::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.thread.take() {
                thread.join().unwrap();
            }
        }
    }
}

Testing Graceful Shutdown

Limited server for testing:

let pool = ThreadPool::new(4);

for stream in listener.incoming().take(2) {
    let stream = stream.unwrap();
    pool.execute(|| {
        handle_connection(stream);
    });
}

println!("Shutting down.");

Expected output:

Worker 0 got a job; executing.
Worker 1 got a job; executing.
Shutting down.
Shutting down worker 0
Worker 0 disconnected; shutting down.
Worker 1 disconnected; shutting down.
Shutting down worker 1
...

Key Patterns

Resource cleanup sequence:

Signal shutdown (close channel)
Wait for workers to finish current tasks
Join all worker threads

Channel closure semantics:

Dropping sender closes channel
recv() returns Err when channel closed
Workers exit loop on channel closure

RAII (Resource Acquisition Is Initialization):

Drop trait ensures cleanup on scope exit
Automatic resource management
Exception safety through destructors

Production considerations:

Timeout for join operations
Graceful vs forceful shutdown
Signal handling for shutdown triggers