Graceful Shutdown and Cleanup - The Rust Programming Language

## Graceful Shutdown and Cleanup

The code in Listing 20-21 is responding to requests asynchronously through the use of a thread pool, as we intended. We get some warnings about fields that we're not using in a direct way, which are a reminder that we're not cleaning anything up. When we use `CTRL-C` to halt the main thread, all the other threads are stopped immediately as well, even if they're in the middle of serving a request.

We're now going to implement the `Drop` trait for `ThreadPool` to call `join` on each of the threads in the pool so that the threads will finish the requests they're working on. Then we'll implement a way for the `ThreadPool` to tell the threads they should stop accepting new requests and shut down. To see this code in action, we'll modify our server to only accept two requests before gracefully shutting down its thread pool.

Let's start with implementing `Drop` for our thread pool. When the pool is dropped, we should join on all of our threads to make sure they finish their work. Listing 20-22 shows a first attempt at a `Drop` implementation; this code won't quite work yet:

Filename: src/lib.rs

    impl Drop for ThreadPool {
        fn drop(&mut self) {
            for worker in &mut self.workers {
                println!("Shutting down worker {}", worker.id);

                worker.thread.join().unwrap();
            }
        }
    }

Listing 20-22: Joining each thread when the thread pool goes out of scope

We loop through each of the thread pool `workers`, using `&mut` because `self` is itself a mutable reference and we also need to be able to mutate `worker`. We print out a message saying that this particular worker is shutting down, and then we call `join` on that worker's thread. If the call to `join` fails, we `unwrap` the error to panic and go into an ungraceful shutdown.

Here's the error we get if we compile this code:

    error[E0507]: cannot move out of borrowed content
      --> src/lib.rs:65:13
       |
    65 |             worker.thread.join().unwrap();
       |             ^^^^^^ cannot move out of borrowed content

Because we only have a mutable borrow of each `worker`, we can't call `join`: `join` takes ownership of its argument. In order to solve this, we need a way to move the `thread` out of the `Worker` instance that owns `thread` so that `join` can consume the thread. We saw a way to do this in Listing 17-15: if the `Worker` holds an `Option<thread::JoinHandle<()>` instead, we can call the `take` method on the `Option` to move the value out of the `Some` variant and leave a `None` variant in its place. In other words, a `Worker` that is running will have a `Some` variant in `thread`, and when we want to clean up a worker, we'll replace `Some` with `None` so the worker doesn't have a thread to run.

So we know we want to update the definition of `Worker` like this:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # use std::thread;
    struct Worker {
        id: usize,
        thread: Option<thread::JoinHandle<()>>,
    }

    #}

Now let's lean on the compiler to find the other places that need to change. We get two errors:

    error: no method named `join` found for type
    `std::option::Option<std::thread::JoinHandle<()>>` in the current scope
      --> src/lib.rs:65:27
       |
    65 |             worker.thread.join().unwrap();
       |                           ^^^^

    error[E0308]: mismatched types
      --> src/lib.rs:89:21
       |
    89 |             thread,
       |             ^^^^^^ expected enum `std::option::Option`, found
       struct `std::thread::JoinHandle`
       |
       = note: expected type `std::option::Option<std::thread::JoinHandle<()>>`
                  found type `std::thread::JoinHandle<_>`

The second error is pointing to the code at the end of `Worker::new`; we need to wrap the `thread` value in `Some` when we create a new `Worker`:

Filename: src/lib.rs

    impl Worker {
        fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
            // ...snip...

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

The first error is in our `Drop` implementation, and we mentioned that we'll be calling `take` on the `Option` value to move `thread` out of `worker`. Here's what that looks like:

Filename: src/lib.rs

    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();
                }
            }
        }
    }

As we saw in Chapter 17, the `take` method on `Option` takes the `Some` variant out and leaves `None` in its place. We're using `if let` to destructure the `Some` and get the thread, then call `join` on the thread. If a worker's thread is already `None`, then we know this worker has already had its thread cleaned up so we don't do anything in that case.

With this, our code compiles without any warnings. Bad news though, this code doesn't function the way we want it to yet. The key is the logic in the closures that the spawned threads of the `Worker` instances run: calling `join` won't shut down the threads since they `loop` forever looking for jobs. If we try to drop our `ThreadPool` with this implementation, the main thread will block forever waiting for the first thread to finish.

To fix this, we're going to modify the threads to listen for either a `Job` to run or a signal that they should stop listening and exit the infinite loop. So instead of `Job` instances, our channel will send one of these two enum variants:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # struct Job;
    enum Message {
        NewJob(Job),
        Terminate,
    }

    #}

This `Message` enum will either be a `NewJob` variant that holds the `Job` the thread should run, or it will be a `Terminate` variant that will cause the thread to exit its loop and stop.

We need to adjust the channel to use values of type `Message` rather than type `Job`, as shown in Listing 20-23:

Filename: src/lib.rs

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

    // ...snip...

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

            let (sender, receiver) = mpsc::channel();

            // ...snip...
        }

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

            self.sender.send(Message::NewJob(job)).unwrap();
        }
    }

    // ...snip...

    impl Worker {
        fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Message>>>) ->
            Worker {

            let thread = thread::spawn(move ||{
                loop {
                    let message = receiver.lock().unwrap().recv().unwrap();

                    match message {
                        Message::NewJob(job) => {
                            println!("Worker {} got a job; executing.", id);

                            job.call_box();
                        },
                        Message::Terminate => {
                            println!("Worker {} was told to terminate.", id);

                            break;
                        },
                    }
                }
            });

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

Listing 20-23: Sending and receiving `Message` values and exiting the loop if a `Worker` receives `Message::Terminate`

We need to change `Job` to `Message` in the definition of `ThreadPool`, in `ThreadPool::new` where we create the channel, and in the signature of `Worker::new`. The `execute` method of `ThreadPool` needs to send jobs wrapped in the `Message::NewJob` variant. Then, in `Worker::new` where we receive a `Message` from the channel, we'll process the job if we get the `NewJob` variant and break out of the loop if we get the `Terminate` variant.

With these changes, the code will compile again and continue to function in the same way as it has been. We'll get a warning, though, because we aren't using the `Terminate` variant in any messages. Let's change our `Drop` implementation to look like Listing 20-24:

Filename: src/lib.rs

    impl Drop for ThreadPool {
        fn drop(&mut self) {
            println!("Sending terminate message to all workers.");

            for _ in &mut self.workers {
                self.sender.send(Message::Terminate).unwrap();
            }

            println!("Shutting down all workers.");

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

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

Listing 20-24: Sending `Message::Terminate` to the workers before calling `join` on each worker thread

We're now iterating over the workers twice, once to send one `Terminate` message for each worker, and once to call `join` on each worker's thread. If we tried to send a message and join immediately in the same loop, it's not guaranteed that the worker in the current iteration will be the one that gets the message from the channel.

To understand better why we need two separate loops, imagine a scenario with two workers. If we iterated through each worker in one loop, on the first iteration where `worker` is the first worker, we'd send a terminate message down the channel and call `join` on the first worker's thread. If the first worker was busy processing a request at that moment, the second worker would pick up the terminate message from the channel and shut down. We're waiting on the first worker to shut down, but it never will since the second thread picked up the terminate message. We're now blocking forever waiting for the first worker to shut down, and we'll never send the second message to terminate. Deadlock!

To prevent this, we first put all of our `Terminate` messages on the channel, and then we join on all the threads. Because each worker will stop receiving requests on the channel once it gets a terminate message, we can be sure that if we send the same number of terminate messages as there are workers, each worker will receive a terminate message before we call `join` on its thread.

In order to see this code in action, let's modify `main` to only accept two requests before gracefully shutting the server down as shown in Listing 20-25:

Filename: src/bin/main.rs

    fn main() {
        let listener = TcpListener::bind("127.0.0.1:8080").unwrap();
        let pool = ThreadPool::new(4);

        let mut counter = 0;

        for stream in listener.incoming() {
            if counter == 2 {
                println!("Shutting down.");
                break;
            }

            counter += 1;

            let stream = stream.unwrap();

            pool.execute(|| {
                handle_connection(stream);
            });
        }
    }

Listing 20-25: Shut down the server after serving two requests by exiting the loop

Only serving two requests isn't behavior you'd like a production web server to have, but this will let us see the graceful shutdown and cleanup working since we won't be stopping the server with `CTRL-C`.

We've added a `counter` variable that we'll increment every time we receive an incoming TCP stream. If that counter reaches 2, we'll stop serving requests and instead break out of the `for` loop. The `ThreadPool` will go out of scope at the end of `main`, and we'll see the `drop` implementation run.

Start the server with `cargo run`, and make three requests. The third request should error, and in your terminal you should see output that looks like:

    $ cargo run
       Compiling hello v0.1.0 (file:///projects/hello)
        Finished dev [unoptimized + debuginfo] target(s) in 1.0 secs
         Running `target/debug/hello`
    Worker 0 got a job; executing.
    Worker 3 got a job; executing.
    Shutting down.
    Sending terminate message to all workers.
    Shutting down all workers.
    Shutting down worker 0
    Worker 1 was told to terminate.
    Worker 2 was told to terminate.
    Worker 0 was told to terminate.
    Worker 3 was told to terminate.
    Shutting down worker 1
    Shutting down worker 2
    Shutting down worker 3

You may get a different ordering, of course. We can see how this works from the messages: workers zero and three got the first two requests, and then on the third request, we stop accepting connections. When the `ThreadPool` goes out of scope at the end of `main`, its `Drop` implementation kicks in, and the pool tells all workers to terminate. The workers each print a message when they see the terminate message, and then the thread pool calls `join` to shut down each worker thread.

One interesting aspect of this particular execution: notice that we sent the terminate messages down the channel, and before any worker received the messages, we tried to join worker zero. Worker zero had not yet gotten the terminate message, so the main thread blocked waiting for worker zero to finish. In the meantime, each of the workers received the termination messages. Once worker zero finished, the main thread waited for the rest of the workers to finish, and they had all received the termination message and were able to shut down at that point.

Congrats! We now have completed our project, and we have a basic web server that uses a thread pool to respond asynchronously. We're able to perform a graceful shutdown of the server, which cleans up all the threads in the pool. Here's the full code for reference:

Filename: src/bin/main.rs

    extern crate hello;
    use hello::ThreadPool;

    use std::io::prelude::*;
    use std::net::TcpListener;
    use std::net::TcpStream;
    use std::fs::File;
    use std::thread;
    use std::time::Duration;

    fn main() {
        let listener = TcpListener::bind("127.0.0.1:8080").unwrap();
        let pool = ThreadPool::new(4);

        let mut counter = 0;

        for stream in listener.incoming() {
            if counter == 2 {
                println!("Shutting down.");
                break;
            }

            counter += 1;

            let stream = stream.unwrap();

            pool.execute(|| {
                handle_connection(stream);
            });
        }
    }

    fn handle_connection(mut stream: TcpStream) {
        let mut buffer = [0; 512];
        stream.read(&mut buffer).unwrap();

        let get = b"GET / HTTP/1.1\r\n";
        let sleep = b"GET /sleep HTTP/1.1\r\n";

        let (status_line, filename) = if buffer.starts_with(get) {
            ("HTTP/1.1 200 OK\r\n\r\n", "hello.html")
        } else if buffer.starts_with(sleep) {
            thread::sleep(Duration::from_secs(5));
            ("HTTP/1.1 200 OK\r\n\r\n", "hello.html")
        } else {
            ("HTTP/1.1 404 NOT FOUND\r\n\r\n", "404.html")
        };

         let mut file = File::open(filename).unwrap();
         let mut contents = String::new();

         file.read_to_string(&mut contents).unwrap();

         let response = format!("{}{}", status_line, contents);

         stream.write(response.as_bytes()).unwrap();
         stream.flush().unwrap();
    }

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    use std::thread;
    use std::sync::mpsc;
    use std::sync::Arc;
    use std::sync::Mutex;

    enum Message {
        NewJob(Job),
        Terminate,
    }

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

    trait FnBox {
        fn call_box(self: Box<Self>);
    }

    impl<F: FnOnce()> FnBox for F {
        fn call_box(self: Box<F>) {
            (*self)()
        }
    }

    type Job = Box<FnBox + 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);

            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, receiver.clone()));
            }

            ThreadPool {
                workers,
                sender,
            }
        }

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

            self.sender.send(Message::NewJob(job)).unwrap();
        }
    }

    impl Drop for ThreadPool {
        fn drop(&mut self) {
            println!("Sending terminate message to all workers.");

            for _ in &mut self.workers {
                self.sender.send(Message::Terminate).unwrap();
            }

            println!("Shutting down all workers.");

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

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

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

    impl Worker {
        fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Message>>>) ->
            Worker {

            let thread = thread::spawn(move ||{
                loop {
                    let message = receiver.lock().unwrap().recv().unwrap();

                    match message {
                        Message::NewJob(job) => {
                            println!("Worker {} got a job; executing.", id);

                            job.call_box();
                        },
                        Message::Terminate => {
                            println!("Worker {} was told to terminate.", id);

                            break;
                        },
                    }
                }
            });

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

    #}

There's more we could do here! If you'd like to continue enhancing this project, here are some ideas:

- Add more documentation to `ThreadPool` and its public methods
- Add tests of the library's functionality
- Change calls to `unwrap` to more robust error handling
- Use `ThreadPool` to perform some other task rather than serving web requests
- Find a thread pool crate on crates.io and implement a similar web server using the crate instead and compare its API and robustness to the thread pool we implemented

## Summary

Well done! You've made it to the end of the book! We'd like to thank you for joining us on this tour of Rust. You're now ready to go out and implement your own Rust projects or help with other people's. Remember there's a community of other Rustaceans who would love to help you with any challenges you encounter on your Rust journey.