优雅停机与清理

ch20-03-graceful-shutdown-and-cleanup.md
commit 322899b375d071e4d96aaf29ce25c1a4b4ec65da

示例 20-20 中的代码如期通过使用线程池异步的响应请求。这里有一些警告说 workers、id 和 thread 字段没有直接被使用，这提醒了我们并没有清理所有的内容。当使用不那么优雅的 ctrl-c 终止主线程时，所有其他线程也会立刻停止，即便它们正处于处理请求的过程中。

现在我们要为 ThreadPool 实现 Drop trait 对线程池中的每一个线程调用 join，这样这些线程将会执行完他们的请求。接着会为 ThreadPool 实现一个告诉线程他们应该停止接收新请求并结束的方式。为了实践这些代码，修改 server 在优雅停机（graceful shutdown）之前只接受两个请求。

为 ThreadPool 实现 Drop Trait

现在开始为线程池实现 Drop。当线程池被丢弃时，应该 join 所有线程以确保他们完成其操作。示例 20-22 展示了 Drop 实现的第一次尝试；这些代码还不能够编译：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}
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);
        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 }
    }
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(job).unwrap();
    }
}
impl Drop for ThreadPool {
    fn drop(&mut self) {
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);
            worker.thread.join().unwrap();
        }
    }
}
struct Worker {
    id: usize,
    thread: thread::JoinHandle<()>,
}
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();
            println!("Worker {} got a job; executing.", id);
            job();
        });
        Worker { id, thread }
    }
}

示例 20-22: 当线程池离开作用域时 join 每个线程

这里首先遍历线程池中的每个 workers。这里使用了 &mut 因为 self 本身是一个可变引用而且也需要能够修改 worker。对于每一个线程，会打印出说明信息表明此特定 worker 正在关闭，接着在 worker 线程上调用 join。如果 join 调用失败，通过 unwrap 使得 panic 并进行不优雅的关闭。

如下是尝试编译代码时得到的错误：

$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0507]: cannot move out of `worker.thread` which is behind a mutable reference
  --> src/lib.rs:52:13
   |
52 |             worker.thread.join().unwrap();
   |             ^^^^^^^^^^^^^ move occurs because `worker.thread` has type `JoinHandle<()>`, which does not implement the `Copy` trait
For more information about this error, try `rustc --explain E0507`.
error: could not compile `hello` due to previous error

这告诉我们并不能调用 join，因为只有每一个 worker 的可变借用，而 join 获取其参数的所有权。为了解决这个问题，需要一个方法将 thread 移动出拥有其所有权的 Worker 实例以便 join 可以消费这个线程。示例 17-15 中我们曾见过这么做的方法：如果 Worker 存放的是 Option，就可以在 Option 上调用 take 方法将值从 Some 成员中移动出来而对 None 成员不做处理。换句话说，正在运行的 Worker 的 thread 将是 Some 成员值，而当需要清理 worker 时，将 Some 替换为 None，这样 worker 就没有可以运行的线程了。

为此需要更新 Worker 的定义为如下：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}
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);
        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 }
    }
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(job).unwrap();
    }
}
impl Drop for ThreadPool {
    fn drop(&mut self) {
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);
            worker.thread.join().unwrap();
        }
    }
}
struct Worker {
    id: usize,
    thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();
            println!("Worker {} got a job; executing.", id);
            job();
        });
        Worker { id, thread }
    }
}

现在依靠编译器来找出其他需要修改的地方。check 代码会得到两个错误：

$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0599]: no method named `join` found for enum `Option` in the current scope
  --> src/lib.rs:52:27
   |
52 |             worker.thread.join().unwrap();
   |                           ^^^^ method not found in `Option<JoinHandle<()>>`
error[E0308]: mismatched types
  --> src/lib.rs:72:22
   |
72 |         Worker { id, thread }
   |                      ^^^^^^ expected enum `Option`, found struct `JoinHandle`
   |
   = note: expected enum `Option<JoinHandle<()>>`
            found struct `JoinHandle<_>`
help: try wrapping the expression in `Some`
   |
72 |         Worker { id, Some(thread) }
   |                      +++++      +
Some errors have detailed explanations: E0308, E0599.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `hello` due to 2 previous errors

让我们修复第二个错误，它指向 Worker::new 结尾的代码；当新建 Worker 时需要将 thread 值封装进 Some。做出如下改变以修复问题：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}
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);
        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 }
    }
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(job).unwrap();
    }
}
impl Drop for ThreadPool {
    fn drop(&mut self) {
        for worker in &mut self.workers {
            println!("Shutting down worker {}", worker.id);
            worker.thread.join().unwrap();
        }
    }
}
struct Worker {
    id: usize,
    thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        // --snip--
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();
            println!("Worker {} got a job; executing.", id);
            job();
        });
        Worker {
            id,
            thread: Some(thread),
        }
    }
}

第一个错误位于 Drop 实现中。之前提到过要调用 Option 上的 take 将 thread 移动出 worker。如下改变会修复问题：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}
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);
        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 }
    }
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(job).unwrap();
    }
}
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();
            }
        }
    }
}
struct Worker {
    id: usize,
    thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();
            println!("Worker {} got a job; executing.", id);
            job();
        });
        Worker {
            id,
            thread: Some(thread),
        }
    }
}

如第十七章我们见过的，Option 上的 take 方法会取出 Some 而留下 None。使用 if let 解构 Some 并得到线程，接着在线程上调用 join。如果 worker 的线程已然是 None，就知道此时这个 worker 已经清理了其线程所以无需做任何操作。

向线程发送信号使其停止接收任务

有了所有这些修改，代码就能编译且没有任何警告。不过也有坏消息，这些代码还不能以我们期望的方式运行。问题的关键在于 Worker 中分配的线程所运行的闭包中的逻辑：调用 join 并不会关闭线程，因为他们一直 loop 来寻找任务。如果采用这个实现来尝试丢弃 ThreadPool ，则主线程会永远阻塞在等待第一个线程结束上。

为了修复这个问题，修改线程既监听是否有 Job 运行也要监听一个应该停止监听并退出无限循环的信号。所以信道将发送这个枚举的两个成员之一而不是 Job 实例：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
enum Message {
    NewJob(Job),
    Terminate,
}
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, Arc::clone(&receiver)));
        }
        ThreadPool { workers, sender }
    }
    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);
        self.sender.send(job).unwrap();
    }
}
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();
            }
        }
    }
}
struct Worker {
    id: usize,
    thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();
            println!("Worker {} got a job; executing.", id);
            job();
        });
        Worker {
            id,
            thread: Some(thread),
        }
    }
}

Message 枚举要么是存放了线程需要运行的 Job 的 NewJob 成员，要么是会导致线程退出循环并终止的 Terminate 成员。

同时需要修改信道来使用 Message 类型值而不是 Job，如示例 20-23 所示：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Message>,
}
// --snip--
type Job = Box<dyn FnOnce() + Send + 'static>;
enum Message {
    NewJob(Job),
    Terminate,
}
impl ThreadPool {
    // --snip--
    /// 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, Arc::clone(&receiver)));
        }
        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();
    }
}
// --snip--
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();
            }
        }
    }
}
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();
                }
                Message::Terminate => {
                    println!("Worker {} was told to terminate.", id);
                    break;
                }
            }
        });
        Worker {
            id,
            thread: Some(thread),
        }
    }
}

示例 20-23: 收发 Message 值并在 Worker 收到 Message::Terminate 时退出循环

为了适用 Message 枚举需要将两个地方的 Job 修改为 Message：ThreadPool 的定义和 Worker::new 的签名。ThreadPool 的 execute 方法需要发送封装进 Message::NewJob 成员的任务。然后，在 Worker::new 中当从信道接收 Message 时，当获取到 NewJob成员会处理任务而收到 Terminate 成员则会退出循环。

通过这些修改，代码再次能够编译并继续按照示例 20-20 之后相同的行为运行。不过还是会得到一个警告，因为并没有创建任何 Terminate 成员的消息。如示例 20-24 所示修改 Drop 实现来修复此问题：

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Message>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
enum Message {
    NewJob(Job),
    Terminate,
}
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, Arc::clone(&receiver)));
        }
        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 &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();
                }
                Message::Terminate => {
                    println!("Worker {} was told to terminate.", id);
                    break;
                }
            }
        });
        Worker {
            id,
            thread: Some(thread),
        }
    }
}

示例 20-24：在对每个 worker 线程调用 join 之前向 worker 发送 Message::Terminate

现在遍历了 worker 两次，一次向每个 worker 发送一个 Terminate 消息，一个调用每个 worker 线程上的 join。如果尝试在同一循环中发送消息并立即 join 线程，则无法保证当前迭代的 worker 是从信道收到终止消息的 worker。

为了更好的理解为什么需要两个分开的循环，想象一下只有两个 worker 的场景。如果在一个单独的循环中遍历每个 worker，在第一次迭代中向信道发出终止消息并对第一个 worker 线程调用 join。如果此时第一个 worker 正忙于处理请求，那么第二个 worker 会收到终止消息并停止。我们会一直等待第一个 worker 结束，不过它永远也不会结束因为第二个线程接收了终止消息。死锁！

为了避免此情况，首先在一个循环中向信道发出所有的 Terminate 消息，接着在另一个循环中 join 所有的线程。每个 worker 一旦收到终止消息即会停止从信道接收消息，意味着可以确保如果发送同 worker 数相同的终止消息，在 join 之前每个线程都会收到一个终止消息。

为了实践这些代码，如示例 20-25 所示修改 main 在优雅停机 server 之前只接受两个请求：

文件名: src/bin/main.rs

use hello::ThreadPool;
use std::fs;
use std::io::prelude::*;
use std::net::TcpListener;
use std::net::TcpStream;
use std::thread;
use std::time::Duration;
fn main() {
    let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
    let pool = ThreadPool::new(4);
    for stream in listener.incoming().take(2) {
        let stream = stream.unwrap();
        pool.execute(|| {
            handle_connection(stream);
        });
    }
    println!("Shutting down.");
}
fn handle_connection(mut stream: TcpStream) {
    let mut buffer = [0; 1024];
    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", "hello.html")
    } else if buffer.starts_with(sleep) {
        thread::sleep(Duration::from_secs(5));
        ("HTTP/1.1 200 OK", "hello.html")
    } else {
        ("HTTP/1.1 404 NOT FOUND", "404.html")
    };
    let contents = fs::read_to_string(filename).unwrap();
    let response = format!(
        "{}\r\nContent-Length: {}\r\n\r\n{}",
        status_line,
        contents.len(),
        contents
    );
    stream.write(response.as_bytes()).unwrap();
    stream.flush().unwrap();
}

示例 20-25: 在处理两个请求之后通过退出循环来停止 server

你不会希望真实世界的 web server 只处理两次请求就停机了，这只是为了展示优雅停机和清理处于正常工作状态。

take 方法定义于 Iterator trait，这里限制循环最多头 2 次。ThreadPool 会在 main 的结尾离开作用域，而且还会看到 drop 实现的运行。

使用 cargo run 启动 server，并发起三个请求。第三个请求应该会失败，而终端的输出应该看起来像这样：

$ cargo run
   Compiling hello v0.1.0 (file:///projects/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 1.0s
     Running `target/debug/main`
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

可能会出现不同顺序的 worker 和信息输出。可以从信息中看到服务是如何运行的： worker 0 和 worker 3 获取了头两个请求，接着在第三个请求时，我们停止接收连接。当 ThreadPool 在 main 的结尾离开作用域时，其 Drop 实现开始工作，线程池通知所有线程终止。每个 worker 在收到终止消息时会打印出一个信息，接着线程池调用 join 来终止每一个 worker 线程。

这个特定的运行过程中一个有趣的地方在于：注意我们向信道中发出终止消息，而在任何线程收到消息之前，就尝试 join worker 0 了。worker 0 还没有收到终止消息，所以主线程阻塞直到 worker 0 结束。与此同时，每一个线程都收到了终止消息。一旦 worker 0 结束，主线程就等待其他 worker 结束，此时他们都已经收到终止消息并能够停止了。

恭喜！现在我们完成了这个项目，也有了一个使用线程池异步响应请求的基础 web server。我们能对 server 执行优雅停机，它会清理线程池中的所有线程。

如下是完整的代码参考：

文件名: src/bin/main.rs

use hello::ThreadPool;
use std::fs;
use std::io::prelude::*;
use std::net::TcpListener;
use std::net::TcpStream;
use std::thread;
use std::time::Duration;
fn main() {
    let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
    let pool = ThreadPool::new(4);
    for stream in listener.incoming() {
        let stream = stream.unwrap();
        pool.execute(|| {
            handle_connection(stream);
        });
    }
    println!("Shutting down.");
}
fn handle_connection(mut stream: TcpStream) {
    let mut buffer = [0; 1024];
    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", "hello.html")
    } else if buffer.starts_with(sleep) {
        thread::sleep(Duration::from_secs(5));
        ("HTTP/1.1 200 OK", "hello.html")
    } else {
        ("HTTP/1.1 404 NOT FOUND", "404.html")
    };
    let contents = fs::read_to_string(filename).unwrap();
    let response = format!(
        "{}\r\nContent-Length: {}\r\n\r\n{}",
        status_line,
        contents.len(),
        contents
    );
    stream.write(response.as_bytes()).unwrap();
    stream.flush().unwrap();
}

文件名: src/lib.rs

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Message>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
enum Message {
    NewJob(Job),
    Terminate,
}
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, Arc::clone(&receiver)));
        }
        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 &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();
                }
                Message::Terminate => {
                    println!("Worker {} was told to terminate.", id);
                    break;
                }
            }
        });
        Worker {
            id,
            thread: Some(thread),
        }
    }
}

这里还有很多可以做的事！如果你希望继续增强这个项目，如下是一些点子：

• 为 ThreadPool 和其公有方法增加更多文档
• 为库的功能增加测试
• 将 unwrap 调用改为更健壮的错误处理
• 使用 ThreadPool 进行其他不同于处理网络请求的任务
• 在 crates.io 上寻找一个线程池 crate 并使用它实现一个类似的 web server，将其 API 和鲁棒性与我们的实现做对比

总结

好极了！你结束了本书的学习！由衷感谢你同我们一道加入这次 Rust 之旅。现在你已经准备好出发并实现自己的 Rust 项目并帮助他人了。请不要忘记我们的社区，这里有其他 Rustaceans 正乐于帮助你迎接 Rust 之路上的任何挑战。