优雅关闭和资源清理

之前的程序，如果使用 ctrl-c 的方法来关闭，所有的线程都会立即停止，这会造成正在请求的用户感知到一个明显的错误。

因此我们需要添加一些优雅关闭( Graceful Shutdown )，以更好的完成资源清理等收尾工作。

为线程池实现 Drop

当线程池被 drop 时，需要等待所有的子线程完成它们的工作，然后再退出，下面是一个初步尝试:

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

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

这里通过实现 Drop 特征来为线程池添加资源收尾工作，代码比较简单，就是依次调用每个线程的 join 方法。编译下试试：

#![allow(unused)]
fn main() {
$ 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();
   |             ^^^^^^^^^^^^^ ------ `worker.thread` moved due to this method call
   |             |
   |             move occurs because `worker.thread` has type `JoinHandle<()>`, which does not implement the `Copy` trait
   |
note: this function takes ownership of the receiver `self`, which moves `worker.thread`

For more information about this error, try `rustc --explain E0507`.
error: could not compile `hello` due to previous error
}

这里的报错很明显，worker.thread 试图拿走所有权，但是 worker 仅仅是一个可变借用，显然是不可行的。

目前来看，只能将 thread 从 worker 中移动出来，一个可行的尝试:

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

对于 Option 类型，可以使用 take 方法拿走内部值的所有权，同时留下一个 None 在风中孤独凌乱。继续尝试编译驱动开发模式:

$ 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<()>>`
   |
note: the method `join` exists on the type `JoinHandle<()>`
help: consider using `Option::expect` to unwrap the `JoinHandle<()>` value, panicking if the value is an `Option::None`
   |
52 |             worker.thread.expect("REASON").join().unwrap();
   |                          +++++++++++++++++

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, thread: Some(thread) }
   |                      +++++++++++++      +

先来解决第二个类型不匹配的错误:

#![allow(unused)]
fn main() {
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        // --snip--

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

简单搞定，回头看看第一个错误，既然换了 Option，就可以用 take 拿走所有权:

#![allow(unused)]
fn main() {
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();
            }
        }
    }
}
}

注意这种 if let 的写法，若 worker.thread 已经是 None，什么都不会发生，符合我们的预期; 若包含一个线程，那就拿走其所有权，然后调用 join。

停止工作线程

虽然调用了 join ，但是目标线程依然不会停止，原因在于它们在无限的 loop 循环等待，看起来需要借用 channel 的 drop 机制：释放 sender发送端后，receiver 接收端会收到报错，然后再退出即可。

#![allow(unused)]
fn main() {
pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: Option<mpsc::Sender<Job>>,
}
// --snip--
impl ThreadPool {
    pub fn new(size: usize) -> ThreadPool {
        // --snip--

        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();
            }
        }
    }
}
}

上面做了两处改变:

为 sender 增加 Option 封装，这样可以用 take 拿走所有权，跟之前的 thread 一样
主动调用 drop 关闭发送端 sender

关闭 sender 后，将关闭对应的 channel，意味着不会再有任何消息被发送。随后，所有的处于无限 loop 的接收端将收到一个错误，我们根据错误再进行进一步的处理。

#![allow(unused)]
fn main() {
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),
        }
    }
}
}

为了快速验证代码是否正确，修改 main 函数，让其只接收前两个请求:

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.");
}

take 是迭代器 Iterator 上的方法，会限制后续的迭代进行最多两次，然后就结束监听，随后 ThreadPool 也将超出作用域并自动触发 drop。

$ cargo run
   Compiling hello v0.1.0 (file:///projects/hello)
    Finished dev [unoptimized + debuginfo] target(s) in 1.0s
     Running `target/debug/hello`
Worker 0 got a job; executing.
Shutting down.
Shutting down worker 0
Worker 3 got a job; executing.
Worker 1 disconnected; shutting down.
Worker 2 disconnected; shutting down.
Worker 3 disconnected; shutting down.
Worker 0 disconnected; shutting down.
Shutting down worker 1
Shutting down worker 2
Shutting down worker 3

可以看到，代码按照我们的设想如期运行，至此，一个基于线程池的简单 Web 服务器已经完成，下面是完整的代码:

完整代码

// src/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_all(response.as_bytes()).unwrap();
    stream.flush().unwrap();
}

#![allow(unused)]
fn main() {
// src/lib.rs
use std::{
    sync::{mpsc, Arc, Mutex},
    thread,
};

pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: Option<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: 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();
            }
        }
    }
}

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 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),
        }
    }
}
}

可以做的更多

事实上，我们还可以做更多，但是受制于篇幅，就不再展开，感兴趣的同学可以自行完成。

增加更多的文档
为线程池增加测试
尽可能移除 unwrap，替换为错误处理
使用线程池完成其它类型的工作，而不仅仅是本章的 Web 服务器
在 crates.io 上找到一个线程池实现，然后使用该包实现一个类似的 Web 服务器

上一章节的遗留问题

在上一章节的末尾，我们提到将 let 替换为 while let 后，多线程的优势将荡然无存，原因藏的很隐蔽：

Mutex 结构体没有提供显式的 unlock，要依赖作用域结束后的 drop 来自动释放
let job = receiver.lock().unwrap().recv().unwrap(); 在这行代码中，由于使用了 let，右边的任何临时变量会在 let 语句结束后立即被 drop，因此锁会自动释放
然而 while let (还包括 if let 和 match) 直到最后一个花括号后，才触发 drop

#![allow(unused)]
fn main() {
impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || {
            while let Ok(job) = receiver.lock().unwrap().recv() {
                println!("Worker {id} got a job; executing.");

                job();
            }
        });

        Worker { id, thread }
    }
}
}

根据之前的分析，上面的代码直到 job() 任务执行结束后，才会释放锁，去执行另一个请求，最终造成请求排队。

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