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20.3 最后的项目：Web服务器的优雅停机与清理

20.3.0. 回顾

在上一篇文章中，我们完成了多线程 Web 服务器，但仍然有一些可以改进之处。这篇文章我们就来完善代码。

注意：本文衔接于 20.2. 最后的项目：多线程Web服务器。如果你想详细了解从零开始构建 Web 服务器的过程，请阅读完第 20 章的所有文章。

20.3.1. 为ThreadPool实现Droptrait

当我们想要关闭服务器（使用不太优雅的 Ctrl + C 方法停止主线程）时，所有其他线程也会立即停止，即使它们仍在处理请求。

用于管理清理的 trait 是Droptrait。我们只需要在本地编写drop函数来覆盖默认实现，让线程能够在关闭之前完成当前正在处理的工作。我们还需要某种方式来阻止线程接收新请求，并为停机做好准备。

让我们为ThreadPool实现Droptrait：

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

逻辑就是遍历每一个worker，然后调用worker里thread字段的join方法（详见 16.1. 使用多线程同时运行代码）。

运行cargo check：

error[E0507]: cannot move out of `worker.thread` which is behind a mutable reference --> src/lib.rs:42:13 | 42 | 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: `JoinHandle::<T>::join` takes ownership of the receiver `self`, which moves `worker.thread` --> /Users/stanyin/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/join_handle.rs:149:17 | 149 | pub fn join(self) -> Result<T> { | ^^^^ For more information about this error, try `rustc --explain E0507`. error: could not compile `web_server` (lib) due to 1 previous error

报错信息显示我们无法把worker的thread字段移出来，因为我们只有每个worker的可变引用，但join需要JoinHandle的所有权（也就是worker.thread的所有权）。

为了满足所有权要求，我们需要修改Worker中thread字段的类型，用Option<T>包裹thread::JoinHandle<()>。这样我们就可以调用Option<T>::take来获得所有权：

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

凡是使用了thread字段的地方，也都必须因Option<T>而更新：

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), } } }

把thread字段的值从thread改为Some(thread)。

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(); } } } }

使用if let模式匹配，在worker.thread为Some时取出其中的值（使用take可以获得所有权，而不是可变引用）。

20.3.2. 向线程发出信号以退出

这样修改后可以编译通过，但仍未达到预期效果。调用drop并不会真正关停线程，因为线程仍然卡在loop中等待工作。

如果用这个drop方法丢弃ThreadPool，主线程会永远阻塞，等待第一个线程结束（因为每个线程都一直在循环寻找工作，不会跳出循环）。

我们需要ThreadPool的sender字段有两种状态：一种是附带任务的存活状态，另一种是终止状态：

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

使用Option<T>可以让它表示这两种状态。

凡是使用了sender字段的地方也都必须修改：

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(); } } } }

添加drop(self.sender.take());来显式丢弃发送端，这样就会关闭通道。发生这种情况时，worker 无限循环中执行的所有recv调用都会返回错误，worker 也会停止运行。

pub fn new(size: usize) -> ThreadPool { assert!(size > 0); let (sender, receiver) = mpsc::channel(); let mut workers = Vec::with_capacity(size); let receiver = Arc::new(Mutex::new(receiver)); for id in 0..size { workers.push(Worker::new(id, Arc::clone(&receiver))); } ThreadPool { workers, sender: Some(sender), } }

用Some包裹返回值中的sender字段。

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(); }

因为sender现在是Option，我们调用as_ref得到Option<&Sender>（对内部发送端的引用），而不会把它从self中移出。随后再unwrap并调用send：send在发送端上接受&self，并把任务移入通道。

这样改仍然不够优雅，因为 worker 无限循环中执行的所有recv调用都会返回错误。最好不要因为错误而退出，所以还需要再改一处：

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

去掉job上最后一个unwrap，转而使用match分支：Ok变体就执行job，Err变体则打印 worker 正在断开连接，然后跳出循环。

20.3.3. 试运行

为了测试修改后的行为，我们修改main.rs，让服务器只接受两个请求（通过take限制迭代次数）：

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."); }

控制台输出如下（某次可能的运行结果；任务分配与交错顺序是不确定的）：

Shutting down. Shutting down worker 0 Worker 0 got a job; executing. 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

你可能会看到不同的 Worker id，以及 “got a job”“disconnected”“Shutting down worker” 行的不同交错顺序，但整体模式应该类似。

20.3.4. 总结

main.rs:

use std::{ fs, io::{prelude::*, BufReader}, net::{TcpListener, TcpStream}, thread, time::Duration, }; use web_server::ThreadPool; 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 buf_reader = BufReader::new(&stream); let request_line = buf_reader.lines().next().unwrap().unwrap(); let (status_line, filename) = match &request_line[..] { "GET / HTTP/1.1" => ("HTTP/1.1 200 OK", "hello.html"), "GET /sleep HTTP/1.1" => { thread::sleep(Duration::from_secs(5)); ("HTTP/1.1 200 OK", "hello.html") } _ => ("HTTP/1.1 404 NOT FOUND", "404.html"), }; let contents = fs::read_to_string(filename).unwrap(); let length = contents.len(); let response = format!("{status_line}\r\nContent-Length: {length}\r\n\r\n{contents}"); stream.write_all(response.as_bytes()).unwrap(); }

lib.rs:

use std::{ sync::{mpsc, Arc, Mutex}, thread, }; pub struct ThreadPool { workers: Vec<Worker>, sender: Option<mpsc::Sender<Job>>, } 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(); } } } } 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 mut workers = Vec::with_capacity(size); let receiver = Arc::new(Mutex::new(receiver)); 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(); } } 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(); match job { Ok(job) => { println!("Worker {} got a job; executing.", id); job(); } Err(_) => { println!("Worker {} disconnected; shutting down.", id); break; } } }); Worker { id, thread: Some(thread), } } }

hello.html:

<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>Hello!</title> </head> <body> <h1>Hello!</h1> <p>Hi from Rust</p> </body> </html>

404.html:

<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>Hello!</title> </head> <body> <h1>Oops!</h1> <p>Sorry, I don't know what you're asking for.</p> </body> </html>