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 Drop trait
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 Drop trait。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 Drop trait：
#![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();
}
}
}
}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：
#![allow(unused)]
fn main() {
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：
#![allow(unused)]
fn main() {
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)。
#![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();
}
}
}
}
}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：
#![allow(unused)]
fn main() {
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：
#![allow(unused)]
fn main() {
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。
#![allow(unused)]
fn main() {
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。
#![allow(unused)]
fn main() {
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：
#![allow(unused)]
fn main() {
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:
#![allow(unused)]
fn main() {
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>