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<h2 id="优雅停机与清理"><a class="header" href="#优雅停机与清理">优雅停机与清理</a></h2>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch21-03-graceful-shutdown-and-cleanup.md">ch21-03-graceful-shutdown-and-cleanup.md</a>
<br>
commit 3e5105b52f7e8d3d95def07ffade4dcb1cfdee27</p>
</blockquote>
<p>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 <code>workers</code>、<code>id</code> 和 <code>thread</code> 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 <span class="keystroke">ctrl-c</span> 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。</p>
<p>现在我们要为 <code>ThreadPool</code> 实现 <code>Drop</code> trait 对线程池中的每一个线程调用 <code>join</code>，这样这些线程将会执行完它们的请求。接着会为 <code>ThreadPool</code> 实现一个告诉线程它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。</p>
<h3 id="为-threadpool-实现-drop-trait"><a class="header" href="#为-threadpool-实现-drop-trait">为 <code>ThreadPool</code> 实现 <code>Drop</code> Trait</a></h3>
<p>现在开始为线程池实现 <code>Drop</code>。当线程池被丢弃时，应该 join 所有线程以确保它们完成其操作。示例 20-22 展示了 <code>Drop</code> 实现的第一次尝试；这些代码还不能够编译：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::{
</span><span class="boring"> sync::{mpsc, Arc, Mutex},
</span><span class="boring"> thread,
</span><span class="boring">};
</span><span class="boring">
</span><span class="boring">pub struct ThreadPool {
</span><span class="boring"> workers: Vec&lt;Worker&gt;,
</span><span class="boring"> sender: mpsc::Sender&lt;Job&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
</span><span class="boring">
</span><span class="boring">impl ThreadPool {
</span><span class="boring"> /// Create a new ThreadPool.
</span><span class="boring"> ///
</span><span class="boring"> /// The size is the number of threads in the pool.
</span><span class="boring"> ///
</span><span class="boring"> /// # Panics
</span><span class="boring"> ///
</span><span class="boring"> /// The `new` function will panic if the size is zero.
</span><span class="boring"> pub fn new(size: usize) -&gt; ThreadPool {
</span><span class="boring"> assert!(size &gt; 0);
</span><span class="boring">
</span><span class="boring"> let (sender, receiver) = mpsc::channel();
</span><span class="boring">
</span><span class="boring"> let receiver = Arc::new(Mutex::new(receiver));
</span><span class="boring">
</span><span class="boring"> let mut workers = Vec::with_capacity(size);
</span><span class="boring">
</span><span class="boring"> for id in 0..size {
</span><span class="boring"> workers.push(Worker::new(id, Arc::clone(&amp;receiver)));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> ThreadPool { workers, sender }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> pub fn execute&lt;F&gt;(&amp;self, f: F)
</span><span class="boring"> where
</span><span class="boring"> F: FnOnce() + Send + 'static,
</span><span class="boring"> {
</span><span class="boring"> let job = Box::new(f);
</span><span class="boring">
</span><span class="boring"> self.sender.send(job).unwrap();
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span>impl Drop for ThreadPool {
fn drop(&amp;mut self) {
for worker in &amp;mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
<span class="boring">
</span><span class="boring">struct Worker {
</span><span class="boring"> id: usize,
</span><span class="boring"> thread: thread::JoinHandle&lt;()&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Worker {
</span><span class="boring"> fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
</span><span class="boring"> let thread = thread::spawn(move || loop {
</span><span class="boring"> let job = receiver.lock().unwrap().recv().unwrap();
</span><span class="boring">
</span><span class="boring"> println!("Worker {id} got a job; executing.");
</span><span class="boring">
</span><span class="boring"> job();
</span><span class="boring"> });
</span><span class="boring">
</span><span class="boring"> Worker { id, thread }
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 20-22: 当线程池离开作用域时 join 每个线程</span></p>
<p>这里首先遍历线程池中的每个 <code>workers</code>。这里使用了 <code>&amp;mut</code> 因为 <code>self</code> 本身是一个可变引用而且也需要能够修改 <code>worker</code>。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 <code>join</code>。如果 <code>join</code> 调用失败，通过 <code>unwrap</code> 使得 panic 并进行不优雅的关闭。</p>
<p>如下是尝试编译代码时得到的错误：</p>
<pre><code class="language-console">$ cargo check
Checking hello v0.1.0 (file:///projects/hello)
error[E0507]: cannot move out of `worker.thread` which is behind a mutable reference
--&gt; 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&lt;()&gt;`, which does not implement the `Copy` trait
|
note: `JoinHandle::&lt;T&gt;::join` takes ownership of the receiver `self`, which moves `worker.thread`
--&gt; /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:1778:17
For more information about this error, try `rustc --explain E0507`.
error: could not compile `hello` (lib) due to 1 previous error
</code></pre>
<p>这里的错误告诉我们并不能调用 <code>join</code>，因为我们只有每一个 <code>worker</code> 的可变借用，而 <code>join</code> 需要获取其参数的所有权。为了解决这个问题，需要一个方法将 <code>thread</code> 移动出拥有其所有权的 <code>Worker</code> 实例以便 <code>join</code> 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 <code>Worker</code> 存放的是 <code>Option&lt;thread::JoinHandle&lt;()&gt;</code>，就可以在 <code>Option</code> 上调用 <code>take</code> 方法将值从 <code>Some</code> 成员中移动出来而对 <code>None</code> 成员不做处理。换句话说，正在运行的 <code>Worker</code> 的 <code>thread</code> 将是 <code>Some</code> 成员值，而当需要清理 worker 时，将 <code>Some</code> 替换为 <code>None</code>，这样 worker 就没有可以运行的线程了。</p>
<p>为此需要更新 <code>Worker</code> 的定义为如下：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::{
</span><span class="boring"> sync::{mpsc, Arc, Mutex},
</span><span class="boring"> thread,
</span><span class="boring">};
</span><span class="boring">
</span><span class="boring">pub struct ThreadPool {
</span><span class="boring"> workers: Vec&lt;Worker&gt;,
</span><span class="boring"> sender: mpsc::Sender&lt;Job&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
</span><span class="boring">
</span><span class="boring">impl ThreadPool {
</span><span class="boring"> /// Create a new ThreadPool.
</span><span class="boring"> ///
</span><span class="boring"> /// The size is the number of threads in the pool.
</span><span class="boring"> ///
</span><span class="boring"> /// # Panics
</span><span class="boring"> ///
</span><span class="boring"> /// The `new` function will panic if the size is zero.
</span><span class="boring"> pub fn new(size: usize) -&gt; ThreadPool {
</span><span class="boring"> assert!(size &gt; 0);
</span><span class="boring">
</span><span class="boring"> let (sender, receiver) = mpsc::channel();
</span><span class="boring">
</span><span class="boring"> let receiver = Arc::new(Mutex::new(receiver));
</span><span class="boring">
</span><span class="boring"> let mut workers = Vec::with_capacity(size);
</span><span class="boring">
</span><span class="boring"> for id in 0..size {
</span><span class="boring"> workers.push(Worker::new(id, Arc::clone(&amp;receiver)));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> ThreadPool { workers, sender }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> pub fn execute&lt;F&gt;(&amp;self, f: F)
</span><span class="boring"> where
</span><span class="boring"> F: FnOnce() + Send + 'static,
</span><span class="boring"> {
</span><span class="boring"> let job = Box::new(f);
</span><span class="boring">
</span><span class="boring"> self.sender.send(job).unwrap();
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Drop for ThreadPool {
</span><span class="boring"> fn drop(&amp;mut self) {
</span><span class="boring"> for worker in &amp;mut self.workers {
</span><span class="boring"> println!("Shutting down worker {}", worker.id);
</span><span class="boring">
</span><span class="boring"> worker.thread.join().unwrap();
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span>struct Worker {
id: usize,
thread: Option&lt;thread::JoinHandle&lt;()&gt;&gt;,
}
<span class="boring">
</span><span class="boring">impl Worker {
</span><span class="boring"> fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
</span><span class="boring"> let thread = thread::spawn(move || loop {
</span><span class="boring"> let job = receiver.lock().unwrap().recv().unwrap();
</span><span class="boring">
</span><span class="boring"> println!("Worker {id} got a job; executing.");
</span><span class="boring">
</span><span class="boring"> job();
</span><span class="boring"> });
</span><span class="boring">
</span><span class="boring"> Worker { id, thread }
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p>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：</p>
<pre><code class="language-console">$ cargo check
Checking hello v0.1.0 (file:///projects/hello)
error[E0599]: no method named `join` found for enum `Option` in the current scope
--&gt; src/lib.rs:52:27
|
52 | worker.thread.join().unwrap();
| ^^^^ method not found in `Option&lt;JoinHandle&lt;()&gt;&gt;`
|
note: the method `join` exists on the type `JoinHandle&lt;()&gt;`
--&gt; /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:1778:5
help: consider using `Option::expect` to unwrap the `JoinHandle&lt;()&gt;` value, panicking if the value is an `Option::None`
|
52 | worker.thread.expect("REASON").join().unwrap();
| +++++++++++++++++
error[E0308]: mismatched types
--&gt; src/lib.rs:72:22
|
72 | Worker { id, thread }
| ^^^^^^ expected `Option&lt;JoinHandle&lt;()&gt;&gt;`, found `JoinHandle&lt;_&gt;`
|
= note: expected enum `Option&lt;JoinHandle&lt;()&gt;&gt;`
found struct `JoinHandle&lt;_&gt;`
help: try wrapping the expression in `Some`
|
72 | Worker { id, thread: Some(thread) }
| +++++++++++++ +
Some errors have detailed explanations: E0308, E0599.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `hello` (lib) due to 2 previous errors
</code></pre>
<p>让我们修复第二个错误，它指向 <code>Worker::new</code> 结尾的代码；当新建 <code>Worker</code> 时需要将 <code>thread</code> 值封装进 <code>Some</code>。做出如下改变以修复问题：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::{
</span><span class="boring"> sync::{mpsc, Arc, Mutex},
</span><span class="boring"> thread,
</span><span class="boring">};
</span><span class="boring">
</span><span class="boring">pub struct ThreadPool {
</span><span class="boring"> workers: Vec&lt;Worker&gt;,
</span><span class="boring"> sender: mpsc::Sender&lt;Job&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
</span><span class="boring">
</span><span class="boring">impl ThreadPool {
</span><span class="boring"> /// Create a new ThreadPool.
</span><span class="boring"> ///
</span><span class="boring"> /// The size is the number of threads in the pool.
</span><span class="boring"> ///
</span><span class="boring"> /// # Panics
</span><span class="boring"> ///
</span><span class="boring"> /// The `new` function will panic if the size is zero.
</span><span class="boring"> pub fn new(size: usize) -&gt; ThreadPool {
</span><span class="boring"> assert!(size &gt; 0);
</span><span class="boring">
</span><span class="boring"> let (sender, receiver) = mpsc::channel();
</span><span class="boring">
</span><span class="boring"> let receiver = Arc::new(Mutex::new(receiver));
</span><span class="boring">
</span><span class="boring"> let mut workers = Vec::with_capacity(size);
</span><span class="boring">
</span><span class="boring"> for id in 0..size {
</span><span class="boring"> workers.push(Worker::new(id, Arc::clone(&amp;receiver)));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> ThreadPool { workers, sender }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> pub fn execute&lt;F&gt;(&amp;self, f: F)
</span><span class="boring"> where
</span><span class="boring"> F: FnOnce() + Send + 'static,
</span><span class="boring"> {
</span><span class="boring"> let job = Box::new(f);
</span><span class="boring">
</span><span class="boring"> self.sender.send(job).unwrap();
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Drop for ThreadPool {
</span><span class="boring"> fn drop(&amp;mut self) {
</span><span class="boring"> for worker in &amp;mut self.workers {
</span><span class="boring"> println!("Shutting down worker {}", worker.id);
</span><span class="boring">
</span><span class="boring"> worker.thread.join().unwrap();
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">struct Worker {
</span><span class="boring"> id: usize,
</span><span class="boring"> thread: Option&lt;thread::JoinHandle&lt;()&gt;&gt;,
</span><span class="boring">}
</span><span class="boring">
</span>impl Worker {
fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
// --snip--
<span class="boring"> let thread = thread::spawn(move || loop {
</span><span class="boring"> let job = receiver.lock().unwrap().recv().unwrap();
</span><span class="boring">
</span><span class="boring"> println!("Worker {id} got a job; executing.");
</span><span class="boring">
</span><span class="boring"> job();
</span><span class="boring"> });
</span><span class="boring">
</span> Worker {
id,
thread: Some(thread),
}
}
}</code></pre>
<p>第一个错误位于 <code>Drop</code> 实现中。之前提到过要调用 <code>Option</code> 上的 <code>take</code> 将 <code>thread</code> 移动出 <code>worker</code>。如下改变会修复问题：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore not_desired_behavior"><span class="boring">use std::{
</span><span class="boring"> sync::{mpsc, Arc, Mutex},
</span><span class="boring"> thread,
</span><span class="boring">};
</span><span class="boring">
</span><span class="boring">pub struct ThreadPool {
</span><span class="boring"> workers: Vec&lt;Worker&gt;,
</span><span class="boring"> sender: mpsc::Sender&lt;Job&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
</span><span class="boring">
</span><span class="boring">impl ThreadPool {
</span><span class="boring"> /// Create a new ThreadPool.
</span><span class="boring"> ///
</span><span class="boring"> /// The size is the number of threads in the pool.
</span><span class="boring"> ///
</span><span class="boring"> /// # Panics
</span><span class="boring"> ///
</span><span class="boring"> /// The `new` function will panic if the size is zero.
</span><span class="boring"> pub fn new(size: usize) -&gt; ThreadPool {
</span><span class="boring"> assert!(size &gt; 0);
</span><span class="boring">
</span><span class="boring"> let (sender, receiver) = mpsc::channel();
</span><span class="boring">
</span><span class="boring"> let receiver = Arc::new(Mutex::new(receiver));
</span><span class="boring">
</span><span class="boring"> let mut workers = Vec::with_capacity(size);
</span><span class="boring">
</span><span class="boring"> for id in 0..size {
</span><span class="boring"> workers.push(Worker::new(id, Arc::clone(&amp;receiver)));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> ThreadPool { workers, sender }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> pub fn execute&lt;F&gt;(&amp;self, f: F)
</span><span class="boring"> where
</span><span class="boring"> F: FnOnce() + Send + 'static,
</span><span class="boring"> {
</span><span class="boring"> let job = Box::new(f);
</span><span class="boring">
</span><span class="boring"> self.sender.send(job).unwrap();
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span>impl Drop for ThreadPool {
fn drop(&amp;mut self) {
for worker in &amp;mut self.workers {
println!("Shutting down worker {}", worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
<span class="boring">
</span><span class="boring">struct Worker {
</span><span class="boring"> id: usize,
</span><span class="boring"> thread: Option&lt;thread::JoinHandle&lt;()&gt;&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Worker {
</span><span class="boring"> fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
</span><span class="boring"> let thread = thread::spawn(move || loop {
</span><span class="boring"> let job = receiver.lock().unwrap().recv().unwrap();
</span><span class="boring">
</span><span class="boring"> println!("Worker {id} got a job; executing.");
</span><span class="boring">
</span><span class="boring"> job();
</span><span class="boring"> });
</span><span class="boring">
</span><span class="boring"> Worker {
</span><span class="boring"> id,
</span><span class="boring"> thread: Some(thread),
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p>如第十八章我们见过的，<code>Option</code> 上的 <code>take</code> 方法会取出 <code>Some</code> 而留下 <code>None</code>。使用 <code>if let</code> 解构 <code>Some</code> 并得到线程，接着在线程上调用 <code>join</code>。如果 worker 的线程已然是 <code>None</code>，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。</p>
<h3 id="向线程发送信号使其停止接收任务"><a class="header" href="#向线程发送信号使其停止接收任务">向线程发送信号使其停止接收任务</a></h3>
<p>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 <code>Worker</code> 中分配的线程所运行的闭包中的逻辑：调用 <code>join</code> 并不会关闭线程，因为它们一直 <code>loop</code> 来寻找任务。如果采用这个实现来尝试丢弃 <code>ThreadPool</code>，则主线程会永远阻塞在等待第一个线程结束上。</p>
<p>为了修复这个问题，我们将修改 <code>ThreadPool</code> 的 <code>drop</code> 实现并修改 <code>Worker</code> 循环。</p>
<p>首先修改 <code>ThreadPool</code> 的 <code>drop</code> 实现在等待线程结束前显式丢弃 <code>sender</code>。示例 20-23 展示了 <code>ThreadPool</code> 显式丢弃 <code>sender</code> 所作的修改。我们使用了与之前处理线程时相同的 <code>Option</code> 和 <code>take</code> 技术以便能从 <code>ThreadPool</code> 中移动 <code>sender</code>：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground not_desired_behavior"><span class="boring">use std::{
</span><span class="boring"> sync::{mpsc, Arc, Mutex},
</span><span class="boring"> thread,
</span><span class="boring">};
</span><span class="boring">
</span>pub struct ThreadPool {
workers: Vec&lt;Worker&gt;,
sender: Option&lt;mpsc::Sender&lt;Job&gt;&gt;,
}
// --snip--
<span class="boring">
</span><span class="boring">type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
</span><span class="boring">
</span>impl ThreadPool {
<span class="boring"> /// Create a new ThreadPool.
</span><span class="boring"> ///
</span><span class="boring"> /// The size is the number of threads in the pool.
</span><span class="boring"> ///
</span><span class="boring"> /// # Panics
</span><span class="boring"> ///
</span><span class="boring"> /// The `new` function will panic if the size is zero.
</span> pub fn new(size: usize) -&gt; ThreadPool {
// --snip--
<span class="boring"> assert!(size &gt; 0);
</span><span class="boring">
</span><span class="boring"> let (sender, receiver) = mpsc::channel();
</span><span class="boring">
</span><span class="boring"> let receiver = Arc::new(Mutex::new(receiver));
</span><span class="boring">
</span><span class="boring"> let mut workers = Vec::with_capacity(size);
</span><span class="boring">
</span><span class="boring"> for id in 0..size {
</span><span class="boring"> workers.push(Worker::new(id, Arc::clone(&amp;receiver)));
</span><span class="boring"> }
</span><span class="boring">
</span> ThreadPool {
workers,
sender: Some(sender),
}
}
pub fn execute&lt;F&gt;(&amp;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(&amp;mut self) {
drop(self.sender.take());
for worker in &amp;mut self.workers {
println!("Shutting down worker {}", worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
<span class="boring">
</span><span class="boring">struct Worker {
</span><span class="boring"> id: usize,
</span><span class="boring"> thread: Option&lt;thread::JoinHandle&lt;()&gt;&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Worker {
</span><span class="boring"> fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
</span><span class="boring"> let thread = thread::spawn(move || loop {
</span><span class="boring"> let job = receiver.lock().unwrap().recv().unwrap();
</span><span class="boring">
</span><span class="boring"> println!("Worker {id} got a job; executing.");
</span><span class="boring">
</span><span class="boring"> job();
</span><span class="boring"> });
</span><span class="boring">
</span><span class="boring"> Worker {
</span><span class="boring"> id,
</span><span class="boring"> thread: Some(thread),
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 20-23: 在 join worker 线程之前显式丢弃 <code>sender</code></span></p>
<p>丢弃 <code>sender</code> 会关闭信道，这表明不会有更多的消息被发送。这时 worker 中的无限循环中的所有 <code>recv</code> 调用都会返回错误。在示例 20-24 中，我们修改 <code>Worker</code> 循环在这种情况下优雅地退出，这意味着当 <code>ThreadPool</code> 的 <code>drop</code> 实现调用 <code>join</code> 时线程会结束。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground"><span class="boring">use std::{
</span><span class="boring"> sync::{mpsc, Arc, Mutex},
</span><span class="boring"> thread,
</span><span class="boring">};
</span><span class="boring">
</span><span class="boring">pub struct ThreadPool {
</span><span class="boring"> workers: Vec&lt;Worker&gt;,
</span><span class="boring"> sender: Option&lt;mpsc::Sender&lt;Job&gt;&gt;,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
</span><span class="boring">
</span><span class="boring">impl ThreadPool {
</span><span class="boring"> /// Create a new ThreadPool.
</span><span class="boring"> ///
</span><span class="boring"> /// The size is the number of threads in the pool.
</span><span class="boring"> ///
</span><span class="boring"> /// # Panics
</span><span class="boring"> ///
</span><span class="boring"> /// The `new` function will panic if the size is zero.
</span><span class="boring"> pub fn new(size: usize) -&gt; ThreadPool {
</span><span class="boring"> assert!(size &gt; 0);
</span><span class="boring">
</span><span class="boring"> let (sender, receiver) = mpsc::channel();
</span><span class="boring">
</span><span class="boring"> let receiver = Arc::new(Mutex::new(receiver));
</span><span class="boring">
</span><span class="boring"> let mut workers = Vec::with_capacity(size);
</span><span class="boring">
</span><span class="boring"> for id in 0..size {
</span><span class="boring"> workers.push(Worker::new(id, Arc::clone(&amp;receiver)));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> ThreadPool {
</span><span class="boring"> workers,
</span><span class="boring"> sender: Some(sender),
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> pub fn execute&lt;F&gt;(&amp;self, f: F)
</span><span class="boring"> where
</span><span class="boring"> F: FnOnce() + Send + 'static,
</span><span class="boring"> {
</span><span class="boring"> let job = Box::new(f);
</span><span class="boring">
</span><span class="boring"> self.sender.as_ref().unwrap().send(job).unwrap();
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Drop for ThreadPool {
</span><span class="boring"> fn drop(&amp;mut self) {
</span><span class="boring"> drop(self.sender.take());
</span><span class="boring">
</span><span class="boring"> for worker in &amp;mut self.workers {
</span><span class="boring"> println!("Shutting down worker {}", worker.id);
</span><span class="boring">
</span><span class="boring"> if let Some(thread) = worker.thread.take() {
</span><span class="boring"> thread.join().unwrap();
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">struct Worker {
</span><span class="boring"> id: usize,
</span><span class="boring"> thread: Option&lt;thread::JoinHandle&lt;()&gt;&gt;,
</span><span class="boring">}
</span><span class="boring">
</span>impl Worker {
fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
let thread = thread::spawn(move || loop {
let message = receiver.lock().unwrap().recv();
match message {
Ok(job) =&gt; {
println!("Worker {id} got a job; executing.");
job();
}
Err(_) =&gt; {
println!("Worker {id} disconnected; shutting down.");
break;
}
}
});
Worker {
id,
thread: Some(thread),
}
}
}</code></pre>
<p><span class="caption">示例 20-24：当 <code>recv</code> 返回错误时显式退出循环</span></p>
<p>为了实践这些代码，如示例 20-25 所示修改 <code>main</code> 在优雅停机 server 之前只接受两个请求：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use hello::ThreadPool;
</span><span class="boring">use std::{
</span><span class="boring"> fs,
</span><span class="boring"> io::{prelude::*, BufReader},
</span><span class="boring"> net::{TcpListener, TcpStream},
</span><span class="boring"> thread,
</span><span class="boring"> time::Duration,
</span><span class="boring">};
</span><span class="boring">
</span>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.");
}
<span class="boring">
</span><span class="boring">fn handle_connection(mut stream: TcpStream) {
</span><span class="boring"> let buf_reader = BufReader::new(&amp;stream);
</span><span class="boring"> let request_line = buf_reader.lines().next().unwrap().unwrap();
</span><span class="boring">
</span><span class="boring"> let (status_line, filename) = match &amp;request_line[..] {
</span><span class="boring"> "GET / HTTP/1.1" =&gt; ("HTTP/1.1 200 OK", "hello.html"),
</span><span class="boring"> "GET /sleep HTTP/1.1" =&gt; {
</span><span class="boring"> thread::sleep(Duration::from_secs(5));
</span><span class="boring"> ("HTTP/1.1 200 OK", "hello.html")
</span><span class="boring"> }
</span><span class="boring"> _ =&gt; ("HTTP/1.1 404 NOT FOUND", "404.html"),
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> let contents = fs::read_to_string(filename).unwrap();
</span><span class="boring"> let length = contents.len();
</span><span class="boring">
</span><span class="boring"> let response =
</span><span class="boring"> format!("{status_line}\r\nContent-Length: {length}\r\n\r\n{contents}");
</span><span class="boring">
</span><span class="boring"> stream.write_all(response.as_bytes()).unwrap();
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 20-25: 在处理两个请求之后通过退出循环来停止 server</span></p>
<p>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。</p>
<p><code>take</code> 方法定义于 <code>Iterator</code> trait，这里限制循环最多头 2 次。<code>ThreadPool</code> 会在 <code>main</code> 的结尾离开作用域，而且还会看到 <code>drop</code> 实现的运行。</p>
<p>使用 <code>cargo run</code> 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：</p>
<pre><code class="language-console">$ 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
</code></pre>
<p>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。server 会在头第二个请求后停止接受请求，<code>ThreadPool</code> 的 <code>Drop</code> 实现甚至会在 worker 3 开始工作之前就开始执行。丢弃 <code>sender</code> 会断开所有 worker 的连接并让它们关闭。每个 worker 在断开时会打印出一个信息，接着线程池调用 <code>join</code> 来等待每一个 worker 线程结束。</p>
<p>这个特定的运行过程中一个有趣的地方在于：<code>ThreadPool</code> 丢弃 <code>sender</code>，而在任何线程收到消息之前，就尝试 join worker 0 了。worker 0 还没有从 <code>recv</code> 获得一个错误，所以主线程阻塞直到 worker 0 结束。与此同时，worker 3 接收到一个任务接着所有线程会收到一个错误。一旦 worker 0 结束，主线程就等待余下其他 worker 结束。此时它们都退出了循环并停止。</p>
<p>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。</p>
<p>如下是完整的代码参考：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore">use hello::ThreadPool;
use std::{
fs,
io::{prelude::*, BufReader},
net::{TcpListener, TcpStream},
thread,
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 buf_reader = BufReader::new(&amp;stream);
let request_line = buf_reader.lines().next().unwrap().unwrap();
let (status_line, filename) = match &amp;request_line[..] {
"GET / HTTP/1.1" =&gt; ("HTTP/1.1 200 OK", "hello.html"),
"GET /sleep HTTP/1.1" =&gt; {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
}
_ =&gt; ("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();
}</code></pre>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec&lt;Worker&gt;,
sender: Option&lt;mpsc::Sender&lt;Job&gt;&gt;,
}
type Job = Box&lt;dyn FnOnce() + Send + 'static&gt;;
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) -&gt; ThreadPool {
assert!(size &gt; 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(&amp;receiver)));
}
ThreadPool {
workers,
sender: Some(sender),
}
}
pub fn execute&lt;F&gt;(&amp;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(&amp;mut self) {
drop(self.sender.take());
for worker in &amp;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&lt;thread::JoinHandle&lt;()&gt;&gt;,
}
impl Worker {
fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
let thread = thread::spawn(move || loop {
let message = receiver.lock().unwrap().recv();
match message {
Ok(job) =&gt; {
println!("Worker {id} got a job; executing.");
job();
}
Err(_) =&gt; {
println!("Worker {id} disconnected; shutting down.");
break;
}
}
});
Worker {
id,
thread: Some(thread),
}
}
}</code></pre>
<p>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：</p>
<ul>
<li>为 <code>ThreadPool</code> 和其公有方法增加更多文档</li>
<li>为库的功能增加测试</li>
<li>将 <code>unwrap</code> 调用改为更健壮的错误处理</li>
<li>使用 <code>ThreadPool</code> 进行其他不同于处理网络请求的任务</li>
<li>在 <a href="https://crates.io/">crates.io</a> 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</li>
</ul>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>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。</p>
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