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<h1 id="优雅关闭和资源清理"><a class="header" href="#优雅关闭和资源清理">优雅关闭和资源清理</a></h1>
<p>之前的程序，如果使用 <code>ctrl-c</code> 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。</p>
<p>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。</p>
<h2 id="为线程池实现-drop"><a class="header" href="#为线程池实现-drop">为线程池实现 Drop</a></h2>
<p>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:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Drop for ThreadPool {
fn drop(&amp;mut self) {
for worker in &amp;mut self.workers {
println!(&quot;Shutting down worker {}&quot;, worker.id);
worker.thread.join().unwrap();
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>这里通过实现 <code>Drop</code> 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 <code>join</code> 方法。编译下试试：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>$ 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: 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
<span class="boring">}</span></code></pre></pre>
<p>这里的报错很明显，<code>worker.thread</code> 试图拿走所有权，但是 <code>worker</code> 仅仅是一个可变借用，显然是不可行的。</p>
<p>目前来看，只能将 <code>thread</code> 从 <code>worker</code> 中移动出来，一个可行的尝试:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>struct Worker {
id: usize,
thread: Option&lt;thread::JoinHandle&lt;()&gt;&gt;,
}
<span class="boring">}</span></code></pre></pre>
<p>对于 <code>Option</code> 类型，可以使用 <code>take</code> 方法拿走内部值的所有权，同时留下一个 <code>None</code> 在风中孤独凌乱。继续尝试编译驱动开发模式:</p>
<pre><code class="language-shell">$ 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;`
help: consider using `Option::expect` to unwrap the `JoinHandle&lt;()&gt;` value, panicking if the value is an `Option::None`
|
52 | worker.thread.expect(&quot;REASON&quot;).join().unwrap();
| +++++++++++++++++
error[E0308]: mismatched types
--&gt; src/lib.rs:72:22
|
72 | Worker { id, thread }
| ^^^^^^ expected enum `Option`, found struct `JoinHandle`
|
= 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) }
| +++++++++++++ +
</code></pre>
<p>先来解决第二个类型不匹配的错误:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Worker {
fn new(id: usize, receiver: Arc&lt;Mutex&lt;mpsc::Receiver&lt;Job&gt;&gt;&gt;) -&gt; Worker {
// --snip--
Worker {
id,
thread: Some(thread),
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>简单搞定，回头看看第一个错误，既然换了 <code>Option</code>，就可以用 <code>take</code> 拿走所有权:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Drop for ThreadPool {
fn drop(&amp;mut self) {
for worker in &amp;mut self.workers {
println!(&quot;Shutting down worker {}&quot;, worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>注意这种 <code>if let</code> 的写法，若 <code>worker.thread</code> 已经是 <code>None</code>，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 <code>join</code>。</p>
<h2 id="停止工作线程"><a class="header" href="#停止工作线程">停止工作线程</a></h2>
<p>虽然调用了 <code>join</code> ，但是目标线程依然不会停止，原因在于它们在无限的 <code>loop</code> 循环等待，看起来需要借用 <code>channel</code> 的 <code>drop</code> 机制：释放 <code>sender</code>发送端后，<code>receiver</code> 接收端会收到报错，然后再退出即可。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct ThreadPool {
workers: Vec&lt;Worker&gt;,
sender: Option&lt;mpsc::Sender&lt;Job&gt;&gt;,
}
// --snip--
impl ThreadPool {
pub fn new(size: usize) -&gt; ThreadPool {
// --snip--
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!(&quot;Shutting down worker {}&quot;, worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>上面做了两处改变:</p>
<ol>
<li>为 <code>sender</code> 增加 <code>Option</code> 封装，这样可以用 <code>take</code> 拿走所有权，跟之前的 <code>thread</code> 一样</li>
<li>主动调用 <code>drop</code> 关闭发送端 <code>sender</code></li>
</ol>
<p>关闭 <code>sender</code> 后，将关闭对应的 <code>channel</code>，意味着不会再有任何消息被发送。随后，所有的处于无限 <code>loop</code> 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。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</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!(&quot;Worker {id} got a job; executing.&quot;);
job();
}
Err(_) =&gt; {
println!(&quot;Worker {id} disconnected; shutting down.&quot;);
break;
}
}
});
Worker {
id,
thread: Some(thread),
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>为了快速验证代码是否正确，修改 <code>main</code> 函数，让其只接收前两个请求:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let listener = TcpListener::bind(&quot;127.0.0.1:7878&quot;).unwrap();
let pool = ThreadPool::new(4);
for stream in listener.incoming().take(2) {
let stream = stream.unwrap();
pool.execute(|| {
handle_connection(stream);
});
}
println!(&quot;Shutting down.&quot;);
}</code></pre></pre>
<p><code>take</code> 是迭代器 <code>Iterator</code> 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 <code>ThreadPool</code> 也将超出作用域并自动触发 <code>drop</code>。</p>
<pre><code class="language-shell">$ 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:</p>
<h2 id="完整代码"><a class="header" href="#完整代码">完整代码</a></h2>
<pre><pre class="playground"><code class="language-rust edition2021">// 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(&quot;127.0.0.1:7878&quot;).unwrap();
let pool = ThreadPool::new(4);
for stream in listener.incoming().take(2) {
let stream = stream.unwrap();
pool.execute(|| {
handle_connection(stream);
});
}
println!(&quot;Shutting down.&quot;);
}
fn handle_connection(mut stream: TcpStream) {
let mut buffer = [0; 1024];
stream.read(&amp;mut buffer).unwrap();
let get = b&quot;GET / HTTP/1.1\r\n&quot;;
let sleep = b&quot;GET /sleep HTTP/1.1\r\n&quot;;
let (status_line, filename) = if buffer.starts_with(get) {
(&quot;HTTP/1.1 200 OK&quot;, &quot;hello.html&quot;)
} else if buffer.starts_with(sleep) {
thread::sleep(Duration::from_secs(5));
(&quot;HTTP/1.1 200 OK&quot;, &quot;hello.html&quot;)
} else {
(&quot;HTTP/1.1 404 NOT FOUND&quot;, &quot;404.html&quot;)
};
let contents = fs::read_to_string(filename).unwrap();
let response = format!(
&quot;{}\r\nContent-Length: {}\r\n\r\n{}&quot;,
status_line,
contents.len(),
contents
);
stream.write_all(response.as_bytes()).unwrap();
stream.flush().unwrap();
}</code></pre></pre>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// src/lib.rs
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!(&quot;Shutting down worker {}&quot;, 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!(&quot;Worker {id} got a job; executing.&quot;);
job();
}
Err(_) =&gt; {
println!(&quot;Worker {id} disconnected; shutting down.&quot;);
break;
}
}
});
Worker {
id,
thread: Some(thread),
}
}
}
<span class="boring">}</span></code></pre></pre>
<h3 id="可以做的更多"><a class="header" href="#可以做的更多">可以做的更多</a></h3>
<p>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。</p>
<ul>
<li>增加更多的文档</li>
<li>为线程池增加测试</li>
<li>尽可能移除 <code>unwrap</code>，替换为错误处理</li>
<li>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</li>
<li>在 <code>crates.io</code> 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</li>
</ul>
<h2 id="上一章节的遗留问题"><a class="header" href="#上一章节的遗留问题">上一章节的遗留问题</a></h2>
<p>在上一章节的末尾，我们提到将 <code>let</code> 替换为 <code>while let</code> 后，多线程的优势将荡然无存，原因藏的很隐蔽：</p>
<ol>
<li><code>Mutex</code> 结构体没有提供显式的 <code>unlock</code>，要依赖作用域结束后的 <code>drop</code> 来自动释放 </li>
<li><code>let job = receiver.lock().unwrap().recv().unwrap();</code> 在这行代码中，由于使用了 <code>let</code>，右边的任何临时变量会在 <code>let</code> 语句结束后立即被 <code>drop</code>，因此锁会自动释放</li>
<li>然而 <code>while let</code> (还包括 <code>if let</code> 和 <code>match</code>) 直到最后一个花括号后，才触发 <code>drop</code></li>
</ol>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</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 || {
while let Ok(job) = receiver.lock().unwrap().recv() {
println!(&quot;Worker {id} got a job; executing.&quot;);
job();
}
});
Worker { id, thread }
}
}
<span class="boring">}</span></code></pre></pre>
<p>根据之前的分析，上面的代码直到 <code>job()</code> 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。</p>
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