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<main>
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<h1 id="select"><a class="header" href="#select">select!</a></h1>
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<p>在实际使用时，一个重要的场景就是同时等待多个异步操作的结果，并且对其结果进行进一步处理，在本章节，我们来看看，强大的 <code>select!</code> 是如何帮助咱们更好的控制多个异步操作并发执行的。</p>
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<h2 id="tokioselect"><a class="header" href="#tokioselect">tokio::select!</a></h2>
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<p><code>select!</code> 允许同时等待多个计算操作，然后当其中一个操作完成时就退出等待:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">use tokio::sync::oneshot;
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#[tokio::main]
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async fn main() {
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let (tx1, rx1) = oneshot::channel();
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let (tx2, rx2) = oneshot::channel();
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tokio::spawn(async {
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let _ = tx1.send("one");
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});
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tokio::spawn(async {
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let _ = tx2.send("two");
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});
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tokio::select! {
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val = rx1 => {
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println!("rx1 completed first with {:?}", val);
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}
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val = rx2 => {
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println!("rx2 completed first with {:?}", val);
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}
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}
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// 任何一个 select 分支结束后，都会继续执行接下来的代码
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}
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</code></pre></pre>
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<p>这里用到了两个 <code>oneshot</code> 消息通道，虽然两个操作的创建在代码上有先后顺序，但在实际执行时却不这样。因此， <code>select</code> 在从两个通道<strong>阻塞等待</strong>接收消息时，<code>rx1</code> 和 <code>rx2</code> 都有可能被先打印出来。</p>
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<p>需要注意，任何一个 <code>select</code> 分支完成后，都会继续执行后面的代码，没被执行的分支会被丢弃( <code>dropped</code> )。</p>
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<h4 id="取消"><a class="header" href="#取消">取消</a></h4>
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<p>对于 <code>Async Rust</code> 来说，释放( drop )掉一个 <code>Future</code> 就意味着取消任务。从上一章节可以得知， <code>async</code> 操作会返回一个 <code>Future</code>，而后者是惰性的，直到被 <code>poll</code> 调用时，才会被执行。一旦 <code>Future</code> 被释放，那操作将无法继续，因为所有相关的状态都被释放。</p>
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<p>对于 Tokio 的 <code>oneshot</code> 的接收端来说，它在被释放时会发送一个关闭通知到发送端，因此发送端可以通过释放任务的方式来终止正在执行的任务。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::sync::oneshot;
|
|
|
|
|
|
|
|
|
|
|
|
async fn some_operation() -> String {
|
|
|
|
|
|
// 在这里执行一些操作...
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (mut tx1, rx1) = oneshot::channel();
|
|
|
|
|
|
let (tx2, rx2) = oneshot::channel();
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async {
|
|
|
|
|
|
// 等待 `some_operation` 的完成
|
|
|
|
|
|
// 或者处理 `oneshot` 的关闭通知
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
val = some_operation() => {
|
|
|
|
|
|
let _ = tx1.send(val);
|
|
|
|
|
|
}
|
|
|
|
|
|
_ = tx1.closed() => {
|
|
|
|
|
|
// 收到了发送端发来的关闭信号
|
|
|
|
|
|
// `select` 即将结束，此时，正在进行的 `some_operation()` 任务会被取消，任务自动完成，
|
|
|
|
|
|
// tx1 被释放
|
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|
|
}
|
|
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|
}
|
|
|
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|
|
});
|
|
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|
|
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|
|
|
|
tokio::spawn(async {
|
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|
|
let _ = tx2.send("two");
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
val = rx1 => {
|
|
|
|
|
|
println!("rx1 completed first with {:?}", val);
|
|
|
|
|
|
}
|
|
|
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|
|
val = rx2 => {
|
|
|
|
|
|
println!("rx2 completed first with {:?}", val);
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>上面代码的重点就在于 <code>tx1.closed</code> 所在的分支，一旦发送端被关闭，那该分支就会被执行，然后 <code>select</code> 会退出，并清理掉还没执行的第一个分支 <code>val = some_operation()</code> ，这其中 <code>some_operation</code> 返回的 <code>Future</code> 也会被清理，根据之前的内容，<code>Future</code> 被清理那相应的任务会立即取消，因此 <code>some_operation</code> 会被取消，不再执行。</p>
|
|
|
|
|
|
<h4 id="future-的实现"><a class="header" href="#future-的实现">Future 的实现</a></h4>
|
|
|
|
|
|
<p>为了更好的理解 <code>select</code> 的工作原理，我们来看看如果使用 <code>Future</code> 该如何实现。当然，这里是一个简化版本，在实际中，<code>select!</code> 会包含一些额外的功能，例如一开始会随机选择一个分支进行 <code>poll</code>。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::sync::oneshot;
|
|
|
|
|
|
use std::future::Future;
|
|
|
|
|
|
use std::pin::Pin;
|
|
|
|
|
|
use std::task::{Context, Poll};
|
|
|
|
|
|
|
|
|
|
|
|
struct MySelect {
|
|
|
|
|
|
rx1: oneshot::Receiver<&'static str>,
|
|
|
|
|
|
rx2: oneshot::Receiver<&'static str>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl Future for MySelect {
|
|
|
|
|
|
type Output = ();
|
|
|
|
|
|
|
|
|
|
|
|
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
|
|
|
|
|
|
if let Poll::Ready(val) = Pin::new(&mut self.rx1).poll(cx) {
|
|
|
|
|
|
println!("rx1 completed first with {:?}", val);
|
|
|
|
|
|
return Poll::Ready(());
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
if let Poll::Ready(val) = Pin::new(&mut self.rx2).poll(cx) {
|
|
|
|
|
|
println!("rx2 completed first with {:?}", val);
|
|
|
|
|
|
return Poll::Ready(());
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
Poll::Pending
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (tx1, rx1) = oneshot::channel();
|
|
|
|
|
|
let (tx2, rx2) = oneshot::channel();
|
|
|
|
|
|
|
|
|
|
|
|
// 使用 tx1 和 tx2
|
|
|
|
|
|
|
|
|
|
|
|
MySelect {
|
|
|
|
|
|
rx1,
|
|
|
|
|
|
rx2,
|
|
|
|
|
|
}.await;
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p><code>MySelect</code> 包含了两个分支中的 <code>Future</code>，当它被 <code>poll</code> 时，第一个分支会先执行。如果执行完成，那取出的值会被使用，然后 <code>MySelect</code> 也随之结束。而另一个分支对应的 <code>Future</code> 会被释放掉，对应的操作也会被取消。</p>
|
|
|
|
|
|
<p>还记得上一章节中很重要的一段话吗？</p>
|
|
|
|
|
|
<blockquote>
|
|
|
|
|
|
<p>当一个 <code>Future</code> 返回 <code>Poll::Pending</code> 时，它必须确保会在某一个时刻通过 <code>Waker</code> 来唤醒，不然该 <code>Future</code> 将永远地被挂起</p>
|
|
|
|
|
|
</blockquote>
|
|
|
|
|
|
<p>但是仔细观察我们之前的代码，里面并没有任何的 <code>wake</code> 调用！事实上，这是因为参数 <code>cx</code> 被传入了内层的 <code>poll</code> 调用。 只要内部的 <code>Future</code> 实现了唤醒并且返回了 <code>Poll::Pending</code>，那 <code>MySelect</code> 也等于实现了唤醒！</p>
|
|
|
|
|
|
<h2 id="语法"><a class="header" href="#语法">语法</a></h2>
|
|
|
|
|
|
<p>目前来说，<code>select!</code> 最多可以支持 64 个分支，每个分支形式如下：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span><模式> = <async 表达式> => <结果处理>,
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>当 <code>select</code> 宏开始执行后，所有的分支会开始并发的执行。当任何一个<strong>表达式</strong>完成时，会将结果跟<strong>模式</strong>进行匹配。若匹配成功，则剩下的表达式会被释放。</p>
|
|
|
|
|
|
<p>最常用的<strong>模式</strong>就是用变量名去匹配表达式返回的值，然后该变量就可以在<strong>结果处理</strong>环节使用。</p>
|
|
|
|
|
|
<p>如果当前的模式不能匹配，剩余的 <code>async</code> 表达式将继续并发的执行，直到下一个完成。</p>
|
|
|
|
|
|
<p>由于 <code>select!</code> 使用的是一个 <code>async</code> 表达式，因此我们可以定义一些更复杂的计算。</p>
|
|
|
|
|
|
<p>例如从在分支中进行 TCP 连接：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::net::TcpStream;
|
|
|
|
|
|
use tokio::sync::oneshot;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (tx, rx) = oneshot::channel();
|
|
|
|
|
|
|
|
|
|
|
|
// 生成一个任务，用于向 oneshot 发送一条消息
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
tx.send("done").unwrap();
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
socket = TcpStream::connect("localhost:3465") => {
|
|
|
|
|
|
println!("Socket connected {:?}", socket);
|
|
|
|
|
|
}
|
|
|
|
|
|
msg = rx => {
|
|
|
|
|
|
println!("received message first {:?}", msg);
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>再比如，在分支中进行 TCP 监听:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::net::TcpListener;
|
|
|
|
|
|
use tokio::sync::oneshot;
|
|
|
|
|
|
use std::io;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let (tx, rx) = oneshot::channel();
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
tx.send(()).unwrap();
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
let mut listener = TcpListener::bind("localhost:3465").await?;
|
|
|
|
|
|
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
_ = async {
|
|
|
|
|
|
loop {
|
|
|
|
|
|
let (socket, _) = listener.accept().await?;
|
|
|
|
|
|
tokio::spawn(async move { process(socket) });
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
// 给予 Rust 类型暗示
|
|
|
|
|
|
Ok::<_, io::Error>(())
|
|
|
|
|
|
} => {}
|
|
|
|
|
|
_ = rx => {
|
|
|
|
|
|
println!("terminating accept loop");
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>分支中接收连接的循环会一直运行，直到遇到错误才停止，或者当 <code>rx</code> 中有值时，也会停止。 <code>_</code> 表示我们并不关心这个值，这样使用唯一的目的就是为了结束第一分支中的循环。</p>
|
|
|
|
|
|
<h2 id="返回值"><a class="header" href="#返回值">返回值</a></h2>
|
|
|
|
|
|
<p><code>select!</code> 还能返回一个值:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">async fn computation1() -> String {
|
|
|
|
|
|
// .. 计算
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
async fn computation2() -> String {
|
|
|
|
|
|
// .. 计算
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let out = tokio::select! {
|
|
|
|
|
|
res1 = computation1() => res1,
|
|
|
|
|
|
res2 = computation2() => res2,
|
|
|
|
|
|
};
|
|
|
|
|
|
|
|
|
|
|
|
println!("Got = {}", out);
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>需要注意的是，此时 <code>select!</code> 的所有分支必须返回一样的类型，否则编译器会报错！</p>
|
|
|
|
|
|
<h2 id="错误传播"><a class="header" href="#错误传播">错误传播</a></h2>
|
|
|
|
|
|
<p>在 Rust 中使用 <code>?</code> 可以对错误进行传播，但是在 <code>select!</code> 中，<code>?</code> 如何工作取决于它是在分支中的 <code>async</code> 表达式使用还是在结果处理的代码中使用:</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>在分支中 <code>async</code> 表达式使用会将该表达式的结果变成一个 <code>Result</code></li>
|
|
|
|
|
|
<li>在结果处理中使用，会将错误直接传播到 <code>select!</code> 之外</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::net::TcpListener;
|
|
|
|
|
|
use tokio::sync::oneshot;
|
|
|
|
|
|
use std::io;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
// [设置 `rx` oneshot 消息通道]
|
|
|
|
|
|
|
|
|
|
|
|
let listener = TcpListener::bind("localhost:3465").await?;
|
|
|
|
|
|
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
res = async {
|
|
|
|
|
|
loop {
|
|
|
|
|
|
let (socket, _) = listener.accept().await?;
|
|
|
|
|
|
tokio::spawn(async move { process(socket) });
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
Ok::<_, io::Error>(())
|
|
|
|
|
|
} => {
|
|
|
|
|
|
res?;
|
|
|
|
|
|
}
|
|
|
|
|
|
_ = rx => {
|
|
|
|
|
|
println!("terminating accept loop");
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p><code>listener.accept().await?</code> 是分支表达式中的 <code>?</code>，因此它会将表达式的返回值变成 <code>Result</code> 类型，然后赋予给 <code>res</code> 变量。</p>
|
|
|
|
|
|
<p>与之不同的是，结果处理中的 <code>res?;</code> 会让 <code>main</code> 函数直接结束并返回一个 <code>Result</code>，可以看出，这里 <code>?</code> 的用法跟我们平时的用法并无区别。</p>
|
|
|
|
|
|
<h2 id="模式匹配"><a class="header" href="#模式匹配">模式匹配</a></h2>
|
|
|
|
|
|
<p>既然是模式匹配，我们需要再来回忆下 <code>select!</code> 的分支语法形式:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span><模式> = <async 表达式> => <结果处理>,
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>迄今为止，我们只用了变量绑定的模式，事实上，<a href="https://course.rs/basic/match-pattern/all-patterns.html">任何 Rust 模式</a>都可以在此处使用。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::sync::mpsc;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (mut tx1, mut rx1) = mpsc::channel(128);
|
|
|
|
|
|
let (mut tx2, mut rx2) = mpsc::channel(128);
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
// 用 tx1 和 tx2 干一些不为人知的事
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
Some(v) = rx1.recv() => {
|
|
|
|
|
|
println!("Got {:?} from rx1", v);
|
|
|
|
|
|
}
|
|
|
|
|
|
Some(v) = rx2.recv() => {
|
|
|
|
|
|
println!("Got {:?} from rx2", v);
|
|
|
|
|
|
}
|
|
|
|
|
|
else => {
|
|
|
|
|
|
println!("Both channels closed");
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>上面代码中，<code>rx</code> 通道关闭后，<code>recv()</code> 方法会返回一个 <code>None</code>，可以看到没有任何模式能够匹配这个 <code>None</code>，那为何不会报错？秘密就在于 <code>else</code> 上：当使用模式去匹配分支时，若之前的所有分支都无法被匹配，那 <code>else</code> 分支将被执行。</p>
|
|
|
|
|
|
<h2 id="借用"><a class="header" href="#借用">借用</a></h2>
|
|
|
|
|
|
<p>当在 Tokio 中生成( spawn )任务时，其 async 语句块必须拥有其中数据的所有权。而 <code>select!</code> 并没有这个限制，它的每个分支表达式可以直接借用数据，然后进行并发操作。只要遵循 Rust 的借用规则，多个分支表达式可以不可变的借用同一个数据，或者在一个表达式可变的借用某个数据。</p>
|
|
|
|
|
|
<p>来看个例子，在这里我们同时向两个 TCP 目标发送同样的数据:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use tokio::io::AsyncWriteExt;
|
|
|
|
|
|
use tokio::net::TcpStream;
|
|
|
|
|
|
use std::io;
|
|
|
|
|
|
use std::net::SocketAddr;
|
|
|
|
|
|
|
|
|
|
|
|
async fn race(
|
|
|
|
|
|
data: &[u8],
|
|
|
|
|
|
addr1: SocketAddr,
|
|
|
|
|
|
addr2: SocketAddr
|
|
|
|
|
|
) -> io::Result<()> {
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
Ok(_) = async {
|
|
|
|
|
|
let mut socket = TcpStream::connect(addr1).await?;
|
|
|
|
|
|
socket.write_all(data).await?;
|
|
|
|
|
|
Ok::<_, io::Error>(())
|
|
|
|
|
|
} => {}
|
|
|
|
|
|
Ok(_) = async {
|
|
|
|
|
|
let mut socket = TcpStream::connect(addr2).await?;
|
|
|
|
|
|
socket.write_all(data).await?;
|
|
|
|
|
|
Ok::<_, io::Error>(())
|
|
|
|
|
|
} => {}
|
|
|
|
|
|
else => {}
|
|
|
|
|
|
};
|
|
|
|
|
|
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>这里其实有一个很有趣的题外话，由于 TCP 连接过程是在模式中发生的，因此当某一个连接过程失败后，它通过 <code>?</code> 返回的 <code>Err</code> 类型并无法匹配 <code>Ok</code>，因此另一个分支会继续被执行，继续连接。</p>
|
|
|
|
|
|
<p>如果你把连接过程放在了结果处理中，那连接失败会直接从 <code>race</code> 函数中返回，而不是继续执行另一个分支中的连接！</p>
|
|
|
|
|
|
<p>还有一个非常重要的点，<strong>借用规则在分支表达式和结果处理中存在很大的不同</strong>。例如上面代码中，我们在两个分支表达式中分别对 <code>data</code> 做了不可变借用，这当然 ok，但是若是两次可变借用，那编译器会立即进行报错。但是转折来了：当在结果处理中进行两次可变借用时，却不会报错，大家可以思考下为什么，提示下：思考下分支在执行完成后会发生什么？</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::sync::oneshot;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (tx1, rx1) = oneshot::channel();
|
|
|
|
|
|
let (tx2, rx2) = oneshot::channel();
|
|
|
|
|
|
|
|
|
|
|
|
let mut out = String::new();
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
_ = rx1 => {
|
|
|
|
|
|
out.push_str("rx1 completed");
|
|
|
|
|
|
}
|
|
|
|
|
|
_ = rx2 => {
|
|
|
|
|
|
out.push_str("rx2 completed");
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
println!("{}", out);
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>例如以上代码，就在两个分支的结果处理中分别进行了可变借用，并不会报错。原因就在于：<code>select!</code>会保证只有一个分支的结果处理会被运行，然后在运行结束后，另一个分支会被直接丢弃。</p>
|
|
|
|
|
|
<h2 id="循环"><a class="header" href="#循环">循环</a></h2>
|
|
|
|
|
|
<p>来看看该如何在循环中使用 <code>select!</code>，顺便说一句，跟循环一起使用是最常见的使用方式。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::sync::mpsc;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (tx1, mut rx1) = mpsc::channel(128);
|
|
|
|
|
|
let (tx2, mut rx2) = mpsc::channel(128);
|
|
|
|
|
|
let (tx3, mut rx3) = mpsc::channel(128);
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
let msg = tokio::select! {
|
|
|
|
|
|
Some(msg) = rx1.recv() => msg,
|
|
|
|
|
|
Some(msg) = rx2.recv() => msg,
|
|
|
|
|
|
Some(msg) = rx3.recv() => msg,
|
|
|
|
|
|
else => { break }
|
|
|
|
|
|
};
|
|
|
|
|
|
|
|
|
|
|
|
println!("Got {}", msg);
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
println!("All channels have been closed.");
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>在循环中使用 <code>select!</code> 最大的不同就是，当某一个分支执行完成后，<code>select!</code> 会继续循环等待并执行下一个分支，直到所有分支最终都完成，最终匹配到 <code>else</code> 分支，然后通过 <code>break</code> 跳出循环。</p>
|
|
|
|
|
|
<p>老生常谈的一句话：<code>select!</code> 中哪个分支先被执行是无法确定的，因此不要依赖于分支执行的顺序！想象一下，在异步编程场景，若 <code>select!</code> 按照分支的顺序来执行会如何：若 <code>rx1</code> 中总是有数据，那每次循环都只会去处理第一个分支，后面两个分支永远不会被执行。</p>
|
|
|
|
|
|
<h4 id="恢复之前的异步操作"><a class="header" href="#恢复之前的异步操作">恢复之前的异步操作</a></h4>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">async fn action() {
|
|
|
|
|
|
// 一些异步逻辑
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (mut tx, mut rx) = tokio::sync::mpsc::channel(128);
|
|
|
|
|
|
|
|
|
|
|
|
let operation = action();
|
|
|
|
|
|
tokio::pin!(operation);
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
_ = &mut operation => break,
|
|
|
|
|
|
Some(v) = rx.recv() => {
|
|
|
|
|
|
if v % 2 == 0 {
|
|
|
|
|
|
break;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>在上面代码中，我们没有直接在 <code>select!</code> 分支中调用 <code>action()</code> ，而是在 <code>loop</code> 循环外面先将 <code>action()</code> 赋值给 <code>operation</code>，因此 <code>operation</code> 是一个 <code>Future</code>。</p>
|
|
|
|
|
|
<p><strong>重点来了</strong>，在 <code>select!</code> 循环中，我们使用了一个奇怪的语法 <code>&mut operation</code>，大家想象一下，如果不加 <code>&mut</code> 会如何？答案是，每一次循环调用的都是一次全新的 <code>action()</code>调用，但是当加了 <code>&mut operatoion</code> 后，每一次循环调用就变成了对同一次 <code>action()</code> 的调用。也就是我们实现了在每次循环中恢复了之前的异步操作！</p>
|
|
|
|
|
|
<p><code>select!</code> 的另一个分支从消息通道收取消息，一旦收到值是偶数，就跳出循环，否则就继续循环。</p>
|
|
|
|
|
|
<p>还有一个就是我们使用了 <code>tokio::pin!</code>，具体的细节这里先不介绍，值得注意的点是：如果要在一个引用上使用 <code>.await</code>，那么引用的值就必须是不能移动的或者实现了 <code>Unpin</code>，关于 <code>Pin</code> 和 <code>Unpin</code> 可以参见<a href="https://course.rs/async/pin-unpin.html">这里</a>。</p>
|
|
|
|
|
|
<p>一旦移除 <code>tokio::pin!</code> 所在行的代码，然后试图编译，就会获得以下错误:</p>
|
|
|
|
|
|
<pre><code class="language-console">error[E0599]: no method named `poll` found for struct
|
|
|
|
|
|
`std::pin::Pin<&mut &mut impl std::future::Future>`
|
|
|
|
|
|
in the current scope
|
|
|
|
|
|
--> src/main.rs:16:9
|
|
|
|
|
|
|
|
|
|
|
|
|
16 | / tokio::select! {
|
|
|
|
|
|
17 | | _ = &mut operation => break,
|
|
|
|
|
|
18 | | Some(v) = rx.recv() => {
|
|
|
|
|
|
19 | | if v % 2 == 0 {
|
|
|
|
|
|
... |
|
|
|
|
|
|
22 | | }
|
|
|
|
|
|
23 | | }
|
|
|
|
|
|
| |_________^ method not found in
|
|
|
|
|
|
| `std::pin::Pin<&mut &mut impl std::future::Future>`
|
|
|
|
|
|
|
|
|
|
|
|
|
= note: the method `poll` exists but the following trait bounds
|
|
|
|
|
|
were not satisfied:
|
|
|
|
|
|
`impl std::future::Future: std::marker::Unpin`
|
|
|
|
|
|
which is required by
|
|
|
|
|
|
`&mut impl std::future::Future: std::future::Future`
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>虽然我们已经学了很多关于 <code>Future</code> 的知识，但是这个错误依然不太好理解。但是它不难解决：当你试图在<strong>一个引用上调用 <code>.await</code> 然后遇到了 <code>Future 未实现</code> 这种错误时</strong>，往往只需要将对应的 <code>Future</code> 进行固定即可: <code> tokio::pin!(operation);</code>。</p>
|
|
|
|
|
|
<h4 id="修改一个分支"><a class="header" href="#修改一个分支">修改一个分支</a></h4>
|
|
|
|
|
|
<p>下面一起来看一个稍微复杂一些的 <code>loop</code> 循环，首先，我们拥有：</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>一个消息通道可以传递 <code>i32</code> 类型的值</li>
|
|
|
|
|
|
<li>定义在 <code>i32</code> 值上的一个异步操作</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<p>想要实现的逻辑是：</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>在消息通道中等待一个偶数出现</li>
|
|
|
|
|
|
<li>使用该偶数作为输入来启动一个异步操作</li>
|
|
|
|
|
|
<li>等待异步操作完成，与此同时监听消息通道以获取更多的偶数</li>
|
|
|
|
|
|
<li>若在异步操作完成前一个新的偶数到来了，终止当前的异步操作，然后接着使用新的偶数开始异步操作</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">async fn action(input: Option<i32>) -> Option<String> {
|
|
|
|
|
|
// 若 input（输入）是None，则返回 None
|
|
|
|
|
|
// 事实上也可以这么写: `let i = input?;`
|
|
|
|
|
|
let i = match input {
|
|
|
|
|
|
Some(input) => input,
|
|
|
|
|
|
None => return None,
|
|
|
|
|
|
};
|
|
|
|
|
|
|
|
|
|
|
|
// 这里定义一些逻辑
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() {
|
|
|
|
|
|
let (mut tx, mut rx) = tokio::sync::mpsc::channel(128);
|
|
|
|
|
|
|
|
|
|
|
|
let mut done = false;
|
|
|
|
|
|
let operation = action(None);
|
|
|
|
|
|
tokio::pin!(operation);
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
let _ = tx.send(1).await;
|
|
|
|
|
|
let _ = tx.send(3).await;
|
|
|
|
|
|
let _ = tx.send(2).await;
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
tokio::select! {
|
|
|
|
|
|
res = &mut operation, if !done => {
|
|
|
|
|
|
done = true;
|
|
|
|
|
|
|
|
|
|
|
|
if let Some(v) = res {
|
|
|
|
|
|
println!("GOT = {}", v);
|
|
|
|
|
|
return;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
Some(v) = rx.recv() => {
|
|
|
|
|
|
if v % 2 == 0 {
|
|
|
|
|
|
// `.set` 是 `Pin` 上定义的方法
|
|
|
|
|
|
operation.set(action(Some(v)));
|
|
|
|
|
|
done = false;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>当第一次循环开始时， 第一个分支会立即完成，因为 <code>operation</code> 的参数是 <code>None</code>。当第一个分支执行完成时，<code>done</code> 会变成 <code>true</code>，此时第一个分支的条件将无法被满足，开始执行第二个分支。</p>
|
|
|
|
|
|
<p>当第二个分支收到一个偶数时，<code>done</code> 会被修改为 <code>false</code>，且 <code>operation</code> 被设置了值。 此后再一次循环时，第一个分支会被执行，且 <code>operation</code> 返回一个 <code>Some(2)</code>，因此会触发 <code>return</code> ，最终结束循环并返回。</p>
|
|
|
|
|
|
<p>这段代码引入了一个新的语法: <code>if !done</code>，在解释之前，先看看去掉后会如何：</p>
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<pre><code class="language-console">thread 'main' panicked at '`async fn` resumed after completion', src/main.rs:1:55
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note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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</code></pre>
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<p><code>'`async fn` resumed after completion'</code> 错误的含义是：<code>async fn</code> 异步函数在完成后，依然被恢复了(继续使用)。</p>
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<p>回到例子中来，这个错误是由于 <code>operation</code> 在它已经调用完成后依然被使用。通常来说，当使用 <code>.await</code> 后，调用 <code>.await</code> 的值会被消耗掉，因此并不存在这个问题。但是在这例子中，我们在引用上调用 <code>.await</code>，因此之后该引用依然可以被使用。</p>
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<p>为了避免这个问题，需要在第一个分支的 <code>operation</code> 完成后禁止再使用该分支。这里的 <code>done</code> 的引入就很好的解决了问题。对于 <code>select!</code> 来说 <code>if !done</code> 的语法被称为预条件( <strong>precondition</strong> )，该条件会在分支被 <code>.await</code> 执行前进行检查。</p>
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<p>那大家肯定有疑问了，既然 <code>operation</code> 不能再被调用了，我们该如何在有偶数值时，再回到第一个分支对其进行调用呢？答案就是 <code>operation.set(action(Some(v)));</code>，该操作会重新使用新的参数设置 <code>operation</code>。</p>
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<h2 id="spawn-和-select-的一些不同"><a class="header" href="#spawn-和-select-的一些不同">spawn 和 select! 的一些不同</a></h2>
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<p>学到现在，相信大家对于 <code>tokio::spawn</code> 和 <code>select!</code> 已经非常熟悉，它们的共同点就是都可以并发的运行异步操作。
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然而它们使用的策略大相径庭。</p>
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<p><code>tokio::spawn</code> 函数会启动新的任务来运行一个异步操作，每个任务都是一个独立的对象可以单独被 Tokio 调度运行，因此两个不同的任务的调度都是独立进行的，甚至于它们可能会运行在两个不同的操作系统线程上。鉴于此，生成的任务和生成的线程有一个相同的限制：不允许对外部环境中的值进行借用。</p>
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<p>而 <code>select!</code> 宏就不一样了，它在同一个任务中并发运行所有的分支。正是因为这样，在同一个任务中，这些分支无法被同时运行。 <code>select!</code> 宏在单个任务中实现了多路复用的功能。</p>
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