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<h1 id="asyncawait-和-stream-流处理"><a class="header" href="#asyncawait-和-stream-流处理">async/await 和 Stream 流处理</a></h1>
<p>在入门章节中，我们简单学习了该如何使用 <code>async/.await</code>， 同时在后面也了解了一些底层原理，现在是时候继续深入了。</p>
<p><code>async/.await</code> 是 Rust 语法的一部分，它在遇到阻塞操作时( 例如 IO )会让出当前线程的所有权而不是阻塞当前线程，这样就允许当前线程继续去执行其它代码，最终实现并发。</p>
<p>有两种方式可以使用 <code>async</code>： <code>async fn</code> 用于声明函数，<code>async { ... }</code> 用于声明语句块，它们会返回一个实现 <code>Future</code> 特征的值:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// `foo()`返回一个`Future&lt;Output = u8&gt;`,
// 当调用`foo().await`时，该`Future`将被运行，当调用结束后我们将获取到一个`u8`值
async fn foo() -&gt; u8 { 5 }
fn bar() -&gt; impl Future&lt;Output = u8&gt; {
// 下面的`async`语句块返回`Future&lt;Output = u8&gt;`
async {
let x: u8 = foo().await;
x + 5
}
}
<span class="boring">}</span></code></pre></pre>
<p><code>async</code> 是懒惰的，直到被执行器 <code>poll</code> 或者 <code>.await</code> 后才会开始运行，其中后者是最常用的运行 <code>Future</code> 的方法。 当 <code>.await</code> 被调用时，它会尝试运行 <code>Future</code> 直到完成，但是若该 <code>Future</code> 进入阻塞，那就会让出当前线程的控制权。当 <code>Future</code> 后面准备再一次被运行时(例如从 <code>socket</code> 中读取到了数据)，执行器会得到通知，并再次运行该 <code>Future</code> ，如此循环，直到完成。</p>
<p>以上过程只是一个简述，详细内容在<a href="https://course.rs/advance/async/future-excuting.html">底层探秘</a>中已经被深入讲解过，因此这里不再赘述。</p>
<h2 id="async-的生命周期"><a class="header" href="#async-的生命周期"><code>async</code> 的生命周期</a></h2>
<p><code>async fn</code> 函数如果拥有引用类型的参数，那它返回的 <code>Future</code> 的生命周期就会被这些参数的生命周期所限制:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn foo(x: &amp;u8) -&gt; u8 { *x }
// 上面的函数跟下面的函数是等价的:
fn foo_expanded&lt;'a&gt;(x: &amp;'a u8) -&gt; impl Future&lt;Output = u8&gt; + 'a {
async move { *x }
}
<span class="boring">}</span></code></pre></pre>
<p>意味着 <code>async fn</code> 函数返回的 <code>Future</code> 必须满足以下条件: 当 <code>x</code> 依然有效时， 该 <code>Future</code> 就必须继续等待( <code>.await</code> ), 也就是说 <code>x</code> 必须比 <code>Future</code> 活得更久。</p>
<p>在一般情况下，在函数调用后就立即 <code>.await</code> 不会存在任何问题，例如<code>foo(&amp;x).await</code>。但是，若 <code>Future</code> 被先存起来或发送到另一个任务或者线程，就可能存在问题了:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::future::Future;
fn bad() -&gt; impl Future&lt;Output = u8&gt; {
let x = 5;
borrow_x(&amp;x) // ERROR: `x` does not live long enough
}
async fn borrow_x(x: &amp;u8) -&gt; u8 { *x }
<span class="boring">}</span></code></pre></pre>
<p>以上代码会报错，因为 <code>x</code> 的生命周期只到 <code>bad</code> 函数的结尾。 但是 <code>Future</code> 显然会活得更久：</p>
<pre><code class="language-shell">error[E0597]: `x` does not live long enough
--&gt; src/main.rs:4:14
|
4 | borrow_x(&amp;x) // ERROR: `x` does not live long enough
| ---------^^-
| | |
| | borrowed value does not live long enough
| argument requires that `x` is borrowed for `'static`
5 | }
| - `x` dropped here while still borrowed
</code></pre>
<p>其中一个常用的解决方法就是将具有引用参数的 <code>async fn</code> 函数转变成一个具有 <code>'static</code> 生命周期的 <code>Future</code> 。 以上解决方法可以通过将参数和对 <code>async fn</code> 的调用放在同一个 <code>async</code> 语句块来实现:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::future::Future;
async fn borrow_x(x: &amp;u8) -&gt; u8 { *x }
fn good() -&gt; impl Future&lt;Output = u8&gt; {
async {
let x = 5;
borrow_x(&amp;x).await
}
}
<span class="boring">}</span></code></pre></pre>
<p>如上所示，通过将参数移动到 <code>async</code> 语句块内， 我们将它的生命周期扩展到 <code>'static</code>， 并跟返回的 <code>Future</code> 保持了一致。</p>
<h2 id="async-move"><a class="header" href="#async-move">async move</a></h2>
<p><code>async</code> 允许我们使用 <code>move</code> 关键字来将环境中变量的所有权转移到语句块内，就像闭包那样，好处是你不再发愁该如何解决借用生命周期的问题，坏处就是无法跟其它代码实现对变量的共享:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// 多个不同的 `async` 语句块可以访问同一个本地变量，只要它们在该变量的作用域内执行
async fn blocks() {
let my_string = &quot;foo&quot;.to_string();
let future_one = async {
// ...
println!(&quot;{my_string}&quot;);
};
let future_two = async {
// ...
println!(&quot;{my_string}&quot;);
};
// 运行两个 Future 直到完成
let ((), ()) = futures::join!(future_one, future_two);
}
// 由于 `async move` 会捕获环境中的变量，因此只有一个 `async move` 语句块可以访问该变量，
// 但是它也有非常明显的好处： 变量可以转移到返回的 Future 中，不再受借用生命周期的限制
fn move_block() -&gt; impl Future&lt;Output = ()&gt; {
let my_string = &quot;foo&quot;.to_string();
async move {
// ...
println!(&quot;{my_string}&quot;);
}
}
<span class="boring">}</span></code></pre></pre>
<h2 id="当await-遇见多线程执行器"><a class="header" href="#当await-遇见多线程执行器">当.await 遇见多线程执行器</a></h2>
<p>需要注意的是，当使用多线程 <code>Future</code> 执行器( <code>executor</code> )时， <code>Future</code> 可能会在线程间被移动，因此 <code>async</code> 语句块中的变量必须要能在线程间传递。 至于 <code>Future</code> 会在线程间移动的原因是：它内部的任何<code>.await</code>都可能导致它被切换到一个新线程上去执行。</p>
<p>由于需要在多线程环境使用，意味着 <code>Rc</code>、 <code>RefCell</code> 、没有实现 <code>Send</code> 的所有权类型、没有实现 <code>Sync</code> 的引用类型，它们都是不安全的，因此无法被使用</p>
<blockquote>
<p>需要注意！实际上它们还是有可能被使用的，只要在 <code>.await</code> 调用期间，它们没有在作用域范围内。</p>
</blockquote>
<p>类似的原因，在 <code>.await</code> 时使用普通的锁也不安全，例如 <code>Mutex</code> 。原因是，它可能会导致线程池被锁：当一个任务获取锁 <code>A</code> 后，若它将线程的控制权还给执行器，然后执行器又调度运行另一个任务，该任务也去尝试获取了锁 <code>A</code> ，结果当前线程会直接卡死，最终陷入死锁中。</p>
<p>因此，为了避免这种情况的发生，我们需要使用 <code>futures</code> 包下的锁 <code>futures::lock</code> 来替代 <code>Mutex</code> 完成任务。</p>
<h2 id="stream-流处理"><a class="header" href="#stream-流处理">Stream 流处理</a></h2>
<p><code>Stream</code> 特征类似于 <code>Future</code> 特征，但是前者在完成前可以生成多个值，这种行为跟标准库中的 <code>Iterator</code> 特征倒是颇为相似。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Stream {
// Stream生成的值的类型
type Item;
// 尝试去解析Stream中的下一个值,
// 若无数据，返回`Poll::Pending`, 若有数据，返回 `Poll::Ready(Some(x))`, `Stream`完成则返回 `Poll::Ready(None)`
fn poll_next(self: Pin&lt;&amp;mut Self&gt;, cx: &amp;mut Context&lt;'_&gt;)
-&gt; Poll&lt;Option&lt;Self::Item&gt;&gt;;
}
<span class="boring">}</span></code></pre></pre>
<p>关于 <code>Stream</code> 的一个常见例子是消息通道（ <code>futures</code> 包中的）的消费者 <code>Receiver</code>。每次有消息从 <code>Send</code> 端发送后，它都可以接收到一个 <code>Some(val)</code> 值， 一旦 <code>Send</code> 端关闭( <code>drop</code> )，且消息通道中没有消息后，它会接收到一个 <code>None</code> 值。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn send_recv() {
const BUFFER_SIZE: usize = 10;
let (mut tx, mut rx) = mpsc::channel::&lt;i32&gt;(BUFFER_SIZE);
tx.send(1).await.unwrap();
tx.send(2).await.unwrap();
drop(tx);
// `StreamExt::next` 类似于 `Iterator::next`, 但是前者返回的不是值，而是一个 `Future&lt;Output = Option&lt;T&gt;&gt;`，
// 因此还需要使用`.await`来获取具体的值
assert_eq!(Some(1), rx.next().await);
assert_eq!(Some(2), rx.next().await);
assert_eq!(None, rx.next().await);
}
<span class="boring">}</span></code></pre></pre>
<h4 id="迭代和并发"><a class="header" href="#迭代和并发">迭代和并发</a></h4>
<p>跟迭代器类似，我们也可以迭代一个 <code>Stream</code>。 例如使用 <code>map</code>，<code>filter</code>，<code>fold</code> 方法，以及它们的<em>遇到错误提前返回</em>的版本： <code>try_map</code>，<code>try_filter</code>，<code>try_fold</code>。</p>
<p>但是跟迭代器又有所不同，<code>for</code> 循环无法在这里使用，但是命令式风格的循环<code>while let</code>是可以用的，同时还可以使用<code>next</code> 和 <code>try_next</code> 方法:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn sum_with_next(mut stream: Pin&lt;&amp;mut dyn Stream&lt;Item = i32&gt;&gt;) -&gt; i32 {
use futures::stream::StreamExt; // 引入 next
let mut sum = 0;
while let Some(item) = stream.next().await {
sum += item;
}
sum
}
async fn sum_with_try_next(
mut stream: Pin&lt;&amp;mut dyn Stream&lt;Item = Result&lt;i32, io::Error&gt;&gt;&gt;,
) -&gt; Result&lt;i32, io::Error&gt; {
use futures::stream::TryStreamExt; // 引入 try_next
let mut sum = 0;
while let Some(item) = stream.try_next().await? {
sum += item;
}
Ok(sum)
}
<span class="boring">}</span></code></pre></pre>
<p>上面代码是一次处理一个值的模式，但是需要注意的是：<strong>如果你选择一次处理一个值的模式，可能会造成无法并发，这就失去了异步编程的意义</strong>。 因此，如果可以的话我们还是要选择从一个 <code>Stream</code> 并发处理多个值的方式，通过 <code>for_each_concurrent</code> 或 <code>try_for_each_concurrent</code> 方法来实现:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn jump_around(
mut stream: Pin&lt;&amp;mut dyn Stream&lt;Item = Result&lt;u8, io::Error&gt;&gt;&gt;,
) -&gt; Result&lt;(), io::Error&gt; {
use futures::stream::TryStreamExt; // 引入 `try_for_each_concurrent`
const MAX_CONCURRENT_JUMPERS: usize = 100;
stream.try_for_each_concurrent(MAX_CONCURRENT_JUMPERS, |num| async move {
jump_n_times(num).await?;
report_n_jumps(num).await?;
Ok(())
}).await?;
Ok(())
}
<span class="boring">}</span></code></pre></pre>
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