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<main>
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<h1 id="io"><a class="header" href="#io">I/O</a></h1>
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<p>本章节中我们将深入学习 Tokio 中的 I/O 操作，了解它的原理以及该如何使用。</p>
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<p>Tokio 中的 I/O 操作和 <code>std</code> 在使用方式上几无区别，最大的区别就是前者是异步的，例如 Tokio 的读写特征分别是 <code>AsyncRead</code> 和 <code>AsyncWrite</code>:</p>
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<ul>
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<li>有部分类型按照自己的所需实现了它们: <code>TcpStream</code>，<code>File</code>，<code>Stdout</code></li>
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<li>还有数据结构也实现了它们：<code>Vec<u8></code>、<code>&[u8]</code>，这样就可以直接使用这些数据结构作为读写器( reader / writer)</li>
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</ul>
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<h2 id="asyncread-和-asyncwrite"><a class="header" href="#asyncread-和-asyncwrite">AsyncRead 和 AsyncWrite</a></h2>
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<p>这两个特征为字节流的异步读写提供了便利，通常我们会使用 <code>AsyncReadExt</code> 和 <code>AsyncWriteExt</code> 提供的工具方法，这些方法都使用 <code>async</code> 声明，且需要通过 <code>.await</code> 进行调用，</p>
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<h4 id="async-fn-read"><a class="header" href="#async-fn-read">async fn read</a></h4>
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<p><code>AsyncReadExt::read</code> 是一个异步方法可以将数据读入缓冲区( <code>buffer</code> )中，然后返回读取的字节数。</p>
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<pre><pre class="playground"><code class="language-rust edition2021">use tokio::fs::File;
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use tokio::io::{self, AsyncReadExt};
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#[tokio::main]
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async fn main() -> io::Result<()> {
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let mut f = File::open("foo.txt").await?;
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let mut buffer = [0; 10];
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// 由于 buffer 的长度限制，当次的 `read` 调用最多可以从文件中读取 10 个字节的数据
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let n = f.read(&mut buffer[..]).await?;
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println!("The bytes: {:?}", &buffer[..n]);
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Ok(())
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}
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</code></pre></pre>
|
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<p>需要注意的是：当 <code>read</code> 返回 <code>Ok(0)</code> 时，意味着字节流( stream )已经关闭，在这之后继续调用 <code>read</code> 会立刻完成，依然获取到返回值 <code>Ok(0)</code>。 例如，字节流如果是 <code>TcpStream</code> 类型，那 <code>Ok(0)</code> 说明该<strong>连接的读取端已经被关闭</strong>(写入端关闭，会报其它的错误)。</p>
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<h4 id="async-fn-read_to_end"><a class="header" href="#async-fn-read_to_end">async fn read_to_end</a></h4>
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<p><code>AsyncReadExt::read_to_end</code> 方法会从字节流中读取所有的字节，直到遇到 <code>EOF</code> ：</p>
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|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::io::{self, AsyncReadExt};
|
|
|
|
|
|
use tokio::fs::File;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let mut f = File::open("foo.txt").await?;
|
|
|
|
|
|
let mut buffer = Vec::new();
|
|
|
|
|
|
|
|
|
|
|
|
// 读取整个文件的内容
|
|
|
|
|
|
f.read_to_end(&mut buffer).await?;
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<h4 id="async-fn-write"><a class="header" href="#async-fn-write">async fn write</a></h4>
|
|
|
|
|
|
<p><code>AsyncWriteExt::write</code> 异步方法会尝试将缓冲区的内容写入到写入器( <code>writer</code> )中，同时返回写入的字节数:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::io::{self, AsyncWriteExt};
|
|
|
|
|
|
use tokio::fs::File;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let mut file = File::create("foo.txt").await?;
|
|
|
|
|
|
|
|
|
|
|
|
let n = file.write(b"some bytes").await?;
|
|
|
|
|
|
|
|
|
|
|
|
println!("Wrote the first {} bytes of 'some bytes'.", n);
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>上面代码很清晰，但是大家可能会疑惑 <code>b"some bytes"</code> 是什么意思。这种写法可以将一个 <code>&str</code> 字符串转变成一个字节数组：<code>&[u8;10]</code>，然后 <code>write</code> 方法又会将这个 <code>&[u8;10]</code> 的数组类型隐式强转为数组切片: <code>&[u8]</code>。</p>
|
|
|
|
|
|
<h4 id="async-fn-write_all"><a class="header" href="#async-fn-write_all">async fn write_all</a></h4>
|
|
|
|
|
|
<p><code>AsyncWriteExt::write_all</code> 将缓冲区的内容全部写入到写入器中：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::io::{self, AsyncWriteExt};
|
|
|
|
|
|
use tokio::fs::File;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let mut file = File::create("foo.txt").await?;
|
|
|
|
|
|
|
|
|
|
|
|
file.write_all(b"some bytes").await?;
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>以上只是部分方法，实际上还有一些实用的方法由于篇幅有限无法列出，大家可以通过 <a href="https://docs.rs/tokio/latest/tokio/io/index.html">API 文档</a> 查看完整的列表。</p>
|
|
|
|
|
|
<h2 id="实用函数"><a class="header" href="#实用函数">实用函数</a></h2>
|
|
|
|
|
|
<p>另外，和标准库一样， <code>tokio::io</code> 模块包含了多个实用的函数或 API，可以用于处理标准输入/输出/错误等。</p>
|
|
|
|
|
|
<p>例如，<code>tokio::io::copy</code> 异步的将读取器( <code>reader</code> )中的内容拷贝到写入器( <code>writer</code> )中。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::fs::File;
|
|
|
|
|
|
use tokio::io;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let mut reader: &[u8] = b"hello";
|
|
|
|
|
|
let mut file = File::create("foo.txt").await?;
|
|
|
|
|
|
|
|
|
|
|
|
io::copy(&mut reader, &mut file).await?;
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>还记得我们之前提到的字节数组 <code>&[u8]</code> 实现了 <code>AsyncRead</code> 吗？正因为这个原因，所以这里可以直接将 <code>&u8</code> 用作读取器。</p>
|
|
|
|
|
|
<h2 id="回声服务-echo-"><a class="header" href="#回声服务-echo-">回声服务( Echo )</a></h2>
|
|
|
|
|
|
<p>就如同写代码必写 <code>hello, world</code>，实现 web 服务器，往往会选择实现一个回声服务。该服务会将用户的输入内容直接返回给用户，就像回声壁一样。</p>
|
|
|
|
|
|
<p>具体来说，就是从用户建立的 TCP 连接的 socket 中读取到数据，然后立刻将同样的数据写回到该 socket 中。因此客户端会收到和自己发送的数据一模一样的回复。</p>
|
|
|
|
|
|
<p>下面我们将使用两种稍有不同的方法实现该回声服务。</p>
|
|
|
|
|
|
<h4 id="使用-iocopy"><a class="header" href="#使用-iocopy">使用 <code>io::copy()</code></a></h4>
|
|
|
|
|
|
<p>先来创建一个新的 bin 文件，用于运行我们的回声服务：</p>
|
|
|
|
|
|
<pre><code class="language-console">touch src/bin/echo-server-copy.rs
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>然后可以通过以下命令运行它(跟上一章节的方式相同)：</p>
|
|
|
|
|
|
<pre><code class="language-console">cargo run --bin echo-server-copy
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>至于客户端，可以简单的使用 <code>telnet</code> 的方式来连接，或者也可以使用 <code>tokio::net::TcpStream</code>，它的<a href="https://docs.rs/tokio/1/tokio/net/struct.TcpStream.html#examples">文档示例</a>非常适合大家进行参考。</p>
|
|
|
|
|
|
<p>先来实现一下基本的服务器框架：通过 loop 循环接收 TCP 连接，然后为每一条连接创建一个单独的任务去处理。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::io;
|
|
|
|
|
|
use tokio::net::TcpListener;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let listener = TcpListener::bind("127.0.0.1:6142").await?;
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
let (mut socket, _) = listener.accept().await?;
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
// 在这里拷贝数据
|
|
|
|
|
|
});
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>下面，来看看重头戏 <code>io::copy</code> ，它有两个参数：一个读取器，一个写入器，然后将读取器中的数据直接拷贝到写入器中，类似的实现代码如下：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>io::copy(&mut socket, &mut socket).await
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>这段代码相信大家一眼就能看出问题，由于我们的读取器和写入器都是同一个 socket，因此需要对其进行两次可变借用，这明显违背了 Rust 的借用规则。</p>
|
|
|
|
|
|
<h5 id="分离读写器"><a class="header" href="#分离读写器">分离读写器</a></h5>
|
|
|
|
|
|
<p>显然，使用同一个 socket 是不行的，为了实现目标功能，必须将 <code>socket</code> 分离成一个读取器和写入器。</p>
|
|
|
|
|
|
<p>任何一个读写器( reader + writer )都可以使用 <code>io::split</code> 方法进行分离，最终返回一个读取器和写入器，这两者可以独自的使用，例如可以放入不同的任务中。</p>
|
|
|
|
|
|
<p>例如，我们的回声客户端可以这样实现，以实现同时并发读写：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
|
|
|
|
|
|
use tokio::net::TcpStream;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let socket = TcpStream::connect("127.0.0.1:6142").await?;
|
|
|
|
|
|
let (mut rd, mut wr) = io::split(socket);
|
|
|
|
|
|
|
|
|
|
|
|
// 创建异步任务，在后台写入数据
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
wr.write_all(b"hello\r\n").await?;
|
|
|
|
|
|
wr.write_all(b"world\r\n").await?;
|
|
|
|
|
|
|
|
|
|
|
|
// 有时，我们需要给予 Rust 一些类型暗示，它才能正确的推导出类型
|
|
|
|
|
|
Ok::<_, io::Error>(())
|
|
|
|
|
|
});
|
|
|
|
|
|
|
|
|
|
|
|
let mut buf = vec![0; 128];
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
let n = rd.read(&mut buf).await?;
|
|
|
|
|
|
|
|
|
|
|
|
if n == 0 {
|
|
|
|
|
|
break;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
println!("GOT {:?}", &buf[..n]);
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
Ok(())
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>实际上，<code>io::split</code> 可以用于任何同时实现了 <code>AsyncRead</code> 和 <code>AsyncWrite</code> 的值，它的内部使用了 <code>Arc</code> 和 <code>Mutex</code> 来实现相应的功能。如果大家觉得这种实现有些重，可以使用 Tokio 提供的 <code>TcpStream</code>，它提供了两种方式进行分离:</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><a href="https://docs.rs/tokio/1.15.0/tokio/net/struct.TcpStream.html#method.split"><code>TcpStream::split</code></a>会获取字节流的引用，然后将其分离成一个读取器和写入器。但由于使用了引用的方式，它们俩必须和 <code>split</code> 在同一个任务中。 优点就是，这种实现没有性能开销，因为无需 <code>Arc</code> 和 <code>Mutex</code>。</li>
|
|
|
|
|
|
<li><a href="https://docs.rs/tokio/1.15.0/tokio/net/struct.TcpStream.html#method.into_split"><code>TcpStream::into_split</code></a>还提供了一种分离实现，分离出来的结果可以在任务间移动，内部是通过 <code>Arc</code> 实现</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<p>再来分析下我们的使用场景，由于 <code>io::copy()</code> 调用时所在的任务和 <code>split</code> 所在的任务是同一个，因此可以使用性能最高的 <code>TcpStream::split</code>:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>tokio::spawn(async move {
|
|
|
|
|
|
let (mut rd, mut wr) = socket.split();
|
|
|
|
|
|
|
|
|
|
|
|
if io::copy(&mut rd, &mut wr).await.is_err() {
|
|
|
|
|
|
eprintln!("failed to copy");
|
|
|
|
|
|
}
|
|
|
|
|
|
});
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>使用 <code>io::copy</code> 实现的完整代码见<a href="https://github.com/tokio-rs/website/blob/master/tutorial-code/io/src/echo-server-copy.rs">此处</a>。</p>
|
|
|
|
|
|
<h4 id="手动拷贝"><a class="header" href="#手动拷贝">手动拷贝</a></h4>
|
|
|
|
|
|
<p>程序员往往拥有一颗手动干翻一切的心，因此如果你不想用 <code>io::copy</code> 来简单实现，还可以自己手动去拷贝数据:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
|
|
|
|
|
|
use tokio::net::TcpListener;
|
|
|
|
|
|
|
|
|
|
|
|
#[tokio::main]
|
|
|
|
|
|
async fn main() -> io::Result<()> {
|
|
|
|
|
|
let listener = TcpListener::bind("127.0.0.1:6142").await?;
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
let (mut socket, _) = listener.accept().await?;
|
|
|
|
|
|
|
|
|
|
|
|
tokio::spawn(async move {
|
|
|
|
|
|
let mut buf = vec![0; 1024];
|
|
|
|
|
|
|
|
|
|
|
|
loop {
|
|
|
|
|
|
match socket.read(&mut buf).await {
|
|
|
|
|
|
// 返回值 `Ok(0)` 说明对端已经关闭
|
|
|
|
|
|
Ok(0) => return,
|
|
|
|
|
|
Ok(n) => {
|
|
|
|
|
|
// Copy the data back to socket
|
|
|
|
|
|
// 将数据拷贝回 socket 中
|
|
|
|
|
|
if socket.write_all(&buf[..n]).await.is_err() {
|
|
|
|
|
|
// 非预期错误，由于我们这里无需再做什么，因此直接停止处理
|
|
|
|
|
|
return;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
Err(_) => {
|
|
|
|
|
|
// 非预期错误，由于我们无需再做什么，因此直接停止处理
|
|
|
|
|
|
return;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
});
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre></pre>
|
|
|
|
|
|
<p>建议这段代码放入一个和之前 <code>io::copy</code> 不同的文件中 <code>src/bin/echo-server.rs</code> ， 然后使用 <code>cargo run --bin echo-server</code> 运行。</p>
|
|
|
|
|
|
<p>下面一起来看看这段代码有哪些值得注意的地方。首先，由于使用了 <code>write_all</code> 和 <code>read</code> 方法，需要先将对应的特征引入到当前作用域内:</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::{self, AsyncReadExt, AsyncWriteExt};
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<h5 id="在堆上分配缓冲区"><a class="header" href="#在堆上分配缓冲区">在堆上分配缓冲区</a></h5>
|
|
|
|
|
|
<p>在上面代码中，我们需要将数据从 <code>socket</code> 中读取到一个缓冲区 <code>buffer</code> 中：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>let mut buf = vec![0; 1024];
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>可以看到，此处的缓冲区是一个 <code>Vec</code> 动态数组，它的数据是存储在堆上，而不是栈上(若改成 <code>let mut buf = [0; 1024];</code>，则存储在栈上)。</p>
|
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<p>在之前，我们提到过一个数据如果想在 <code>.await</code> 调用过程中存在，那它必须存储在当前任务内。在我们的代码中，<code>buf</code> 会在 <code>.await</code> 调用过程中被使用，因此它必须要存储在任务内。</p>
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<p>若该缓冲区数组创建在栈上，那每条连接所对应的任务的内部数据结构看上去可能如下所示：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>struct Task {
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task: enum {
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AwaitingRead {
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socket: TcpStream,
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buf: [BufferType],
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},
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AwaitingWriteAll {
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socket: TcpStream,
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buf: [BufferType],
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}
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}
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>可以看到，栈数组要被使用，就必须存储在相应的结构体内，其中两个结构体分别持有了不同的栈数组 <code>[BufferType]</code>，这种方式会导致任务结构变得很大。特别地，我们选择缓冲区长度往往会使用分页长度(page size)，因此使用栈数组会导致任务的内存大小变得很奇怪甚至糟糕：<code>$page-size + 一些额外的字节</code>。</p>
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<p>当然，编译器会帮助我们做一些优化。例如，会进一步优化 <code>async</code> 语句块的布局，而不是像上面一样简单的使用 <code>enum</code>。在实践中，变量也不会在枚举成员间移动。</p>
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<p>但是再怎么优化，任务的结构体至少也会跟其中的栈数组一样大，因此通常情况下，使用堆上的缓冲区会高效实用的多。</p>
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<blockquote>
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<p>当任务因为调度在线程间移动时，存储在栈上的数据需要进行保存和恢复，过大的栈上变量会带来不小的数据拷贝开销</p>
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<p>因此，存储大量数据的变量最好放到堆上</p>
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</blockquote>
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<h5 id="处理-eof"><a class="header" href="#处理-eof">处理 EOF</a></h5>
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<p>当 TCP 连接的读取端关闭后，再调用 <code>read</code> 方法会返回 <code>Ok(0)</code>。此时，再继续下去已经没有意义，因此我们需要退出循环。忘记在 EOF 时退出读取循环，是网络编程中一个常见的 bug :</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>loop {
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match socket.read(&mut buf).await {
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Ok(0) => return,
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// ... 其余错误处理
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}
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>大家不妨深入思考下，如果没有退出循环会怎么样？之前我们提到过，一旦读取端关闭后，那后面的 <code>read</code> 调用就会立即返回 <code>Ok(0)</code>，而不会阻塞等待，因此这种无阻塞循环会最终导致 CPU 立刻跑到 100% ，并将一直持续下去，直到程序关闭。</p>
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