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< h1 id = "深入-tokio-背后的异步原理" > < a class = "header" href = "#深入-tokio-背后的异步原理" > 深入 Tokio 背后的异步原理< / a > < / h1 >
< p > 在经过多个章节的深入学习后， < / p >
< p > 但从现在开始，如果想要进一步的深入 Tokio ，首先需要深入理解 < code > async< / code > 的原理，其实我们在< a href = "https://course.rs/async/intro.html" > 之前的章节< / a > 已经深入学习过，这里结合 Tokio 再来回顾下。< / p >
< h2 id = "future" > < a class = "header" href = "#future" > Future< / a > < / h2 >
< p > 先来回顾一下 < code > async fn< / code > 异步函数 :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use tokio::net::TcpStream;
async fn my_async_fn() {
println!("hello from async");
// 通过 .await 创建 socket 连接
let _socket = TcpStream::connect("127.0.0.1:3000").await.unwrap();
println!("async TCP operation complete");
// 关闭socket
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 接着对它进行调用获取一个返回值，再在返回值上调用 < code > .await< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[tokio::main]
async fn main() {
let what_is_this = my_async_fn();
// 上面的调用不会产生任何效果
// ... 执行一些其它代码
what_is_this.await;
// 直到 .await 后， ，
}< / code > < / pre >
< p > 在上面代码中 < code > my_async_fn< / code > 函数为何可以惰性执行( 直到 .await 调用时才执行)？秘密就在于 < code > async fn< / code > 声明的函数返回一个 < code > Future< / code > 。< / p >
< p > < code > Future< / code > 是一个实现了 < a href = "https://doc.rust-lang.org/std/future/trait.Future.html" > < code > std::future::Future< / code > < / a > 特征的值，该值包含了一系列异步计算过程，而这个过程直到 < code > .await< / code > 调用时才会被执行。< / p >
< p > < code > std::future::Future< / code > 的定义如下所示:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::pin::Pin;
use std::task::{Context, Poll};
pub trait Future {
type Output;
fn poll(self: Pin< & mut Self> , cx: & mut Context)
-> Poll< Self::Output> ;
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 代码中有几个关键点：< / p >
< ul >
< li > < a href = "https://course.rs/basic/trait/advance-trait.html#关联类型" > 关联类型< / a > < code > Output< / code > 是 < code > Future< / code > 执行完成后返回的值的类型< / li >
< li > < code > Pin< / code > 类型是在异步函数中进行借用的关键，在< a href = "https://course.rs/advance/async/pin-unpin.html" > 这里< / a > 有非常详细的介绍< / li >
< / ul >
< p > 和其它语言不同， < code > Future< / code > 不代表一个发生在后台的计算，而是 < code > Future< / code > 就代表了计算本身，因此
< code > Future< / code > 的所有者有责任去推进该计算过程的执行，例如通过 < code > Future::poll< / code > 函数。听上去好像还挺复杂？但是大家不必担心，因为这些都在 Tokio 中帮你自动完成了 :)< / p >
< h4 id = "实现-future" > < a class = "header" href = "#实现-future" > 实现 Future< / a > < / h4 >
< p > 下面来一起实现个五脏俱全的 < code > Future< / code > ， ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};
struct Delay {
when: Instant,
}
// 为我们的 Delay 类型实现 Future 特征
impl Future for Delay {
type Output = & 'static str;
fn poll(self: Pin< & mut Self> , cx: & mut Context< '_> )
-> Poll< & 'static str>
{
if Instant::now() > = self.when {
// 时间到了，
println!("Hello world");
// Future 执行结束并返回 "done" 字符串
Poll::Ready("done")
} else {
// 目前先忽略下面这行代码
cx.waker().wake_by_ref();
Poll::Pending
}
}
}
#[tokio::main]
async fn main() {
let when = Instant::now() + Duration::from_millis(10);
let future = Delay { when };
// 运行并等待 Future 的完成
let out = future.await;
// 判断 Future 返回的字符串是否是 "done"
assert_eq!(out, "done");
}< / code > < / pre >
< p > 以上代码很清晰的解释了如何自定义一个 < code > Future< / code > ，并指定它如何通过 < code > poll< / code > 一步一步执行，直到最终完成返回 < code > "done"< / code > 字符串。< / p >
< h4 id = "async-fn-作为-future" > < a class = "header" href = "#async-fn-作为-future" > async fn 作为 Future< / a > < / h4 >
< p > 大家有没有注意到，上面代码我们在 < code > main< / code > 函数中初始化一个 < code > Future< / code > 并使用 < code > .await< / code > 对其进行调用执行，如果你是在 < code > fn main< / code > 中这么做，是会报错的。< / p >
< p > 原因是 < code > .await< / code > 只能用于 < code > async fn< / code > 函数中，因此我们将 < code > main< / code > 函数声明成 < code > async fn main< / code > 同时使用 < code > #[tokio::main]< / code > 进行了标注，此时 < code > async fn main< / code > 生成的代码类似下面：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};
enum MainFuture {
// 初始化，但永远不会被 poll
State0,
// 等待 `Delay` 运行，例如 `future.await` 代码行
State1(Delay),
// Future 执行完成
Terminated,
}
impl Future for MainFuture {
type Output = ();
fn poll(mut self: Pin< & mut Self> , cx: & mut Context< '_> )
-> Poll< ()>
{
use MainFuture::*;
loop {
match *self {
State0 => {
let when = Instant::now() +
Duration::from_millis(10);
let future = Delay { when };
*self = State1(future);
}
State1(ref mut my_future) => {
match Pin::new(my_future).poll(cx) {
Poll::Ready(out) => {
assert_eq!(out, "done");
*self = Terminated;
return Poll::Ready(());
}
Poll::Pending => {
return Poll::Pending;
}
}
}
Terminated => {
panic!("future polled after completion")
}
}
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 可以看出，编译器会将 < code > Future< / code > 变成状态机， 其中 < code > MainFuture< / code > 包含了 < code > Future< / code > 可能处于的状态：从 < code > State0< / code > 状态开始，当 < code > poll< / code > 被调用时， < code > Future< / code > 会尝试去尽可能的推进内部的状态，若它可以被完成时，就会返回 < code > Poll::Ready< / code > ，其中还会包含最终的输出结果。< / p >
< p > 若 < code > Future< / code > 无法被完成，例如它所等待的资源还没有准备好，此时就会返回 < code > Poll::Pending< / code > ，该返回值会通知调用者： < code > Future< / code > 会在稍后才能完成。< / p >
< p > 同时可以看到：当一个 < code > Future< / code > 由其它 < code > Future< / code > 组成时，调用外层 < code > Future< / code > 的 < code > poll< / code > 函数会同时调用一次内部 < code > Future< / code > 的 < code > poll< / code > 函数。< / p >
< h2 id = "执行器-excecutor-" > < a class = "header" href = "#执行器-excecutor-" > 执行器( Excecutor )< / a > < / h2 >
< p > < code > async fn< / code > 返回 < code > Future< / code > ，而后者需要通过被不断的 < code > poll< / code > 才能往前推进状态，同时该 < code > Future< / code > 还能包含其它 < code > Future< / code > ，那么问题来了谁来负责调用最外层 < code > Future< / code > 的 < code > poll< / code > 函数？< / p >
< p > 回一下之前的内容，为了运行一个异步函数，我们必须使用 < code > tokio::spawn< / code > 或 通过 < code > #[tokio::main]< / code > 标注的 < code > async fn main< / code > 函数。它们有一个非常重要的作用：将最外层 < code > Future< / code > 提交给 Tokio 的执行器。该执行器负责调用 < code > poll< / code > 函数，然后推动 < code > Future< / code > 的执行，最终直至完成。< / p >
< h4 id = "mini-tokio" > < a class = "header" href = "#mini-tokio" > mini tokio< / a > < / h4 >
< p > 为了更好理解相关的内容，我们一起来实现一个迷你版本的 Tokio， < a href = "https://github.com/tokio-rs/website/blob/master/tutorial-code/mini-tokio/src/main.rs" > 这里< / a > 。< / p >
< p > 先来看一段基础代码:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::collections::VecDeque;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};
use futures::task;
fn main() {
let mut mini_tokio = MiniTokio::new();
mini_tokio.spawn(async {
let when = Instant::now() + Duration::from_millis(10);
let future = Delay { when };
let out = future.await;
assert_eq!(out, "done");
});
mini_tokio.run();
}
struct MiniTokio {
tasks: VecDeque< Task> ,
}
type Task = Pin< Box< dyn Future< Output = ()> + Send> > ;
impl MiniTokio {
fn new() -> MiniTokio {
MiniTokio {
tasks: VecDeque::new(),
}
}
/// 生成一个 Future并放入 mini-tokio 实例的任务队列中
fn spawn< F> (& mut self, future: F)
where
F: Future< Output = ()> + Send + 'static,
{
self.tasks.push_back(Box::pin(future));
}
fn run(& mut self) {
let waker = task::noop_waker();
let mut cx = Context::from_waker(& waker);
while let Some(mut task) = self.tasks.pop_front() {
if task.as_mut().poll(& mut cx).is_pending() {
self.tasks.push_back(task);
}
}
}
}< / code > < / pre >
< p > 以上代码运行了一个 < code > async< / code > 语句块 < code > mini_tokio.spawn(async {...})< / code > ， < code > Delay< / code > 实例用于等待所需的时间。看上去相当不错，但这个实现有一个 < strong > 重大缺陷< / strong > ：我们的执行器永远也不会休眠。执行器会持续的循环遍历所有的 < code > Future< / code > ，然后不停的 < code > poll< / code > 它们，但是事实上，大多数 < code > poll< / code > 都是没有用的，因为此时 < code > Future< / code > 并没有准备好，因此会继续返回 < code > Poll::Pending< / code > ，最终这个循环遍历会让你的 CPU 疲于奔命，真打工人！< / p >
< p > 鉴于此，我们的 mini-tokio 只应该在 < code > Future< / code > 准备好可以进一步运行后，才去 < code > poll< / code > 它，例如该 < code > Future< / code > 之前阻塞等待的< strong > 资源< / strong > 已经准备好并可以被使用了，就可以对其进行 < code > poll< / code > 。再比如，如果一个 < code > Future< / code > 任务在阻塞等待从 TCP socket 中读取数据，那我们只想在 < code > socket< / code > 中有数据可以读取后才去 < code > poll< / code > 它，而不是没事就 < code > poll< / code > 着玩。< / p >
< p > 回到上面的代码中， < code > poll< / code > 它。 为了实现这个功能，我们需要 < code > 通知 -> 运行< / code > 机制：当任务可以进一步被推进运行时，它会主动通知执行器，然后执行器再来 < code > poll< / code > 。< / p >
< h2 id = "waker" > < a class = "header" href = "#waker" > Waker< / a > < / h2 >
< p > 一切的答案都在 < code > Waker< / code > 中，资源可以用它来通知正在等待的任务：该资源已经准备好，可以继续运行了。< / p >
< p > 再来看下 < code > Future::poll< / code > 的定义：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn poll(self: Pin< & mut Self> , cx: & mut Context)
-> Poll< Self::Output> ;
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > Context< / code > 参数中包含有 < code > waker()< / code > 方法。该方法返回一个绑定到当前任务上的 < code > Waker< / code > ，然后 < code > Waker< / code > 上定义了一个 < code > wake()< / code > 方法，用于通知执行器相关的任务可以继续执行。< / p >
< p > 准确来说，当 < code > Future< / code > 阻塞等待的资源已经准备好时(例如 socket 中有了可读取的数据)，该资源可以调用 < code > wake()< / code > 方法，来通知执行器可以继续调用该 < code > Future< / code > 的 < code > poll< / code > 函数来推进任务的执行。< / p >
< h4 id = "发送-wake-通知" > < a class = "header" href = "#发送-wake-通知" > 发送 wake 通知< / a > < / h4 >
< p > 现在，为 < code > Delay< / code > 添加下 < code > Waker< / code > 支持：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};
use std::thread;
struct Delay {
when: Instant,
}
impl Future for Delay {
type Output = & 'static str;
fn poll(self: Pin< & mut Self> , cx: & mut Context< '_> )
-> Poll< & 'static str>
{
if Instant::now() > = self.when {
println!("Hello world");
Poll::Ready("done")
} else {
// 为当前任务克隆一个 waker 的句柄
let waker = cx.waker().clone();
let when = self.when;
// 生成一个计时器线程
thread::spawn(move || {
let now = Instant::now();
if now < when {
thread::sleep(when - now);
}
waker.wake();
});
Poll::Pending
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 此时，计时器用来模拟一个阻塞等待的资源，一旦计时结束(该资源已经准备好)，资源会通过 < code > waker.wake()< / code > 调用通知执行器我们的任务再次被调度执行了。< / p >
< p > 当然，现在的实现还较为粗糙，等会我们会来进一步优化，在此之前，先来看看如何监听这个 < code > wake< / code > 通知。< / p >
< blockquote >
< p > 当 Future 会返回 < code > Poll::Pending< / code > 时，一定要确保 < code > wake< / code > 能被正常调用，否则会导致任务永远被挂起，再也不会被执行器 < code > poll< / code > 。< / p >
< p > < strong > 忘记在返回 < code > Poll::Pending< / code > 时调用 < code > wake< / code > 是很多难以发现 bug 的潜在源头！< / strong > < / p >
< / blockquote >
< p > 再回忆下最早实现的 < code > Delay< / code > 代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl Future for Delay {
type Output = & 'static str;
fn poll(self: Pin< & mut Self> , cx: & mut Context< '_> )
-> Poll< & 'static str>
{
if Instant::now() > = self.when {
// 时间到了，
println!("Hello world");
// Future 执行结束并返回 "done" 字符串
Poll::Ready("done")
} else {
// 目前先忽略下面这行代码
cx.waker().wake_by_ref();
Poll::Pending
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在返回 < code > Poll::Pending< / code > 之前，先调用了 < code > cx.waker().wake_by_ref()< / code > ，由于此时我们还没有模拟计时资源，因此这里直接调用了 < code > wake< / code > 进行通知，这样做会导致当前的 < code > Future< / code > 被立即再次调度执行。< / p >
< p > 由此可见，这种通知的控制权是在你手里的，甚至可以像上面代码这样，还没准备好资源，就直接进行 < code > wake< / code > 通知，但是总归意义不大，而且浪费了 CPU， < code > 执行 -> 立即通知再调度 -> 执行< / code > 的方式会造成一个非常繁忙的循环。< / p >
< h4 id = "处理-wake-通知" > < a class = "header" href = "#处理-wake-通知" > 处理 wake 通知< / a > < / h4 >
< p > 下面，让我们更新 mini-tokio 服务，让它能接收 wake 通知：当 < code > waker.wake()< / code > 被调用后，相关联的任务会被放入执行器的队列中，然后等待执行器的调用执行。< / p >
< p > 为了实现这一点，我们将使用消息通道来排队存储这些被唤醒并等待调度的任务。有一点需要注意，从消息通道接收消息的线程(执行器所在的线程)和发送消息的线程（唤醒任务时所在的线程）可能是不同的，因此消息( < code > Waker< / code > )必须要实现 < code > Send< / code > 和 < code > Sync< / code > ，才能跨线程使用。< / p >
< blockquote >
< p > 关于 < code > Send< / code > 和 < code > Sync< / code > 的具体讲解见< a href = "https://course.rs/advance/concurrency-with-threads/send-sync.html" > 这里< / a > < / p >
< / blockquote >
< p > 基于以上理由，选择 < code > std::sync::mpsc< / code > 重新实现 < code > MiniTokio< / code > 结构体：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::sync::mpsc;
use std::sync::Arc;
struct MiniTokio {
scheduled: mpsc::Receiver< Arc< Task> > ,
sender: mpsc::Sender< Arc< Task> > ,
}
struct Task {
// 先空着，后面会填充代码
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > Waker< / code > 实现了 < code > Sync< / code > 特征，同时还可以被克隆，当 < code > wake< / code > 被调用时，任务就会被调度执行。< / p >
< p > 为了实现上述的目的，我们引入了消息通道，当 < code > waker.wake()< / code > 函数被调用时，任务会被发送到该消息通道中:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::sync::{Arc, Mutex};
struct Task {
// `Mutex` 是为了让 `Task` 实现 `Sync` 特征，它能保证同一时间只有一个线程可以访问 `Future`。
// 事实上 `Mutex` 并没有在 Tokio 中被使用，这里我们只是为了简化： Tokio 的真实代码实在太长了 :D
future: Mutex< Pin< Box< dyn Future< Output = ()> + Send> > > ,
executor: mpsc::Sender< Arc< Task> > ,
}
impl Task {
fn schedule(self: & Arc< Self> ) {
self.executor.send(self.clone());
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 接下来，我们需要让 < code > std::task::Waker< / code > 能准确的找到所需的调度函数 关联起来，对此标准库中提供了一个底层的 API < a href = "https://doc.rust-lang.org/std/task/struct.RawWakerVTable.html" > < code > std::task::RawWakerVTable< / code > < / a > 可以用于手动的访问 < code > vtable< / code > ，这种实现提供了最大的灵活性，但是需要大量 < code > unsafe< / code > 的代码。< / p >
< p > 因此我们选择更加高级的实现：由 < code > futures< / code > 包提供的 < a href = "https://docs.rs/futures/0.3.19/futures/task/trait.ArcWake.html" > < code > ArcWake< / code > < / a > 特征，只要简单实现该特征，就可以将我们的 < code > Task< / code > 转变成一个 < code > waker< / code > 。在 < code > Cargo.toml< / code > 中添加以下包：< / p >
< pre > < code class = "language-toml" > futures = "0.3"
< / code > < / pre >
< p > 然后为我们的任务 < code > Task< / code > 实现 < code > ArcWake< / code > :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use futures::task::{self, ArcWake};
use std::sync::Arc;
impl ArcWake for Task {
fn wake_by_ref(arc_self: & Arc< Self> ) {
arc_self.schedule();
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当之前的计时器线程调用 < code > waker.wake()< / code > 时，所在的任务会被推入到消息通道中。因此接下来，我们需要实现接收端的功能，然后 < code > MiniTokio::run()< / code > 函数中执行该任务:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl MiniTokio {
// 从消息通道中接收任务，然后通过 poll 来执行
fn run(& self) {
while let Ok(task) = self.scheduled.recv() {
task.poll();
}
}
/// 初始化一个新的 mini-tokio 实例
fn new() -> MiniTokio {
let (sender, scheduled) = mpsc::channel();
MiniTokio { scheduled, sender }
}
/// 在下面函数中，通过参数传入的 future 被 `Task` 包裹起来，然后会被推入到调度队列中，当 `run` 被调用时，该 future 将被执行
fn spawn< F> (& self, future: F)
where
F: Future< Output = ()> + Send + 'static,
{
Task::spawn(future, & self.sender);
}
}
impl Task {
fn poll(self: Arc< Self> ) {
// 基于 Task 实例创建一个 waker, 它使用了之前的 `ArcWake`
let waker = task::waker(self.clone());
let mut cx = Context::from_waker(& waker);
// 没有其他线程在竞争锁时，我们将获取到目标 future
let mut future = self.future.try_lock().unwrap();
// 对 future 进行 poll
let _ = future.as_mut().poll(& mut cx);
}
// 使用给定的 future 来生成新的任务
//
// 新的任务会被推到 `sender` 中，接着该消息通道的接收端就可以获取该任务，然后执行
fn spawn< F> (future: F, sender: & mpsc::Sender< Arc< Task> > )
where
F: Future< Output = ()> + Send + 'static,
{
let task = Arc::new(Task {
future: Mutex::new(Box::pin(future)),
executor: sender.clone(),
});
let _ = sender.send(task);
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 首先，我们实现了 < code > MiniTokio::run()< / code > 函数，它会持续从消息通道中接收被唤醒的任务，然后通过 < code > poll< / code > 来推动其继续执行。< / p >
< p > 其次，< code > MiniTokio::new()< / code > 和 < code > MiniTokio::spawn()< / code > 使用了消息通道而不是一个 < code > VecDeque< / code > 。当新任务生成后，这些任务中会携带上消息通道的发送端，当任务中的资源准备就绪时，会使用该发送端将该任务放入消息通道的队列中，等待执行器 < code > poll< / code > 。< / p >
< p > < code > Task::poll()< / code > 函数使用 < code > futures< / code > 包提供的 < code > ArcWake< / code > 创建了一个 < code > waker< / code > ，后者可以用来创建 < code > task::Context< / code > ，最终该 < code > Context< / code > 会被传给执行器调用的 < code > poll< / code > 函数。< / p >
< blockquote >
< p > 注意， < / p >
< / blockquote >
< h2 id = "一些遗留问题" > < a class = "header" href = "#一些遗留问题" > 一些遗留问题< / a > < / h2 >
< p > 至此，我们的程序已经差不多完成，还剩几个遗留问题需要解决下。< / p >
< h4 id = "在异步函数中生成异步任务" > < a class = "header" href = "#在异步函数中生成异步任务" > 在异步函数中生成异步任务< / a > < / h4 >
< p > 之前实现 < code > Delay Future< / code > 时， < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use futures::future::poll_fn;
use std::future::Future;
use std::pin::Pin;
#[tokio::main]
async fn main() {
let when = Instant::now() + Duration::from_millis(10);
let mut delay = Some(Delay { when });
poll_fn(move |cx| {
let mut delay = delay.take().unwrap();
let res = Pin::new(& mut delay).poll(cx);
assert!(res.is_pending());
tokio::spawn(async move {
delay.await;
});
Poll::Ready(())
}).await;
}< / code > < / pre >
< p > 首先，< code > poll_fn< / code > 函数使用闭包创建了一个 < code > Future< / code > ，其次，上面代码还创建一个 < code > Delay< / code > 实例，然后在闭包中，对其进行了一次 < code > poll< / code > ，接着再将该 < code > Delay< / code > 实例发送到一个新的任务，在此任务中使用 < code > .await< / code > 进行了执行。< / p >
< p > 在例子中，< code > Delay:poll< / code > 被调用了不止一次，且使用了不同的 < code > Waker< / code > 实例，在这种场景下，你必须确保调用最近一次 < code > poll< / code > 函数中的 < code > Waker< / code > 参数中的< code > wake< / code > 方法。也就是调用最内层 < code > poll< / code > 函数参数( < code > Waker< / code > )上的 < code > wake< / code > 方法。< / p >
< p > 当实现一个 < code > Future< / code > 时，很关键的一点就是要假设每次 < code > poll< / code > 调用都会应用到一个不同的 < code > Waker< / code > 实例上。因此 < code > poll< / code > 函数必须要使用一个新的 < code > waker< / code > 去更新替代之前的 < code > waker< / code > 。< / p >
< p > 我们之前的 < code > Delay< / code > 实现中，会在每一次 < code > poll< / code > 调用时都生成一个新的线程。这么做问题不大，但是当 < code > poll< / code > 调用较多时会出现明显的性能问题！一个解决方法就是记录你是否已经生成了一个线程，然后只有在没有生成时才去创建一个新的线程。但是一旦这么做，就必须确保线程的 < code > Waker< / code > 在后续 < code > poll< / code > 调用中被正确更新，否则你无法唤醒最近的 < code > Waker< / code > ！ < / p >
< p > 这一段大家可能会看得云里雾里的，没办法，原文就绕来绕去，好在终于可以看代码了。。我们可以通过代码来解决疑惑：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::future::Future;
use std::pin::Pin;
use std::sync::{Arc, Mutex};
use std::task::{Context, Poll, Waker};
use std::thread;
use std::time::{Duration, Instant};
struct Delay {
when: Instant,
// 用于说明是否已经生成一个线程
// Some 代表已经生成， None 代表还没有
waker: Option< Arc< Mutex< Waker> > > ,
}
impl Future for Delay {
type Output = ();
fn poll(mut self: Pin< & mut Self> , cx: & mut Context< '_> ) -> Poll< ()> {
// 若这是 Future 第一次被调用，那么需要先生成一个计时器线程。
// 若不是第一次调用(该线程已在运行)，那要确保已存储的 `Waker` 跟当前任务的 `waker` 匹配
if let Some(waker) = & self.waker {
let mut waker = waker.lock().unwrap();
// 检查之前存储的 `waker` 是否跟当前任务的 `waker` 相匹配.
// 这是必要的，原因是 `Delay Future` 的实例可能会在两次 `poll` 之间被转移到另一个任务中，然后
// 存储的 waker 被该任务进行了更新。
// 这种情况一旦发生，`Context` 包含的 `waker` 将不同于存储的 `waker`。
// 因此我们必须对存储的 `waker` 进行更新
if !waker.will_wake(cx.waker()) {
*waker = cx.waker().clone();
}
} else {
let when = self.when;
let waker = Arc::new(Mutex::new(cx.waker().clone()));
self.waker = Some(waker.clone());
// 第一次调用 `poll`，生成计时器线程
thread::spawn(move || {
let now = Instant::now();
if now < when {
thread::sleep(when - now);
}
// 计时结束，通过调用 `waker` 来通知执行器
let waker = waker.lock().unwrap();
waker.wake_by_ref();
});
}
// 一旦 waker 被存储且计时器线程已经开始，我们就需要检查 `delay` 是否已经完成
// 若计时已完成，则当前 Future 就可以完成并返回 `Poll::Ready`
if Instant::now() > = self.when {
Poll::Ready(())
} else {
// 计时尚未结束，
//
// `Future` 特征要求当 `Pending` 被返回时，那我们要确保当资源准备好时，必须调用 `waker` 以通
// 知执行器。 在我们的例子中，会通过生成的计时线程来保证
//
// 如果忘记调用 waker，
Poll::Pending
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这着实有些复杂(原文。。)，但是简单来看就是：在每次 < code > poll< / code > 调用时，都会检查 < code > Context< / code > 中提供的 < code > waker< / code > 和我们之前记录的 < code > waker< / code > 是否匹配。若匹配，就什么都不用做，若不匹配，那之前存储的就必须进行更新。< / p >
< h4 id = "notify" > < a class = "header" href = "#notify" > Notify< / a > < / h4 >
< p > 我们之前证明了如何用手动编写的 < code > waker< / code > 来实现 < code > Delay Future< / code > 。 < code > Waker< / code > 是 Rust 异步编程的基石，因此绝大多数时候，我们并不需要直接去使用它。例如，在 < code > Delay< / code > 的例子中， 可以使用 < a href = "https://docs.rs/tokio/1.16.0/tokio/sync/struct.Notify.html" > < code > tokio::sync::Notify< / code > < / a > 去实现。< / p >
< p > 该 < code > Notify< / code > 提供了一个基础的任务通知机制，它会处理这些 < code > waker< / code > 的细节，包括确保两次 < code > waker< / code > 的匹配:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use tokio::sync::Notify;
use std::sync::Arc;
use std::time::{Duration, Instant};
use std::thread;
async fn delay(dur: Duration) {
let when = Instant::now() + dur;
let notify = Arc::new(Notify::new());
let notify2 = notify.clone();
thread::spawn(move || {
let now = Instant::now();
if now < when {
thread::sleep(when - now);
}
notify2.notify_one();
});
notify.notified().await;
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当使用 < code > Notify< / code > 后，我们就可以轻松的实现如上的 < code > delay< / code > 函数。< / p >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 在看完这么长的文章后，我们来总结下，否则大家可能还会遗忘:< / p >
< ul >
< li > 在 Rust 中，< code > async< / code > 是惰性的，直到执行器 < code > poll< / code > 它们时，才会开始执行< / li >
< li > < code > Waker< / code > 是 < code > Future< / code > 被执行的关键，它可以链接起 < code > Future< / code > 任务和执行器< / li >
< li > 当资源没有准备时，会返回一个 < code > Poll::Pending< / code > < / li >
< li > 当资源准备好时，会通过 < code > waker.wake< / code > 发出通知< / li >
< li > 执行器会收到通知，然后调度该任务继续执行，此时由于资源已经准备好，因此任务可以顺利往前推进了< / li >
< / ul >
< / main >
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