Future for &mut F {\n type Output = F::Output;\n\n fn poll(mut self: Pin, cx: &mut Context) -> Poll<:output> {\n F::poll(Pin::new(&mut **self), cx)\n }\n}\n\nimpl Future for Pin
\nwhere\n P: Unpin + ops::DerefMut,\n{\n type Output = <::Target as Future>::Output;\n\n fn poll(self: Pin, cx: &mut Context) -> Poll<:output> {\n Pin::get_mut(self).as_mut().poll(cx)\n }\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Pin 类型能够让数据在内存中固定。withoutboats 有一篇 "},{"type":"link","attrs":{"href":"https://boats.gitlab.io/blog/post/2018-01-25-async-i-self-referential-structs/","title":null},"content":[{"type":"text","text":"blog"}]},{"type":"text","text":"，详细说明了自引用结构在 MOVE 之后产生的问题。Pin 能够解决这类问题。在上面的代码里面，看到针对 unpin 的类型，也增加了 Pin::new 来保障数据在内存中固定"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Future 的 poll 的第二个参数是 Context 类型，也就是 waker 的封装。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"fn poll(self: Pin, cx: &mut Context) -> Poll<:output>;"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个 Context 用来保存 future 的状态，在 poll 是 Pending 的时候保存 future 的信息和状态，在 poll 到 Ready 的时候，再执行。所以在每次 poll 之后，如果发现不是 Ready 的状态，都会重新把这个 Context 带入到一个新的 future 等待下次事件触发调用。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Future 和 Pin 构成了 rust async/await 的基础。在函数前面加上 async ，就把函数包装称为了一个 Future；Future 后面加上 .await，就执行 Future 的 poll 操作。例如："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"async fn read_file(path: &str) -> io::Result {\n let mut file = File::open(path).await?;\n let mut contentx = String::new();\n file.read_to_string(&mut contexts).await?;\n Ok(contents)\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"async 在函数前面，把函数包装为一个"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"Future
{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"异步开发应该是从 python 和 nodejs 开始的，带来了极大的性能提升。Rust 充分利用异步模型的框架，在Web Framework Benchmark 性能榜单上，长期排名前十。异步不光带来性能的提升，还有很多更好的机制。我们通过 Tokio 和 async-std 两个 Rust 异步框架，来学习一下异步开发模型"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"先了解 std::Waker"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"task 就是未来要执行的任务，称为异步任务。对于异步的操作，有一个点，就是能够控制任务。让任务能够在合适的时候启动，以及在没有事的时候 pending。为了达到这个目标，rust 设计了 Waker 这个机制"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Waker 包含以下几个内容："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Context : 上下文，在这里，就是包装 Waker 的入口"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"RawWaker : 生成 Waker"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"RawWakerVTable : 是一个虚函数指针表(Virtual Function Pointer Table, vtable)，指向 RawWaker 对象的指针的列表"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Waker : 通知任务准备启动的入口"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上几个的关系是这样："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e2/e214b136cc26d52906cb80f46f651057.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Context 结构里面定义一个 Waker，实现了和 Waker 的相互转换。RawWaker 定义了一个指向任何数据类型的指针，和虚函数表，用来实现和虚函数的绑定、RawWakerVTable 定义了虚函数的几种操作，其中wake 和 wake_by_ref 和Waker 的实例对应"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Waker 里面有两个重要的操作：wake() 和 clone()"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"wake()的代码如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"pub fn wake(self) {\n // The actual wakeup call is delegated through a virtual function call\n // to the implementation which is defined by the executor.\n let wake = self.waker.vtable.wake;\n let data = self.waker.data;\n\n // Don't call `drop` -- the waker will be consumed by `wake`.\n crate::mem::forget(self);\n\n // SAFETY: This is safe because `Waker::from_raw` is the only way\n // to initialize `wake` and `data` requiring the user to acknowledge\n // that the contract of `RawWaker` is upheld.\n unsafe { (wake)(data) };\n }"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"wake() 的作用是让 task 再次启动，并且获得 pending 之前的信息(堆栈和函数操作)。这个代码里面的 vtable 和 data 就是 RawWaker 里面的数据。中间有一个操作 mem::forget(self)，forget 的作用是让 self 实例进入一个不能操作的状态，但是实例还在，可以下次调用。forget 是调用了 mem crate 里面 ManuallyDrop::new() 。ManuallyDrop 能够控制编译器，让其不要自动去调用实例的解构函数 (destructor)，这样就能保存之前函数运行的信息。forget 调用 ManuallDrop::new() ，就是把 self 实例创建成一个 ManuallyDrop，获得之前 Waker 的信息，这样在切换到另外一个runtime 或者 thread 的时候，之前的 Waker 信息就同步过去，不会随着进程结束而解构。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"clone() 的代码如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"impl Clone for Waker {\n #[inline]\n fn clone(&self) -> Self {\n Waker {\n // SAFETY: This is safe because `Waker::from_raw` is the only way\n // to initialize `clone` and `data` requiring the user to acknowledge\n // that the contract of [`RawWaker`] is upheld.\n waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },\n }\n }\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"clone() 的作用是 clone 这个 waker ，这个用在 task 在要进入 Pending 之前，把获得数据 clone 一份到一个全局的数据结构里面，下一次调用的时候，不会丢失。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么会设计一个 Waker ? 在 task 进入 pending 的时候用一个全局的方法来保存状态，下次启动再调用不就可以了吗？我猜测主要是为了灵活的和性能。还有一个原因，task 很多时候是以闭包的形式出现，闭包可以获取所在进程的环境变量，但是自己没有 heap allocation 。这样的话，其实就很难通过全局或者参数传递的方法来实现。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Withoutboats 有一篇文章："},{"type":"link","attrs":{"href":"https://boats.gitlab.io/blog/post/wakers-i/","title":null},"content":[{"type":"text","text":"What does a waker do?"}]},{"type":"text","text":"。说了一个意思是，在异步过程中，有一个 Poll(这个下一篇会讲到)，传入 Future 的值(data和vtable)会连续传递，就是 Poll 的第二个参数，Context，里面实际是 Waker。直到有一个事件触发 Future 重新启动，这个值也重新使用。但是这个传递过程是动态的，因为也可能是多线程的的等待获取。所以重新设计了 Waker。Waker 有一个关键操作， Clone()。在事件触发，Waker 启动的时候，一定要 impl Clone。还是上面的原因，Waker 存在的运行时可能动态的，也可能是多派发的。所以 Waker 必须要自我 clone。然后 Clone 操作的返回必须是 Waker，而不是 self。也就是重新建立一个 Waker，而不是引用到之前的 Waker。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"总结一下，Waker 是为了支持异步任务的灵活控制而设计的。让任务能够在需要的时候启动，和空闲的时候停止。那任务是什么呢？我们需要理解两个概念 Future 和 Task，让我们先理解一下 Future 是什么？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"task 是 rust 的 libcore 的模块，也就是说 task 是 rust 的核心库"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"Future ：异步的核心概念"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Future 意思就是未来要执行的动作 。按照 Aaron Turon 的定义，future 本质上表达一个还没有准备好的值。通常，future 的完成(值准备好了)取决于事件在某处完成了某个事件。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"In essence ,afuture represents a value tha might not be ready yet. Usually, the future becomes complete (the value is ready ) due to an event happening somewhere else."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://aturon.github.io/blog/2016/09/07/futures-design/","title":null},"content":[{"type":"text","text":"原文"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"future 这个 trait 定义如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"pub trait Future {\n type Output;\n fn poll(self:Pin,cx: &mut Context ) -> Poll<:output>;\n}\nenum Poll