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 類型能夠讓數據在內存中固定。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":"，詳細說明了自引用結構在 MOVE 之後產生的問題。Pin 能夠解決這類問題。在上面的代碼裏面，看到針對 unpin 的類型，也增加了 Pin::new 來保障數據在內存中固定"}]},{"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 的第二個參數是 Context 類型，也就是 waker 的封裝。"}]},{"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":"這個 Context 用來保存 future 的狀態，在 poll 是 Pending 的時候保存 future 的信息和狀態，在 poll 到 Ready 的時候，再執行。所以在每次 poll 之後，如果發現不是 Ready 的狀態，都會重新把這個 Context 帶入到一個新的 future 等待下次事件觸發調用。"}]},{"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 構成了 rust async/await 的基礎。在函數前面加上 async ，就把函數包裝稱爲了一個 Future；Future 後面加上 .await，就執行 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 在函數前面，把函數包裝爲一個"}]},{"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":"異步開發應該是從 python 和 nodejs 開始的，帶來了極大的性能提升。Rust 充分利用異步模型的框架，在Web Framework Benchmark 性能榜單上，長期排名前十。異步不光帶來性能的提升，還有很多更好的機制。我們通過 Tokio 和 async-std 兩個 Rust 異步框架，來學習一下異步開發模型"}]},{"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 就是未來要執行的任務，稱爲異步任務。對於異步的操作，有一個點，就是能夠控制任務。讓任務能夠在合適的時候啓動，以及在沒有事的時候 pending。爲了達到這個目標，rust 設計了 Waker 這個機制"}]},{"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 包含以下幾個內容："}]},{"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，在這裏，就是包裝 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 : 是一個虛函數指針表(Virtual Function Pointer Table, vtable)，指向 RawWaker 對象的指針的列表"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Waker : 通知任務準備啓動的入口"}]}]}]},{"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":"以上幾個的關係是這樣："}]},{"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 結構裏面定義一個 Waker，實現了和 Waker 的相互轉換。RawWaker 定義了一個指向任何數據類型的指針，和虛函數表，用來實現和虛函數的綁定、RawWakerVTable 定義了虛函數的幾種操作，其中wake 和 wake_by_ref 和Waker 的實例對應"}]},{"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 裏面有兩個重要的操作：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()的代碼如下："}]},{"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() 的作用是讓 task 再次啓動，並且獲得 pending 之前的信息(堆棧和函數操作)。這個代碼裏面的 vtable 和 data 就是 RawWaker 裏面的數據。中間有一個操作 mem::forget(self)，forget 的作用是讓 self 實例進入一個不能操作的狀態，但是實例還在，可以下次調用。forget 是調用了 mem crate 裏面 ManuallyDrop::new() 。ManuallyDrop 能夠控制編譯器，讓其不要自動去調用實例的解構函數 (destructor)，這樣就能保存之前函數運行的信息。forget 調用 ManuallDrop::new() ，就是把 self 實例創建成一個 ManuallyDrop，獲得之前 Waker 的信息，這樣在切換到另外一個runtime 或者 thread 的時候，之前的 Waker 信息就同步過去，不會隨着進程結束而解構。"}]},{"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() 的代碼如下："}]},{"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 這個 waker ，這個用在 task 在要進入 Pending 之前，把獲得數據 clone 一份到一個全局的數據結構裏面，下一次調用的時候，不會丟失。"}]},{"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":"爲什麼會設計一個 Waker ? 在 task 進入 pending 的時候用一個全局的方法來保存狀態，下次啓動再調用不就可以了嗎？我猜測主要是爲了靈活的和性能。還有一個原因，task 很多時候是以閉包的形式出現，閉包可以獲取所在進程的環境變量，但是自己沒有 heap allocation 。這樣的話，其實就很難通過全局或者參數傳遞的方法來實現。"}]},{"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":"。說了一個意思是，在異步過程中，有一個 Poll(這個下一篇會講到)，傳入 Future 的值(data和vtable)會連續傳遞，就是 Poll 的第二個參數，Context，裏面實際是 Waker。直到有一個事件觸發 Future 重新啓動，這個值也重新使用。但是這個傳遞過程是動態的，因爲也可能是多線程的的等待獲取。所以重新設計了 Waker。Waker 有一個關鍵操作， Clone()。在事件觸發，Waker 啓動的時候，一定要 impl Clone。還是上面的原因，Waker 存在的運行時可能動態的，也可能是多派發的。所以 Waker 必須要自我 clone。然後 Clone 操作的返回必須是 Waker，而不是 self。也就是重新建立一個 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":"總結一下，Waker 是爲了支持異步任務的靈活控制而設計的。讓任務能夠在需要的時候啓動，和空閒的時候停止。那任務是什麼呢？我們需要理解兩個概念 Future 和 Task，讓我們先理解一下 Future 是什麼？"}]},{"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 的模塊，也就是說 task 是 rust 的核心庫"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"Future ：異步的核心概念"}]},{"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 意思就是未來要執行的動作 。按照 Aaron Turon 的定義，future 本質上表達一個還沒有準備好的值。通常，future 的完成(值準備好了)取決於事件在某處完成了某個事件。"}]},{"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 這個 trait 定義如下："}]},{"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