.NET6中的await原理浅析已关闭评论
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摘要

看过不少关于 await 的原理的文章,也知道背后是编译器给转成了状态机实现的,但是具体是怎么完成的,回调又是如何衔接的,一直都没有搞清楚,这次下定决心把源码自己跑了下,终于豁然开朗了

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前言

看过不少关于 await 的原理的文章,也知道背后是编译器给转成了状态机实现的,但是具体是怎么完成的,回调又是如何衔接的,一直都没有搞清楚,这次下定决心把源码自己跑了下,终于豁然开朗了

本文的演示代码基于 VS2022 + .NET 6

示例

public class Program {     static int Work()     {         Console.WriteLine("In Task.Run");         return 1;     }      static async Task TestAsync()     {         Console.WriteLine("Before Task.Run");         await Task.Run(Work);         Console.WriteLine("After Task.Run");     }      static void Main()     {         _ = TestAsync();         Console.WriteLine("End");         Console.ReadKey();     } }

很简单的异步代码,我们来看下,编译器把它变成了啥

class Program {     static int Work()     {         Console.WriteLine("In Task.Run");         return 1;     }      static Task TestAsync()     {         var stateMachine = new StateMachine()         {             _builder = AsyncTaskMethodBuilder.Create(),             _state = -1         };         stateMachine._builder.Start(ref stateMachine);         return stateMachine._builder.Task;     }      static void Main()     {         _ = TestAsync();         Console.WriteLine("End");         Console.ReadKey();     }      class StateMachine : IAsyncStateMachine     {         public int _state;         public AsyncTaskMethodBuilder _builder;         private TaskAwaiter<int> _awaiter;          void IAsyncStateMachine.MoveNext()         {             int num = _state;             try             {                 TaskAwaiter<int> awaiter;                 if (num != 0)                 {                     Console.WriteLine("Before Task.Run");                     awaiter = Task.Run(Work).GetAwaiter();                     if (!awaiter.IsCompleted)                     {                         _state = 0;                         _awaiter = awaiter;                         StateMachine stateMachine = this;                         _builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);                         return;                     }                 }                 else                 {                     awaiter = _awaiter;                     _awaiter = default;                     _state = -1;                 }                 awaiter.GetResult();                 Console.WriteLine("After Task.Run");             }             catch (Exception exception)             {                 _state = -2;                 _builder.SetException(exception);                 return;             }             _state = -2;             _builder.SetResult();         }          void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine) { }     } }

编译后的代码经过我的整理,命名简化了,更容易理解

状态机实现

我们看到实际是生成了一个隐藏的状态机类 StateMachine
把状态机的初始状态 _state 设置 -1
stateMachine._builder.Start(ref stateMachine); 启动状态机,内部实际调用的就是状态机的 MoveNext 方法
Task.Run 创建一个任务, 把委托放在 Task.m_action 字段,丢到线程池,等待调度
任务在线程池内被调度完成后,是怎么回到这个状态机继续执行后续代码的呢?
_builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine); 就是关键了, 跟下去,到了如下的代码:
```
if (!this.AddTaskContinuation(stateMachineBox, false)) {     ThreadPool.UnsafeQueueUserWorkItemInternal(stateMachineBox, true); } bool AddTaskContinuation(object tc, bool addBeforeOthers) {     return !this.IsCompleted && ((this.m_continuationObject == null && Interlocked.CompareExchange(ref this.m_continuationObject, tc, null) == null) || this.AddTaskContinuationComplex(tc, addBeforeOthers)); } 
```
- 这里很清楚的看到,尝试把状态机对象(实际是状态机的包装类),赋值到 Task.m_continuationObject, 如果操作失败,则把状态机对象丢进线程池等待调度,这里为什么这么实现,看一下线程池是怎么执行的就清楚了

线程池实现

.NET6 的线程池实现,实际是放到了 PortableThreadPool, 具体调试步骤我就不放了,直接说结果就是, 线程池线程从任务队列中拿到任务后都执行了 DispatchWorkItem 方法

static void DispatchWorkItem(object workItem, Thread currentThread) {     Task task = workItem as Task;     if (task != null)     {         task.ExecuteFromThreadPool(currentThread);         return;     }     Unsafe.As<IThreadPoolWorkItem>(workItem).Execute(); } virtual void ExecuteFromThreadPool(Thread threadPoolThread) {     this.ExecuteEntryUnsafe(threadPoolThread); }

我们看到, 线程池队列中的任务都是 object 类型的, 这里进行了类型判断, 如果是 Task , 直接执行 task.ExecuteFromThreadPool, 更有意思的这个方法是个虚方法,后面说明

ExecuteFromThreadPool 继续追下去,我们来到了这里,代码做了简化

private void ExecuteWithThreadLocal(ref Task currentTaskSlot, Thread threadPoolThread = null) {     this.InnerInvoke();     this.Finish(true); }  virtual void InnerInvoke() {     Action action = this.m_action as Action;     if (action != null)     {         action();         return;     } }

很明显 this.InnerInvoke 就是执行了最开始 Task.Run(Work) 封装的委托了, 在 m_action 字段

this.Finish(true); 跟下去会发现会调用 FinishStageTwo 设置任务的完成状态,异常等, 继续调用 FinishStageThree 就来了重点: FinishContinuations 这个方法就是衔接后续回调的核心

internal void FinishContinuations() {     object obj = Interlocked.Exchange(ref this.m_continuationObject, Task.s_taskCompletionSentinel);     if (obj != null)     {         this.RunContinuations(obj);     } }

还记得状态机实现么, Task.m_continuationObject 字段实际存储的就是状态机的包装类,这里线程池线程也会判断这个字段有值的话,就直接使用它执行后续代码了

void RunContinuations(object continuationObject) {     var asyncStateMachineBox = continuationObject as IAsyncStateMachineBox;     if (asyncStateMachineBox != null)     {         AwaitTaskContinuation.RunOrScheduleAction(asyncStateMachineBox, flag2);         return;     } }  static void RunOrScheduleAction(IAsyncStateMachineBox box, bool allowInlining) {     if (allowInlining && AwaitTaskContinuation.IsValidLocationForInlining)     {         box.MoveNext();         return;     } }

总结

Task.Run 创建 Task, 把委托放在 m_action 字段, 把 Task 压入线程池队列,等待调度
_builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine); 尝试把状态机对象放在 Task.m_continuationObject 字段上,等待线程池线程调度完成任务后使用(用来执行后续),若操作失败,直接把状态机对象压入线程池队列,等待调度
线程池线程调度任务完成后,会判断 Task.m_continuationObject 有值,直接执行它的 MoveNext

备注

状态机实现中,尝试修改 Task.m_continuationObject,可能会失败,
就会直接把状态机对象压入线程池, 但是线程池调度,不都是判断是不是 Task 类型么, 其实状态机的包装类是 Task 的子类,哈哈,是不是明白了

class AsyncStateMachineBox<TStateMachine> : Task<TResult>, IAsyncStateMachineBox where TStateMachine : IAsyncStateMachine  static void DispatchWorkItem(object workItem, Thread currentThread) {     Task task = workItem as Task;     if (task != null)     {         task.ExecuteFromThreadPool(currentThread);         return;     }     Unsafe.As<IThreadPoolWorkItem>(workItem).Execute(); }

还有就是状态机包装类,重写了 Task.ExecuteFromThreadPool,所以线程池调用 task.ExecuteFromThreadPool 就是直接调用了状态机的 MoveNext 了, Soga ^_^
```
override void ExecuteFromThreadPool(Thread threadPoolThread) {     this.MoveNext(threadPoolThread); } 
```