取消令牌 - F# 异步.同步运行与超时和取消令牌

Question

当使用超时和取消令牌调用Async.RunSyncly时，超时值似乎被忽略了。我可以通过在 CancelToken 上调用 CancelAfter 来解决此问题，但理想情况下，我希望能够区分工作流中发生的异常、超时异常和操作取消异常。

我相信下面的示例代码证明了这一点。

open System
open System.Threading
let work = 
    async {
        let endTime = DateTime.UtcNow.AddMilliseconds(100.0)
        while DateTime.UtcNow < endTime do
            do! Async.Sleep(10)
            Console.WriteLine "working..."
        raise ( Exception "worked for more than 100 millis" )
    }

[<EntryPoint>]
let main argv = 
    try
        Async.RunSynchronously(work, 50)
    with
        | e -> Console.WriteLine (e.GetType().Name + ": " + e.Message)
    let cts = new CancellationTokenSource()
    try
        Async.RunSynchronously(work, 50, cts.Token)
    with
        | e -> Console.WriteLine (e.GetType().Name + ": " + e.Message)  

    cts.CancelAfter(80)
    try
        Async.RunSynchronously(work, 50, cts.Token)
    with
        | e -> Console.WriteLine (e.GetType().Name + ": " + e.Message)  
    Console.ReadKey(true) |> ignore
    0

输出以下内容，显示超时仅在第一种情况下有效（未指定取消令牌）

working...
working...
TimeoutException: The operation has timed out.
working...
working...
working...
working...
working...
working...
working...
Exception: worked for more than 100 millis
working...
working...
working...
working...
working...
working...
OperationCanceledException: The operation was canceled.

这是预期的行为吗？有什么办法得到我所追求的行为吗？

谢谢！

Answer 1

我不确定这是否是预期的行为 - 至少，我看不出任何理由会这样。但是，此行为直接在处理RunSynchronously参数时实现。如果您查看库源代码，您可以看到：

static member RunSynchronously (p:Async<'T>,?timeout,?cancellationToken) =
  let timeout,token =
    match cancellationToken with
    | None -> timeout,(!defaultCancellationTokenSource).Token                
    | Some token when not token.CanBeCanceled -> timeout, token                
    | Some token -> None, token

在您的情况下（超时和可以取消的取消令牌），代码会通过最后一个分支并忽略超时。我认为这要么是一个错误，要么是应该在文档中提及的内容。

作为解决方法，您可以创建一个单独的CancellationTokenSource来指定超时并将其链接到主取消源，以便调用方提供（使用 CreateLinkedTokenSource ）。当您得到OperationCancelledException 时，您可以检测源是实际取消还是超时：

type Microsoft.FSharp.Control.Async with
  static member RunSynchronouslyEx(a:Async<'T>, timeout:int, cancellationToken) =
    // Create cancellation token that is cancelled after 'timeout'
    let timeoutCts = new CancellationTokenSource()
    timeoutCts.CancelAfter(timeout)
    // Create a combined token that is cancelled either when 
    // 'cancellationToken' is cancelled, or after a timeout
    let combinedCts = 
      CancellationTokenSource.CreateLinkedTokenSource
        (cancellationToken, timeoutCts.Token)
    // Run synchronously with the combined token
    try Async.RunSynchronously(a, cancellationToken = combinedCts.Token)
    with :? OperationCanceledException as e ->
      // If the timeout occurred, then we throw timeout exception instead
      if timeoutCts.IsCancellationRequested then
        raise (new System.TimeoutException())
      else reraise()