F# Async.RunSynchronously 与 timeout 和 cancelToken

Question

当使用超时和 CancellationToken 调用 Async.RunSynchronously 时，超时值似乎被忽略了。我可以通过在 CancellationToken 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 TimeOutExceptions 和 OperationCanceledExceptions。

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

输出如下，说明超时只在第一种情况下有效（没有指定CancelationToken）

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？有什么办法可以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，我看不出有任何原因。但是，这种行为是在处理RunSynchronously 的参数时直接实现的。如果你看library source code，你可以看到：

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（使用超时和可以取消的取消令牌），代码将通过最后一个分支并忽略超时。我认为这要么是一个错误，要么应该在文档中提及。

作为一种解决方法，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，以便调用者提供（使用CreateLinkedTokenSource）。当您收到OperationCancelledException 时，您可以检测来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()