监视同步方法的超时

Question

如果同步方法执行时间过长，我正在寻找一种有效的方法来引发超时异常。我看过一些样本，但没有一个完全符合我的要求。

我需要做的是

1. 检查同步方法是否超出其 SLA
2. 如果确实抛出超时异常

如果同步方法执行时间过长，我确实不必须终止同步方法。 （多次故障会使断路器跳闸并防止级联故障）

到目前为止，我的解决方案如下所示。请注意，我确实将 CancellationToken 传递给同步方法，希望它能够在超时时兑现取消请求。我的解决方案还返回一个任务，然后可以根据我的调用代码的需要等待等。

我担心的是，这段代码会为每个正在监控的方法创建两个任务。我认为 TPL 会很好地解决这个问题，但我想确认一下。

这有意义吗？有没有更好的方法来做到这一点？

private Task TimeoutSyncMethod( Action<CancellationToken> syncAction, TimeSpan timeout )
{
  var cts = new CancellationTokenSource();

  var outer = Task.Run( () =>
  {
     try
     {
        //Start the synchronous method - passing it a cancellation token
        var inner = Task.Run( () => syncAction( cts.Token ), cts.Token );

        if( !inner.Wait( timeout ) )
        {
            //Try give the sync method a chance to abort grecefully
            cts.Cancel();
            //There was a timeout regardless of what the sync method does - so throw
            throw new TimeoutException( "Timeout waiting for method after " + timeout );
        }
     }
     finally
     {
        cts.Dispose();
     }
  }, cts.Token );

  return outer;
}

---

编辑：

使用@Timothy 的回答我现在正在使用它。虽然代码并没有显着减少，但它更清晰。谢谢！

  private Task TimeoutSyncMethod( Action<CancellationToken> syncAction, TimeSpan timeout )
  {
    var cts = new CancellationTokenSource();

    var inner = Task.Run( () => syncAction( cts.Token ), cts.Token );
    var delay = Task.Delay( timeout, cts.Token );

    var timeoutTask = Task.WhenAny( inner, delay ).ContinueWith( t => 
      {
        try
        {
          if( !inner.IsCompleted )
          {
            cts.Cancel();
            throw new TimeoutException( "Timeout waiting for method after " + timeout );
          }
        }
        finally
        {
          cts.Dispose();
        }
      }, cts.Token );

    return timeoutTask;
  }

Answer 1

如果您有一个名为task 的Task，您可以这样做：

var delay = Task.Delay(TimeSpan.FromSeconds(3));
var timeoutTask = Task.WhenAny(task, delay);

如果timeoutTask.Result 最终成为task，那么它不会超时。否则，它是delay，它确实超时了。

我不知道这是否会与您实现的行为相同，但这是执行此操作的内置方式。

Answer 2

我已经为.NET 4.0 重新编写了这个解决方案，其中某些方法不可用，例如Delay。此版本正在监视返回object 的方法。如何在.NET 4.0中实现Delay来自这里：How to put a task to sleep (or delay) in C# 4.0?

public class OperationWithTimeout
{
    public Task<object> Execute(Func<CancellationToken, object> operation, TimeSpan timeout)
    {
        var cancellationToken = new CancellationTokenSource();

        // Two tasks are created. 
        // One which starts the requested operation and second which starts Timer. 
        // Timer is set to AutoReset = false so it runs only once after given 'delayTime'. 
        // When this 'delayTime' has elapsed then TaskCompletionSource.TrySetResult() method is executed. 
        // This method attempts to transition the 'delayTask' into the RanToCompletion state.
        Task<object> operationTask = Task<object>.Factory.StartNew(() => operation(cancellationToken.Token), cancellationToken.Token);
        Task delayTask = Delay(timeout.TotalMilliseconds);

        // Then WaitAny() waits for any of the provided task objects to complete execution.
        Task[] tasks = new Task[]{operationTask, delayTask};
        Task.WaitAny(tasks);

        try
        {
            if (!operationTask.IsCompleted)
            {
                // If operation task didn't finish within given timeout call Cancel() on token and throw 'TimeoutException' exception.
                // If Cancel() was called then in the operation itself the property 'IsCancellationRequested' will be equal to 'true'.
                cancellationToken.Cancel();
                throw new TimeoutException("Timeout waiting for method after " + timeout + ". Method was to slow :-)");
            }
        }
        finally
        {
            cancellationToken.Dispose();
        }

        return operationTask;
    }

    public static Task Delay(double delayTime)
    {
        var completionSource = new TaskCompletionSource<bool>();
        Timer timer = new Timer();
        timer.Elapsed += (obj, args) => completionSource.TrySetResult(true);
        timer.Interval = delayTime;
        timer.AutoReset = false;
        timer.Start();
        return completionSource.Task;
    }
}

如何在控制台应用程序中使用它。

    public static void Main(string[] args)
    {
        var operationWithTimeout = new OperationWithTimeout();
        TimeSpan timeout = TimeSpan.FromMilliseconds(10000);

        Func<CancellationToken, object> operation = token =>
        {
            Thread.Sleep(9000); // 12000

            if (token.IsCancellationRequested)
            {
                Console.Write("Operation was cancelled.");
                return null;
            }

            return 123456;
        };

        try
        {
            var t = operationWithTimeout.Execute(operation, timeout);
            var result = t.Result;
            Console.WriteLine("Operation returned '" + result + "'");
        }
        catch (TimeoutException tex)
        {
            Console.WriteLine(tex.Message);
        }

        Console.WriteLine("Press enter to exit");
        Console.ReadLine();
    }

Answer 3

详细说明 Timothy Shields 清洁解决方案：

        if (task == await Task.WhenAny(task, Task.Delay(TimeSpan.FromSeconds(3))))
        {
            return await task;
        }
        else
            throw new TimeoutException();

我发现，这个解决方案也可以处理 Task 有返回值的情况——即：

async Task<T>

更多内容请点击此处：MSDN: Crafting a Task.TimeoutAfter Method

Answer 4

Jasper 的回答让我得到了大部分的帮助，但我特别想要一个 void 函数来调用具有超时的非任务同步方法。这是我最终得到的结果：

public static void RunWithTimeout(Action action, TimeSpan timeout)
{
    var task = Task.Run(action);
    try
    {
        var success = task.Wait(timeout);
        if (!success)
        {
            throw new TimeoutException();
        }
    }
    catch (AggregateException ex)
    {
        throw ex.InnerException;
    }
}

这样称呼：

RunWithTimeout(() => File.Copy(..), TimeSpan.FromSeconds(3));