C# 任务忽略取消超时

Question

我正在尝试为任意代码编写一个包装器，它将在给定的超时期限后取消(或至少停止等待)代码。

我有以下测试和实现

[Test]
public void Policy_TimeoutExpires_DoStuff_TaskShouldNotContinue()
{
    var cts = new CancellationTokenSource();
    var fakeService = new Mock<IFakeService>();
    IExecutionPolicy policy = new TimeoutPolicy(new ExecutionTimeout(20), new DefaultExecutionPolicy());
    Assert.Throws<TimeoutException>(async () => await policy.ExecuteAsync(() => DoStuff(3000, fakeService.Object), cts.Token));

    fakeService.Verify(f=>f.DoStuff(),Times.Never);
}

和“DoStuff”方法

private static async Task DoStuff(int sleepTime, IFakeService fakeService)
{

    await Task.Delay(sleepTime).ConfigureAwait(false);
    var result = await Task.FromResult("bob");
    var test = result + "test";
    fakeService.DoStuff();
}

以及IExecutionPolicy.ExecuteAsync的实现

public async Task ExecuteAsync(Action action, CancellationToken token)
{
    var cts = new CancellationTokenSource();//TODO: resolve ignoring the token we were given!

    var task = _decoratedPolicy.ExecuteAsync(action, cts.Token);
    cts.CancelAfter(_timeout);

    try
    {
        await task.ConfigureAwait(false);
    }
    catch(OperationCanceledException err)
    {
        throw new TimeoutException("The task did not complete within the TimeoutExecutionPolicy window of" + _timeout + "ms", err);
    }
}

应该发生的是测试方法尝试花费 >3000 毫秒，超时应该发生在 20 毫秒，但这并没有发生。为什么我的代码没有按预期超时？

编辑:

按要求——decoratedPolicy如下

public async Task ExecuteAsync(Action action, CancellationToken token)
{
    token.ThrowIfCancellationRequested();
    await Task.Factory.StartNew(action.Invoke, token);  
}

Answer 1

如果我理解正确，您正试图为不支持超时/取消的方法支持超时。

通常这是通过启动具有所需超时值的计时器来完成的。如果计时器先触发，那么您可以抛出异常。使用 TPL，您可以使用 Task.Delay(_timeout) 而不是计时器。

public async Task ExecuteAsync(Action action, CancellationToken token)
{
    var task = _decoratedPolicy.ExecuteAsync(action, token);

    var completed = await Task.WhenAny(task, Task.Delay(_timeout));
    if (completed != task)
    {
        throw new TimeoutException("The task did not complete within the TimeoutExecutionPolicy window of" + _timeout + "ms");
    }
}

注意:这不会停止 _decoratedPolicy.ExecuteAsync 方法的执行，而是会忽略它。

如果您的方法确实支持取消(但不及时)，那么最好在超时后取消任务。您可以通过创建链接 token 来实现。

public async Task ExecuteAsync(Action action, CancellationToken token)
{
    using(var linkedTokenSource = CancellationTokenSource.CreateLinkedTokenSource(token))
    {
        var task = _decoratedPolicy.ExecuteAsync(action, linkedTokenSource.Token);

        var completed = await Task.WhenAny(task, Task.Delay(_timeout));
        if (completed != task)
        {
            linkedTokenSource.Cancel();//Try to cancel the method
            throw new TimeoutException("The task did not complete within the TimeoutExecutionPolicy window of" + _timeout + "ms");
        }
    }
}