Propagation time of cancellation request to all tasks (TPL)

Question

With TPL we have CancellationTokenSource which provides tokens, useful to cooperatively cancellation of current task (or its start).

Question:

How long it take to propagate cancellation request to all hooked running tasks? Is there any place, where code could look to check that: "from now" every interested Task , will find that cancellation has been requested?

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Why there is need for it?

I would like to have stable unit test, to show that cancellation works in our code.

Problem details:

We have "Executor" which produces tasks, these task wrap some long running actions. Main job of executor is to limit how many concurrent actions were started. All of these tasks can be cancelled individually, and also these actions will respect CancellationToken internally.

I would like to provide unit test, which shows that when cancellation occurred while task is waiting for slot to start given action , that task will cancel itself (eventually) and does not start execution of given action .

So, idea was to prepare LimitingExecutor with single slot . Then start blocking action , which would request cancellation when unblocked. Then "enqueue" test action , which should fail when executed. With that setup, tests would call unblock and then assert that task of test action will throw TaskCanceledException when awaited.

[Test]
public void RequestPropagationTest()
{
    using (var setupEvent = new ManualResetEvent(initialState: false))
    using (var cancellation = new CancellationTokenSource())
    using (var executor = new LimitingExecutor())
    {
        // System-state setup action:
        var cancellingTask = executor.Do(() =>
        {
            setupEvent.WaitOne();
            cancellation.Cancel();
        }, CancellationToken.None);

        // Main work action:
        var actionTask = executor.Do(() =>
        {
            throw new InvalidOperationException(
                "This action should be cancelled!");
        }, cancellation.Token);

        // Let's wait until this `Task` starts, so it will got opportunity
        // to cancel itself, and expected later exception will not come
        // from just starting that action by `Task.Run` with token:
        while (actionTask.Status < TaskStatus.Running)
            Thread.Sleep(millisecondsTimeout: 1);

        // Let's unblock slot in Executor for the 'main work action'
        // by finalizing the 'system-state setup action' which will
        // finally request "global" cancellation:
        setupEvent.Set();

        Assert.DoesNotThrowAsync(
            async () => await cancellingTask);

        Assert.ThrowsAsync<TaskCanceledException>(
            async () => await actionTask);
    }
}

public class LimitingExecutor : IDisposable
{
    private const int UpperLimit = 1;
    private readonly Semaphore _semaphore
        = new Semaphore(UpperLimit, UpperLimit);

    public Task Do(Action work, CancellationToken token)
        => Task.Run(() =>
        {
            _semaphore.WaitOne();
            try
            {
                token.ThrowIfCancellationRequested();
                work();
            }
            finally
            {
                _semaphore.Release();
            }
        }, token);

    public void Dispose()
        => _semaphore.Dispose();
}

Executable demo (via NUnit) of this problem could be found at GitHub .

However, implementation of that test sometimes fails (no expected TaskCanceledException ), on my machin maybe 1 in 10 runs. Kind of "solution" to this problem is to insert Thread.Sleep right after request of cancellation. Even with sleep for 3 seconds this test sometimes fails (found after 20-ish runs), and when it passes, that long waiting is usually unnecessary (I guess). For reference, please see diff .

"Other problem", was to ensure that cancellation comes from "waiting time" and not from Task.Run , because ThreadPool could be busy (other executing tests), and it cold postpone start of second task after request of cancellation - that would render this test "falsy-green". The "easy fix by hack" was to actively wait until second task starts - its Status becomes TaskStatus.Running . Please check version under this branch and see that test without this hack will be sometimes "green" - so exampled bug could pass through it.

Answer 1

Your test method assumes that cancellingTask always takes the slot (enters the semaphore) in LimitingExecutor before the actionTask . Unfortunatelly, this assumption is wrong, LimitingExecutor does not guarantee this and it's just a matter of luck, which of the two task takes the slot (actually on my computer it only happens in something like 5% of runs).

To resolve this problem, you need another ManualResetEvent , that will allow main thread to wait until cancellingTask actually occupies the slot:

using (var slotTaken = new ManualResetEvent(initialState: false))
using (var setupEvent = new ManualResetEvent(initialState: false))
using (var cancellation = new CancellationTokenSource())
using (var executor = new LimitingExecutor())
{
    // System-state setup action:
    var cancellingTask = executor.Do(() =>
    {
        // This is called from inside the semaphore, so it's
        // certain that this task occupies the only available slot.
        slotTaken.Set();

        setupEvent.WaitOne();
        cancellation.Cancel();
    }, CancellationToken.None);

    // Wait until cancellingTask takes the slot
    slotTaken.WaitOne();

    // Now it's guaranteed that cancellingTask takes the slot, not the actionTask

    // ...
}

.NET Framework doesn't provide API to detect task transition to the Running state, so if you don't like polling the State property + Thread.Sleep() in a loop, you'll need to modify LimitingExecutor.Do() to provide this information, probably using another ManualResetEvent , eg:

public Task Do(Action work, CancellationToken token, ManualResetEvent taskRunEvent = null)
    => Task.Run(() =>
    {
        // Optional notification to the caller that task is now running
        taskRunEvent?.Set();

        // ...
    }, token);