How can I prevent synchronous continuations on a Task?

I have some library (socket networking) code that provides a Task-based API for pending responses to requests, based on TaskCompletionSource<T>. However, there's an annoyance in the TPL in that it seems to be impossible to prevent synchronous continuations. What I would like to be able to do is either:

• tell a TaskCompletionSource<T> that is should not allow callers to attach with TaskContinuationOptions.ExecuteSynchronously, or
• set the result (SetResult / TrySetResult) in a way that specifies that TaskContinuationOptions.ExecuteSynchronously should be ignored, using the pool instead

Specifically, the issue I have is that the incoming data is being processed by a dedicated reader, and if a caller can attach with TaskContinuationOptions.ExecuteSynchronously they can stall the reader (which affects more than just them). Previously, I have worked around this by some hackery that detects whether any continuations are present, and if they are it pushes the completion onto the ThreadPool, however this has significant impact if the caller has saturated their work queue, as the completion will not get processed in a timely fashion. If they are using Task.Wait() (or similar), they will then essentially deadlock themselves. Likewise, this is why the reader is on a dedicated thread rather than using workers.

So; before I try and nag the TPL team: am I missing an option?

Key points:

• I don't want external callers to be able to hijack my thread
• I can't use the ThreadPool as an implementation, as it needs to work when the pool is saturated

The example below produces output (ordering may vary based on timing):

Continuation on: Main thread
Press [return]
Continuation on: Thread pool

The problem is the fact that a random caller managed to get a continuation on "Main thread". In the real code, this would be interrupting the primary reader; bad things!

Code:

using System;
using System.Threading;
using System.Threading.Tasks;

static class Program
{
    static void Identify()
    {
        var thread = Thread.CurrentThread;
        string name = thread.IsThreadPoolThread
            ? "Thread pool" : thread.Name;
        if (string.IsNullOrEmpty(name))
            name = "#" + thread.ManagedThreadId;
        Console.WriteLine("Continuation on: " + name);
    }
    static void Main()
    {
        Thread.CurrentThread.Name = "Main thread";
        var source = new TaskCompletionSource<int>();
        var task = source.Task;
        task.ContinueWith(delegate {
            Identify();
        });
        task.ContinueWith(delegate {
            Identify();
        }, TaskContinuationOptions.ExecuteSynchronously);
        source.TrySetResult(123);
        Console.WriteLine("Press [return]");
        Console.ReadLine();
    }
}

New in .NET 4.6:

.NET 4.6 contains a new TaskCreationOptions: RunContinuationsAsynchronously.

---

Since you're willing to use Reflection to access private fields...

You can mark the TCS's Task with the TASK_STATE_THREAD_WAS_ABORTED flag, which would cause all continuations not to be inlined.

const int TASK_STATE_THREAD_WAS_ABORTED = 134217728;

var stateField = typeof(Task).GetField("m_stateFlags", BindingFlags.NonPublic | BindingFlags.Instance);
stateField.SetValue(task, (int) stateField.GetValue(task) | TASK_STATE_THREAD_WAS_ABORTED);

Edit:

Instead of using Reflection emit, I suggest you use expressions. This is much more readable and has the advantage of being PCL-compatible:

var taskParameter = Expression.Parameter(typeof (Task));
const string stateFlagsFieldName = "m_stateFlags";
var setter =
    Expression.Lambda<Action<Task>>(
        Expression.Assign(Expression.Field(taskParameter, stateFlagsFieldName),
            Expression.Or(Expression.Field(taskParameter, stateFlagsFieldName),
                Expression.Constant(TASK_STATE_THREAD_WAS_ABORTED))), taskParameter).Compile();

Without using Reflection:

If anyone's interested, I've figured out a way to do this without Reflection, but it is a bit "dirty" as well, and of course carries a non-negligible perf penalty:

try
{
    Thread.CurrentThread.Abort();
}
catch (ThreadAbortException)
{
    source.TrySetResult(123);
    Thread.ResetAbort();
}

I don't think there's anything in TPL which would provides explicit API control over TaskCompletionSource.SetResult continuations. I decided to keep my initial answer for controlling this behavior for async/await scenarios.

Here is another solution which imposes asynchronous upon ContinueWith, if the tcs.SetResult-triggered continuation takes place on the same thread the SetResult was called on:

public static class TaskExt
{
    static readonly ConcurrentDictionary<Task, Thread> s_tcsTasks =
        new ConcurrentDictionary<Task, Thread>();

    // SetResultAsync
    static public void SetResultAsync<TResult>(
        this TaskCompletionSource<TResult> @this,
        TResult result)
    {
        s_tcsTasks.TryAdd(@this.Task, Thread.CurrentThread);
        try
        {
            @this.SetResult(result);
        }
        finally
        {
            Thread thread;
            s_tcsTasks.TryRemove(@this.Task, out thread);
        }
    }

    // ContinueWithAsync, TODO: more overrides
    static public Task ContinueWithAsync<TResult>(
        this Task<TResult> @this,
        Action<Task<TResult>> action,
        TaskContinuationOptions continuationOptions = TaskContinuationOptions.None)
    {
        return @this.ContinueWith((Func<Task<TResult>, Task>)(t =>
        {
            Thread thread = null;
            s_tcsTasks.TryGetValue(t, out thread);
            if (Thread.CurrentThread == thread)
            {
                // same thread which called SetResultAsync, avoid potential deadlocks

                // using thread pool
                return Task.Run(() => action(t));

                // not using thread pool (TaskCreationOptions.LongRunning creates a normal thread)
                // return Task.Factory.StartNew(() => action(t), TaskCreationOptions.LongRunning);
            }
            else
            {
                // continue on the same thread
                var task = new Task(() => action(t));
                task.RunSynchronously();
                return Task.FromResult(task);
            }
        }), continuationOptions).Unwrap();
    }
}

Updated to address the comment:

> I don't control the caller - I can't get them to use a specific > continue-with variant: if I could, the problem would not exist in the > first place

I wasn't aware you don't control the caller. Nevertheless, if you don't control it, you're probably not passing the TaskCompletionSource object directly to the caller, either. Logically, you'd be passing the token part of it, i.e. tcs.Task. In which case, the solution might be even easier, by adding another extension method to the above:

// ImposeAsync, TODO: more overrides
static public Task<TResult> ImposeAsync<TResult>(this Task<TResult> @this)
{
    return @this.ContinueWith(new Func<Task<TResult>, Task<TResult>>(antecedent =>
    {
        Thread thread = null;
        s_tcsTasks.TryGetValue(antecedent, out thread);
        if (Thread.CurrentThread == thread)
        {
            // continue on a pool thread
            return antecedent.ContinueWith(t => t, 
                TaskContinuationOptions.None).Unwrap();
        }
        else
        {
            return antecedent;
        }
    }), TaskContinuationOptions.ExecuteSynchronously).Unwrap();
}

Use:

// library code
var source = new TaskCompletionSource<int>();
var task = source.Task.ImposeAsync();
// ... 

// client code
task.ContinueWith(delegate
{
    Identify();
}, TaskContinuationOptions.ExecuteSynchronously);

// ...
// library code
source.SetResultAsync(123);

This actually works for both await and ContinueWith (fiddle) and is free of reflection hacks.

What about instead of doing

var task = source.Task;

you do this instead

var task = source.Task.ContinueWith<Int32>( x => x.Result );

Thus you are always adding one continuation which will be executed asynchronously and then it doesn't matter if the subscribers want a continuation in the same context. It's sort of currying the task, isn't it?

The simulate abort approach looked really good, but led to the TPL hijacking threads in some scenarios.

I then had an implementation that was similar to checking the continuation object, but just checking for any continuation since there are actually too many scenarios for the given code to work well, but that meant that even things like Task.Wait resulted in a thread-pool lookup.

Ultimately, after inspecting lots and lots of IL, the only safe and useful scenario is the SetOnInvokeMres scenario (manual-reset-event-slim continuation). There are lots of other scenarios:

• some aren't safe, and lead to thread hijacking
• the rest aren't useful, as they ultimately lead to the thread-pool

So in the end, I opted to check for a non-null continuation-object; if it is null, fine (no continuations); if it is non-null, special-case check for SetOnInvokeMres - if it is that: fine (safe to invoke); otherwise, let the thread-pool perform the TrySetComplete, without telling the task to do anything special like spoofing abort. Task.Wait uses the SetOnInvokeMres approach, which is the specific scenario we want to try really hard not to deadlock.

Type taskType = typeof(Task);
FieldInfo continuationField = taskType.GetField("m_continuationObject", BindingFlags.Instance | BindingFlags.NonPublic);
Type safeScenario = taskType.GetNestedType("SetOnInvokeMres", BindingFlags.NonPublic);
if (continuationField != null && continuationField.FieldType == typeof(object) && safeScenario != null)
{
    var method = new DynamicMethod("IsSyncSafe", typeof(bool), new[] { typeof(Task) }, typeof(Task), true);
    var il = method.GetILGenerator();
    var hasContinuation = il.DefineLabel();
    il.Emit(OpCodes.Ldarg_0);
    il.Emit(OpCodes.Ldfld, continuationField);
    Label nonNull = il.DefineLabel(), goodReturn = il.DefineLabel();
    // check if null
    il.Emit(OpCodes.Brtrue_S, nonNull);
    il.MarkLabel(goodReturn);
    il.Emit(OpCodes.Ldc_I4_1);
    il.Emit(OpCodes.Ret);

    // check if is a SetOnInvokeMres - if so, we're OK
    il.MarkLabel(nonNull);
    il.Emit(OpCodes.Ldarg_0);
    il.Emit(OpCodes.Ldfld, continuationField);
    il.Emit(OpCodes.Isinst, safeScenario);
    il.Emit(OpCodes.Brtrue_S, goodReturn);

    il.Emit(OpCodes.Ldc_I4_0);
    il.Emit(OpCodes.Ret);

    IsSyncSafe = (Func<Task, bool>)method.CreateDelegate(typeof(Func<Task, bool>));

Updated, I posted a separate answer to deal with ContinueWith as opposed to await (because ContinueWith doesn't care about the current synchronization context).

You could use a dumb synchronization context to impose asynchrony upon continuation triggered by calling SetResult/SetCancelled/SetException on TaskCompletionSource. I believe the current synchronization context (at the point of await tcs.Task) is the criteria TPL uses to decide whether to make such continuation synchronous or asynchronous.

The following works for me:

if (notifyAsync)
{
    tcs.SetResultAsync(null);
}
else
{
    tcs.SetResult(null);
}

SetResultAsync is implemented like this:

public static class TaskExt
{
    static public void SetResultAsync<T>(this TaskCompletionSource<T> tcs, T result)
    {
        FakeSynchronizationContext.Execute(() => tcs.SetResult(result));
    }

    // FakeSynchronizationContext
    class FakeSynchronizationContext : SynchronizationContext
    {
        private static readonly ThreadLocal<FakeSynchronizationContext> s_context =
            new ThreadLocal<FakeSynchronizationContext>(() => new FakeSynchronizationContext());

        private FakeSynchronizationContext() { }

        public static FakeSynchronizationContext Instance { get { return s_context.Value; } }

        public static void Execute(Action action)
        {
            var savedContext = SynchronizationContext.Current;
            SynchronizationContext.SetSynchronizationContext(FakeSynchronizationContext.Instance);
            try
            {
                action();
            }
            finally
            {
                SynchronizationContext.SetSynchronizationContext(savedContext);
            }
        }

        // SynchronizationContext methods

        public override SynchronizationContext CreateCopy()
        {
            return this;
        }

        public override void OperationStarted()
        {
            throw new NotImplementedException("OperationStarted");
        }

        public override void OperationCompleted()
        {
            throw new NotImplementedException("OperationCompleted");
        }

        public override void Post(SendOrPostCallback d, object state)
        {
            throw new NotImplementedException("Post");
        }

        public override void Send(SendOrPostCallback d, object state)
        {
            throw new NotImplementedException("Send");
        }
    }
}

SynchronizationContext.SetSynchronizationContext is very cheap in terms of the overhead it adds. In fact, a very similar approach is taken by the implementation of WPF Dispatcher.BeginInvoke.

TPL compares the target synchronization context at the point of await to that of the point of tcs.SetResult. If the synchronization context is the same (or there is no synchronization context at both places), the continuation is called directly, synchronously. Otherwise, it's queued using SynchronizationContext.Post on the target synchronization context, i.e., the normal await behavior. What this approach does is always impose the SynchronizationContext.Post behavior (or a pool thread continuation if there's no target synchronization context).

Updated, this won't work for task.ContinueWith, because ContinueWith doesn't care about the current synchronization context. It however works for await task (fiddle). It also does work for await task.ConfigureAwait(false).

OTOH, this approach works for ContinueWith.

if you can and are ready to use reflection, this should do it;

public static class MakeItAsync
{
    static public void TrySetAsync<T>(this TaskCompletionSource<T> source, T result)
    {
        var continuation = typeof(Task).GetField("m_continuationObject", BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance);
        var continuations = (List<object>)continuation.GetValue(source.Task);

        foreach (object c in continuations)
        {
            var option = c.GetType().GetField("m_options", BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance);
            var options = (TaskContinuationOptions)option.GetValue(c);

            options &= ~TaskContinuationOptions.ExecuteSynchronously;
            option.SetValue(c, options);
        }

        source.TrySetResult(result);
    }        
}