How can I prevent synchronous continuations on a Task?

Question

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();     } } 

Answer 1

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.

Answer 2

New in .NET 4.6:

.NET 4.6 contains a new TaskCreationOptions: RunContinuationsAsynchronously.

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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(); }