Task sequencing and re-entracy

Question

I've got the following scenario, which I think might be quite common:

1. There is a task (a UI command handler) which can complete either synchronously or asynchronously.

2. Commands may arrive faster than they are getting processed.

3. If there is already a pending task for a command, the new command handler task should be queued and processed sequentially.

4. Each new task's result may depend on the result of the previous task.

Cancellation should be observed, but I'd like to leave it outside the scope of this question for simplicity. Also, thread-safety (concurrency) is not a requirement, but re-entrancy must be supported.

Here's a basic example of what I'm trying to achieve (as a console app, for simplicity):

using System;
using System.Threading.Tasks;

namespace ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            var asyncOp = new AsyncOp<int>();

            Func<int, Task<int>> handleAsync = async (arg) =>
            {
                Console.WriteLine("this task arg: " + arg);

                //await Task.Delay(arg); // make it async

                return await Task.FromResult(arg); // sync
            };

            Console.WriteLine("Test #1...");
            asyncOp.RunAsync(() => handleAsync(1000));
            asyncOp.RunAsync(() => handleAsync(900));
            asyncOp.RunAsync(() => handleAsync(800));
            asyncOp.CurrentTask.Wait();

            Console.WriteLine("\nPress any key to continue to test #2...");
            Console.ReadLine();

            asyncOp.RunAsync(() =>
            {
                asyncOp.RunAsync(() => handleAsync(200));
                return handleAsync(100);
            });

            asyncOp.CurrentTask.Wait();
            Console.WriteLine("\nPress any key to exit...");
            Console.ReadLine();
        }

        // AsyncOp
        class AsyncOp<T>
        {
            Task<T> _pending = Task.FromResult(default(T));

            public Task<T> CurrentTask { get { return _pending; } }

            public Task<T> RunAsync(Func<Task<T>> handler)
            {
                var pending = _pending;
                Func<Task<T>> wrapper = async () =>
                {
                    // await the prev task
                    var prevResult = await pending;
                    Console.WriteLine("\nprev task result:  " + prevResult);
                    // start and await the handler
                    return await handler();
                };

                _pending = wrapper();
                return _pending;
            }
        }

    }
}

The output:

Test #1...

prev task result:  0
this task arg: 1000

prev task result:  1000
this task arg: 900

prev task result:  900
this task arg: 800

Press any key to continue to test #2...


prev task result:  800

prev task result:  800
this task arg: 200
this task arg: 100

Press any key to exit...

It works in accordance with the requirements, until re-entrancy is introduced in test #2:

asyncOp.RunAsync(() =>
{
    asyncOp.RunAsync(() => handleAsync(200));
    return handleAsync(100);
});

The desired output should be 100, 200, rather than 200, 100, because there's already a pending outer task for 100. That's obviously because the inner task executes synchronously, breaking the logic var pending = _pending; /* ... */ _pending = wrapper() for the outer task.

How to make it work for test #2, too?

One solution would be to enforce asynchrony for every task, with Task.Factory.StartNew(..., TaskScheduler.FromCurrentSynchronizationContext(). However, I don't want to impose asynchronous execution upon the command handlers which might be synchronous internally. Also, I don't want to depend on the behavior of any particular synchronization context (i.e. relying upon that Task.Factory.StartNew should return before the created task has been actually started).

In the real-life project, I'm responsible for what AsyncOp is above, but have no control over the command handlers (i.e., whatever is inside handleAsync).

Answer 1

I almost forgot it's possible to construct a Task manually, without starting or scheduling it. Then, "Task.Factory.StartNew" vs "new Task(...).Start" put me back on track. I think this is one of those few cases when the Task<TResult> constructor may actually be useful, along with nested tasks (Task<Task<T>>) and Task.Unwrap():

// AsyncOp
class AsyncOp<T>
{
    Task<T> _pending = Task.FromResult(default(T));

    public Task<T> CurrentTask { get { return _pending; } }

    public Task<T> RunAsync(Func<Task<T>> handler, bool useSynchronizationContext = false)
    {
        var pending = _pending;
        Func<Task<T>> wrapper = async () =>
        {
            // await the prev task
            var prevResult = await pending;
            Console.WriteLine("\nprev task result:  " + prevResult);
            // start and await the handler
            return await handler();
        };

        var task = new Task<Task<T>>(wrapper);
        var inner = task.Unwrap();
        _pending = inner;

        task.RunSynchronously(useSynchronizationContext ?
            TaskScheduler.FromCurrentSynchronizationContext() :
            TaskScheduler.Current);

        return inner;
    }
}

The output:

Test #1...

prev task result:  0
this task arg: 1000

prev task result:  1000
this task arg: 900

prev task result:  900
this task arg: 800

Press any key to continue to test #2...


prev task result:  800
this task arg: 100

prev task result:  100
this task arg: 200

It's now also very easy to make AsyncOp thread-safe by adding a lock to protect _pending, if needed.

Updated, this has been further improved with cancel/restart logic.

Answer 2

Here is a solution that is worse on every aspect compared to the accepted answer, except from being thread-safe (which is not a requirement of the question). Disadvantages:

1. All lambdas are executed asynchronously (there is no fast path).
2. The executeOnCurrentContext configuration effects all lambdas (it's not a per-lambda configuration).

This solution uses as processing engine an ActionBlock from the TPL Dataflow library.

public class AsyncOp<T>
{
    private readonly ActionBlock<Task<Task<T>>> _actionBlock;

    public AsyncOp(bool executeOnCurrentContext = false)
    {
        var options = new ExecutionDataflowBlockOptions();
        if (executeOnCurrentContext)
            options.TaskScheduler = TaskScheduler.FromCurrentSynchronizationContext();

        _actionBlock = new ActionBlock<Task<Task<T>>>(async taskTask =>
        {
            try
            {
                taskTask.RunSynchronously();
                await await taskTask;
            }
            catch { } // Ignore exceptions
        }, options);
    }

    public Task<T> RunAsync(Func<Task<T>> taskFactory)
    {
        var taskTask = new Task<Task<T>>(taskFactory);
        if (!_actionBlock.Post(taskTask))
            throw new InvalidOperationException("Not accepted"); // Should never happen
        return taskTask.Unwrap();
    }
}