How to force an ActionBlock to complete fast

问题

According to the documentation:

A dataflow block is considered completed when it is not currently processing a message and when it has guaranteed that it will not process any more messages.

This behavior is not ideal in my case. I want to be able to cancel the job at any time, but the processing of each individual action takes a long time. So when I cancel the token, the effect is not immediate. I must wait for the currently processed item to complete. I have no way to cancel the actions directly, because the API I use is not cancelable. Can I do anything to make the block ignore the currently running action, and complete instantly?

Here is an example that demonstrates my problem. The token is canceled after 500 msec, and the duration of each action is 1000 msec:

static async Task Main()
{
    var cts = new CancellationTokenSource(500);
    var block = new ActionBlock<int>(async x =>
    {
        await Task.Delay(1000);
    }, new ExecutionDataflowBlockOptions() { CancellationToken = cts.Token });
    block.Post(1); // I must wait for this one to complete
    block.Post(2); // This one is ignored
    block.Complete();
    var stopwatch = Stopwatch.StartNew();
    try
    {
        await block.Completion;
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine($"Canceled after {stopwatch.ElapsedMilliseconds} msec");
    }
}

Output:

Canceled after 1035 msec

The desired output would be a cancellation after ~500 msec.

回答1:

Based on this excerpt from your comment...:

What I want to happen in case of a cancellation request is to ignore the currently running workitem. I don't care about it any more, so why I have to wait for it?

...and assuming you are truly OK with leaving the Task running, you can simply wrap the job you wish to call inside another Task which will constantly poll for cancellation or completion, and cancel that Task instead. Take a look at the following "proof-of-concept" code that wraps a "long-running" task inside another Task "tasked" with constantly polling the wrapped task for completion, and a CancellationToken for cancellation (completely "spur-of-the-moment" status, you will want to re-adapt it a bit of course):

public class LongRunningTaskSource
{
    public Task LongRunning(int milliseconds)
    {
        return Task.Run(() =>
        {
            Console.WriteLine("Starting long running task");
            Thread.Sleep(3000);
            Console.WriteLine("Finished long running task");
        });
    }

    public Task LongRunningTaskWrapper(int milliseconds, CancellationToken token)
    {
        Task task = LongRunning(milliseconds);

        Task wrapperTask = Task.Run(() =>
        {
            while (true)
            {
                //Check for completion (you could, of course, do different things
                //depending on whether it is faulted or completed).
                if (!(task.Status == TaskStatus.Running))
                    break;

                //Check for cancellation.
                if (token.IsCancellationRequested)
                {
                    Console.WriteLine("Aborting Task.");

                    token.ThrowIfCancellationRequested();
                }
            }

        }, token);

        return wrapperTask;
    }
}

Using the following code:

static void Main()
{
    LongRunningTaskSource longRunning = new LongRunningTaskSource();

    CancellationTokenSource cts = new CancellationTokenSource(1500);

    Task task = longRunning.LongRunningTaskWrapper(3000, cts.Token);

    //Sleep long enough to let things roll on their own.
    Thread.Sleep(5000);

    Console.WriteLine("Ended Main");
}

...produces the following output:

Starting long running task
Aborting Task.
Exception thrown: 'System.OperationCanceledException' in mscorlib.dll
Finished long running task
Ended Main

The wrapped Task obviously completes in its own good time. If you don't have a problem with that, which is often not the case, hopefully, this should fit your needs.

As a supplementary example, running the following code (letting the wrapped Task finish before time-out):

static void Main()
{
    LongRunningTaskSource longRunning = new LongRunningTaskSource();

    CancellationTokenSource cts = new CancellationTokenSource(3000);

    Task task = longRunning.LongRunningTaskWrapper(1500, cts.Token);

    //Sleep long enough to let things roll on their own.
    Thread.Sleep(5000);

    Console.WriteLine("Ended Main");
}

...produces the following output:

Starting long running task
Finished long running task
Ended Main

So the task started and finished before timeout and nothing had to be cancelled. Of course nothing is blocked while waiting. As you probably already know, of course, if you know what is being used behind the scenes in the long-running code, it would be good to clean up if necessary.

Hopefully, you can adapt this example to pass something like this to your ActionBlock.

Disclaimer & Notes

I am not familiar with the TPL Dataflow library, so this is just a workaround, of course. Also, if all you have is, for example, a synchronous method call that you do not have any influence on at all, then you will obviously need two tasks. One wrapper task to wrap the synchronous call and another one to wrap the wrapper task to include continuous status polling and cancellation checks.

来源：https://stackoverflow.com/questions/58330380/how-to-force-an-actionblock-to-complete-fast