ShellStream.EndExpect continues to wait for a match until the timeout is reached

[vesuvian]

I am using the published stable Renci SSH.NET nuget package (2016.1.0)

I am successfully using ShellStreams BeginExpect method to wait asynchronously for one of several patterns to match until a timeout is reached. Additionally, I would like to be able to cancel the expect at any time arbitrarily.

I had hoped EndExpect would immediately stop the expect thread, but instead the behaviour appears to be that EndExpect blocks until a match is found.

I have tried setting the AsyncResult as completed using SetAsCompleted before calling EndExpect to bypass this behaviour, but the thread will continue to search for matches in the background until it eventually calls SetAsCompleted itself and throws an exception (InvalidOperationException - You can set a result only once).

I would like to propose a CancelExpect method that quickly sets the AsyncResult as cancelled and causes the expect thread to quickly exit.

I'm very happy to implement this feature myself, I would just like to know if there is already a better approach for what I am trying to do, and which branch I should be basing my work on.

[IgorMilavec]

@vesuvian before proposing a change in the API I'd like to better understand usage patterns related to Expect API. Can you please explain how you use it and if possible provide a proto code for it?

[gbrunton]

I wrote the following extension method because I wanted to be able to cancel an expect on demand. I just started using it and it seems to be working good for now. Unfortunately I don't have tests for it and I can't say that it's perfect but I thought I'd share because it took me a while to write and maybe it can be helpful.

using System;
using System.Threading;
using System.Threading.Tasks;
using Renci.SshNet;

namespace core.ExtensionMethods;

public static class ShellStreamExtensions
{
    public static Task ExpectAsync(this ShellStream source, ExpectAction expectAction, CancellationToken cancellationToken = default)
    {
        // Using the TaskCreationOptions.RunContinuationsAsynchronously because without it I noticed hangs when an exception is thrown within the 
        // BeginExpect delegate. The following discusses thoughts on this.
        // https://devblogs.microsoft.com/premier-developer/the-danger-of-taskcompletionsourcet-class/
        var tcs = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);

        // The registration needs to be unregistered when this task is complete, regardless of completion reason.
        // Note that we are using the cancellationTokenRegistration.Unregister() method instead of cancellationTokenRegistration.Dispose().
        // This is because I've observed the Dispose method hang at times.
        var cancellationTokenRegistration = cancellationToken.Register(() =>
        {
            // we might be able to write something to the shell here but I'm not sure what.
            //shellStream.WriteLine();
            tcs.TrySetCanceled(cancellationToken);
        });
        source.BeginExpect(ar =>
        {
            try
            {
                if (cancellationToken.IsCancellationRequested)
                {
                    cancellationTokenRegistration.Unregister();
                    tcs.TrySetCanceled(cancellationToken);
                    return;
                }

                if (ar.IsCompleted)
                {
                    cancellationTokenRegistration.Unregister();
                    tcs.TrySetResult(source.EndExpect(ar));
                }
            }
            // I don't expect anything to come here since we can't pass the cancellation token into the shellStream. If we decide to use
            // cancellationToken.ThrowIfCancellationRequested() above instead of explicitly checking cancellationToken.IsCancellationRequested 
            // then this would be used.
            catch (Exception e) when (e is TaskCanceledException or OperationCanceledException)
            {
                cancellationTokenRegistration.Unregister();
                tcs.TrySetCanceled(cancellationToken);
            }
            catch (Exception e)
            {
                cancellationTokenRegistration.Unregister();
                tcs.TrySetException(e);
            }
        }, expectAction);

        return tcs.Task;
    }
}