actions-runner/src/Runner.Common/AsyncManualResetEvent.cs

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

namespace GitHub.Runner.Common
{
    //Stephen Toub: http://blogs.msdn.com/b/pfxteam/archive/2012/02/11/10266920.aspx
    
    public class AsyncManualResetEvent
    {
        private volatile TaskCompletionSource<bool> m_tcs = new TaskCompletionSource<bool>();

        public Task WaitAsync() { return m_tcs.Task; }

        public void Set()
        {
            var tcs = m_tcs;
            Task.Factory.StartNew(s => ((TaskCompletionSource<bool>)s).TrySetResult(true),
                tcs, CancellationToken.None, TaskCreationOptions.PreferFairness, TaskScheduler.Default);
            tcs.Task.Wait();
        }

        public void Reset()
        {
            while (true)
            {
                var tcs = m_tcs;
                if (!tcs.Task.IsCompleted ||
                    Interlocked.CompareExchange(ref m_tcs, new TaskCompletionSource<bool>(), tcs) == tcs)
                    return;
            }
        }
    }
}