LongRun.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Web;

namespace WebApplication1
{
    public class Run : HttpTaskAsyncHandler
    {

        static Dictionary<string, Tuple<DateTime, TaskCompletionSource<object>>> list = new Dictionary<string, Tuple<DateTime, TaskCompletionSource<object>>>();

        public static Tuple<string, DateTime>[] List
        {
            get {
                lock (list) {
                    return list.Select(x => Tuple.Create(x.Key, x.Value.Item1)).ToArray();
                }
            }
        }

        public static void SetResult(string key)
        {
            if (!ValidateKey(key)) {
                return;
            }            

            Tuple<DateTime, TaskCompletionSource<object>> t;
            lock (list) {
                if (!list.TryGetValue(key, out t)) {
                    return;
                }

                list.Remove(key);
            }

            t.Item2.SetResult(null);
        }

        static Timer timer = new Timer((_) => {
            var now = DateTime.Now;

            TaskCompletionSource<object>[] tcs; 
            lock (list) {

                var expired = list.Where(x => now.Subtract(x.Value.Item1).TotalSeconds >= 5).ToArray();

                tcs = expired.Select(x => x.Value.Item2).ToArray();
                foreach (var x in expired) {
                    list.Remove(x.Key);
                }
            }

            foreach (var x in tcs) {
                x.SetResult(null);
            }

        }, null, 3000, 3000);

        static Task Listen(string key)
        {
            var tcs = new TaskCompletionSource<object>();
            
            lock (list) {
                list.Add(key, Tuple.Create(DateTime.Now, tcs));
            }

            return tcs.Task;
        }

        static string CreateKey()
        {
            return Guid.NewGuid().ToString("N");
        }

        static bool ValidateKey(string s)
        {
            Guid _;
            return Guid.TryParseExact(s, "N", out _);
        }


        public override async Task ProcessRequestAsync(HttpContext context)
        {
            var key = CreateKey();

            await Listen(key);

            if (!context.Response.IsClientConnected) {
                return;
            }
        }
    }
}