Prefer using CancellationToken.WaitHandle again

We should trust CancellationToken.WaitHandle again, since the early days of .NET Core passed. This helps to avoid using timers unnecessarily on a fully synchronous path, and we now also have a protection agains problems with waiting.

src/Hangfire.Core/Common/CancellationTokenExtentions.cs

@@ -28,6 +28,7 @@ public static class CancellationTokenExtentions
         /// on cancellation token registration and avoids using the <see cref="CancellationToken.WaitHandle"/>
         /// property as it may lead to high CPU issues.
         /// </summary>
+        [Obsolete("CancellationToken.WaitHandle is now preferred, since early days of .NET Core passed. Will be removed in 2.0.0.")]
         public static CancellationEvent GetCancellationEvent(this CancellationToken cancellationToken)
         {
             return new CancellationEvent(cancellationToken);
@@ -56,10 +57,8 @@ public static void WaitOrThrow(this CancellationToken cancellationToken, TimeSpa
         /// </summary>
         public static bool Wait(this CancellationToken cancellationToken, TimeSpan timeout)
         {
-            using var cancellationEvent = GetCancellationEvent(cancellationToken);
-
             var stopwatch = Stopwatch.StartNew();
-            var waitResult = cancellationEvent.WaitHandle.WaitOne(timeout);
+            var waitResult = cancellationToken.WaitHandle.WaitOne(timeout);
             stopwatch.Stop();
 
             var timeoutThreshold = TimeSpan.FromMilliseconds(1000);
@@ -84,6 +83,7 @@ public static bool Wait(this CancellationToken cancellationToken, TimeSpan timeo
             return waitResult;
         }
 
+        [Obsolete("CancellationToken.WaitHandle is now preferred, since early days of .NET Core passed. Will be removed in 2.0.0.")]
         public sealed class CancellationEvent : IDisposable
         {
             private static readonly Action<object> SetEventCallback = SetEvent;

src/Hangfire.Core/Processing/TaskExtensions.cs

@@ -36,9 +36,7 @@ public static bool WaitOne([NotNull] this WaitHandle waitHandle, TimeSpan timeou
 
             token.ThrowIfCancellationRequested();
 
-            using var ev = token.GetCancellationEvent();
-
-            var waitHandles = new[] { waitHandle, ev.WaitHandle };
+            var waitHandles = new[] { waitHandle, token.WaitHandle };
 
             var stopwatch = Stopwatch.StartNew();
             var waitResult = WaitHandle.WaitAny(waitHandles, timeout);

src/Hangfire.Core/Server/CoreBackgroundJobPerformer.cs

@@ -182,7 +182,6 @@ private static object InvokeOnTaskPump(PerformContext context, Tuple<MethodInfo,
             try
             {
                 using (var syncContext = new InlineSynchronizationContext())
-                using (var cancellationEvent = context.CancellationToken.ShutdownToken.GetCancellationEvent())
                 {
                     SynchronizationContext.SetSynchronizationContext(syncContext);
 
@@ -192,7 +191,7 @@ private static object InvokeOnTaskPump(PerformContext context, Tuple<MethodInfo,
                     var task = getTaskFunc(result);
                     var asyncResult = (IAsyncResult)task;
 
-                    var waitHandles = new[] { syncContext.WaitHandle, asyncResult.AsyncWaitHandle, cancellationEvent.WaitHandle };
+                    var waitHandles = new[] { syncContext.WaitHandle, asyncResult.AsyncWaitHandle, context.CancellationToken.ShutdownToken.WaitHandle };
 
                     while (!asyncResult.IsCompleted && WaitHandle.WaitAny(waitHandles) == 0)
                     {