Fix inconsistent JTF dependency logic in the code.

The refcount skips completed tasks, but dependency walk through them. It will lead the dependency graph computation to go wrong, and leave stale dependencies to finished tasks when there is a cicular dependency loop. Ths problem is exposed through a new unit test.

Author: Lifeng Lu

src/Microsoft.VisualStudio.Threading/JoinableTask.cs

@@ -940,6 +940,9 @@ internal void CompleteOnCurrentThread()
                             }
                             else if (tryAgainAfter is object)
                             {
+                                // prevent referencing tasks which may be GCed during the waiting cycle.
+                                visited?.Clear();
+
                                 ThreadingEventSource.Instance.WaitSynchronouslyStart();
                                 this.owner.WaitSynchronously(tryAgainAfter);
                                 ThreadingEventSource.Instance.WaitSynchronouslyStop();
@@ -1070,7 +1073,7 @@ private static bool TryDequeueSelfOrDependencies(IJoinableTaskDependent currentN
 
                 if (work is null)
                 {
-                    if (joinableTask?.IsFullyCompleted != true)
+                    if (joinableTask?.IsCompleteRequested != true)
                     {
                         foreach (IJoinableTaskDependent? item in JoinableTaskDependencyGraph.GetDirectDependentNodes(currentNode))
                         {

src/Microsoft.VisualStudio.Threading/JoinableTaskDependencyGraph.cs

@@ -453,7 +453,7 @@ internal static void AddSelfAndDescendentOrJoinedJobs(IJoinableTaskDependent tas
 
                 if (taskOrCollection is JoinableTask thisJoinableTask)
                 {
-                    if (thisJoinableTask.IsFullyCompleted || !joinables.Add(thisJoinableTask))
+                    if (thisJoinableTask.IsFullyCompleted || !joinables.Add(thisJoinableTask) || thisJoinableTask.IsCompleteRequested)
                     {
                         return;
                     }
@@ -535,6 +535,11 @@ internal static void ComputeSelfAndDescendentOrJoinedJobsAndRemainTasks(IJoinabl
                             return;
                         }
 
+                        if ((taskOrCollection as JoinableTask)?.IsCompleteRequested == true)
+                        {
+                            return;
+                        }
+
                         WeakKeyDictionary<IJoinableTaskDependent, int>? dependencies = taskOrCollection.GetJoinableTaskDependentData().childDependentNodes;
                         if (dependencies is object)
                         {