Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/Util/CountdownTask.cs

@@ -0,0 +1,54 @@
+//------------------------------------------------------------------------------
+// <copyright file="CountdownTask.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//------------------------------------------------------------------------------
+
+namespace System.Web.Util {
+    using System;
+    using System.Threading;
+    using System.Threading.Tasks;
+
+    // This class is similar to CountdownEvent, but it uses Task under the covers
+    // instead of ManualResetEvent. When the internal counter hits zero, the Task
+    // is marked as complete.
+    //
+    // This type is thread-safe. Error checking is not performed in RET builds.
+
+    internal sealed class CountdownTask {
+
+        private int _pendingCount;
+        private readonly TaskCompletionSource<object> _tcs = new TaskCompletionSource<object>();
+
+        public CountdownTask(int initialCount) {
+            AddCount(initialCount);
+        }
+
+        public int CurrentCount {
+            get { return _pendingCount; }
+        }
+
+        public Task Task {
+            get { return _tcs.Task; }
+        }
+
+        private void AddCount(int delta) {
+            int newCount = Interlocked.Add(ref _pendingCount, delta);
+            Debug.Assert(newCount >= 0, "The counter should never hit a negative value.");
+
+            if (newCount == 0) {
+                bool success = _tcs.TrySetResult(null);
+                Debug.Assert(success, "TrySetResult shouldn't have been already called.");
+            }
+        }
+
+        public void MarkOperationCompleted() {
+            AddCount(-1);
+        }
+
+        public void MarkOperationPending() {
+            AddCount(+1);
+        }
+
+    }
+}