Adding AsyncLock.

AsyncRx.NET/System.Reactive.Async/System/Threading/AsyncLock.cs

@@ -0,0 +1,63 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System.Diagnostics;
+using System.Threading.Tasks;
+
+namespace System.Threading
+{
+    public sealed class AsyncLock
+    {
+        private readonly object gate = new object();
+        private readonly SemaphoreSlim semaphore = new SemaphoreSlim(1, 1);
+        private readonly AsyncLocal<int> recursionCount = new AsyncLocal<int>();
+
+        public Task<Releaser> LockAsync()
+        {
+            var shouldAcquire = false;
+
+            lock (gate)
+            {
+                if (recursionCount.Value == 0)
+                {
+                    shouldAcquire = true;
+                    recursionCount.Value = 1;
+                }
+                else
+                {
+                    recursionCount.Value++;
+                }
+            }
+
+            if (shouldAcquire)
+            {
+                return semaphore.WaitAsync().ContinueWith(_ => new Releaser(this));
+            }
+
+            return Task.FromResult(new Releaser(this));
+        }
+
+        private void Release()
+        {
+            lock (gate)
+            {
+                Debug.Assert(recursionCount.Value > 0);
+
+                if (--recursionCount.Value == 0)
+                {
+                    semaphore.Release();
+                }
+            }
+        }
+
+        public struct Releaser : IDisposable
+        {
+            private readonly AsyncLock parent;
+
+            public Releaser(AsyncLock parent) => this.parent = parent;
+
+            public void Dispose() => parent.Release();
+        }
+    }
+}