AsyncLock - import

src/Umbraco.Core/AsyncLock.cs

@@ -0,0 +1,198 @@
+using System;
+using System.Runtime.ConstrainedExecution;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Umbraco.Core
+{
+    // http://blogs.msdn.com/b/pfxteam/archive/2012/02/12/10266988.aspx
+    //
+    // notes:
+    // - this is NOT a reader/writer lock
+    // - this is NOT a recursive lock
+    //
+    internal class AsyncLock
+    {
+        private readonly SemaphoreSlim _semaphore;
+        private readonly Semaphore _semaphore2;
+        private readonly IDisposable _releaser;
+        private readonly Task<IDisposable> _releaserTask;
+
+        public AsyncLock()
+            : this (null)
+        { }
+
+        public AsyncLock(string name)
+        {
+            // WaitOne() waits until count > 0 then decrements count
+            // Release() increments count
+            // initial count: the initial count value
+            // maximum count: the max value of count, and then Release() throws
+
+            if (string.IsNullOrWhiteSpace(name))
+            {
+                // anonymous semaphore
+                // use one unique releaser, that will not release the semaphore when finalized
+                // because the semaphore is destroyed anyway if the app goes down
+
+                _semaphore = new SemaphoreSlim(1, 1); // create a local (to the app domain) semaphore
+                _releaser = new SemaphoreSlimReleaser(_semaphore);
+                _releaserTask = Task.FromResult(_releaser);
+            }
+            else
+            {
+                // named semaphore
+                // use dedicated releasers, that will release the semaphore when finalized
+                // because the semaphore is system-wide and we cannot leak counts
+
+                _semaphore2 = new Semaphore(1, 1, name); // create a system-wide named semaphore
+            }
+        }
+
+        private IDisposable CreateReleaser()
+        {
+            // for anonymous semaphore, use the unique releaser, else create a new one
+            return _semaphore != null
+                ? _releaser // (IDisposable)new SemaphoreSlimReleaser(_semaphore)
+                : (IDisposable)new NamedSemaphoreReleaser(_semaphore2);
+        }
+
+        public Task<IDisposable> LockAsync()
+        {
+            var wait = _semaphore != null 
+                ? _semaphore.WaitAsync() 
+                : WaitOneAsync(_semaphore2);
+
+            return wait.IsCompleted 
+                ? _releaserTask ?? Task.FromResult(CreateReleaser()) // anonymous vs named
+                : wait.ContinueWith((_, state) => (((AsyncLock) state).CreateReleaser()),
+                    this, CancellationToken.None,
+                    TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
+        }
+
+        public IDisposable Lock()
+        {
+            if (_semaphore != null)
+                _semaphore.Wait();
+            else
+                _semaphore2.WaitOne();
+            return _releaser ?? CreateReleaser(); // anonymous vs named
+        }
+
+        public IDisposable Lock(int millisecondsTimeout)
+        {
+            var entered = _semaphore != null
+                ? _semaphore.Wait(millisecondsTimeout)
+                : _semaphore2.WaitOne(millisecondsTimeout);
+            if (entered == false)
+                throw new TimeoutException("Failed to enter the lock within timeout.");
+            return _releaser ?? CreateReleaser(); // anonymous vs named
+        }
+
+        // note - before making those classes some structs, read 
+        // about "impure methods" and mutating readonly structs...
+
+        private class NamedSemaphoreReleaser : CriticalFinalizerObject, IDisposable
+        {
+            private readonly Semaphore _semaphore;
+
+            internal NamedSemaphoreReleaser(Semaphore semaphore)
+            {
+                _semaphore = semaphore;
+            }
+
+            public void Dispose()
+            {
+                Dispose(true);
+                GC.SuppressFinalize(this);
+            }
+
+            private void Dispose(bool disposing)
+            {
+                // critical
+                _semaphore.Release();
+            }
+
+            // we WANT to release the semaphore because it's a system object
+            // ie a critical non-managed resource - so we inherit from CriticalFinalizerObject
+            // which means that the finalizer "should" run in all situations
+
+            // however... that can fail with System.ObjectDisposedException because the
+            // underlying handle was closed... because we cannot guarantee that the semaphore
+            // is not gone already... unless we get a GCHandle = GCHandle.Alloc(_semaphore);
+            // which should keep it around and then we free the handle?
+
+            // so... I'm not sure this is safe really...
+
+            ~NamedSemaphoreReleaser()
+            {
+                Dispose(false);
+            }
+        }
+
+        private class SemaphoreSlimReleaser : IDisposable
+        {
+            private readonly SemaphoreSlim _semaphore;
+
+            internal SemaphoreSlimReleaser(SemaphoreSlim semaphore)
+            {
+                _semaphore = semaphore;
+            }
+
+            public void Dispose()
+            {
+                Dispose(true);
+                GC.SuppressFinalize(this);
+            }
+
+            private void Dispose(bool disposing)
+            {
+                if (disposing)
+                {
+                    // normal
+                    _semaphore.Release();
+                }
+            }
+
+            ~SemaphoreSlimReleaser()
+            {
+                Dispose(false);
+            }
+        }
+
+        // http://stackoverflow.com/questions/25382583/waiting-on-a-named-semaphore-with-waitone100-vs-waitone0-task-delay100
+        // http://blog.nerdbank.net/2011/07/c-await-for-waithandle.html
+        // F# has a AwaitWaitHandle method that accepts a time out... and seems pretty complex...
+        // version below should be OK
+
+        private static Task WaitOneAsync(WaitHandle handle)
+        {
+            var tcs = new TaskCompletionSource<object>();
+            var callbackHandleInitLock = new object();
+            lock (callbackHandleInitLock)
+            {
+                RegisteredWaitHandle callbackHandle = null;
+                // ReSharper disable once RedundantAssignment
+                callbackHandle = ThreadPool.RegisterWaitForSingleObject(
+                    handle,
+                    (state, timedOut) =>
+                    {
+                        tcs.SetResult(null);
+
+                        // we take a lock here to make sure the outer method has completed setting the local variable callbackHandle.
+                        lock (callbackHandleInitLock)
+                        {
+                            // ReSharper disable once PossibleNullReferenceException
+                            // ReSharper disable once AccessToModifiedClosure
+                            callbackHandle.Unregister(null);
+                        }
+                    },
+                    /*state:*/ null,
+                    /*millisecondsTimeOutInterval:*/ Timeout.Infinite,
+                    /*executeOnlyOnce:*/ true);
+            }
+
+            return tcs.Task;
+        }
+    }
+}

src/Umbraco.Core/Umbraco.Core.csproj

@@ -136,6 +136,7 @@
     <Compile Include="ApplicationContext.cs" />
     <Compile Include="ApplicationEventHandler.cs" />
     <Compile Include="AssemblyExtensions.cs" />
+    <Compile Include="AsyncLock.cs" />
     <Compile Include="Attempt{T}.cs" />
     <Compile Include="Auditing\AuditTrail.cs" />
     <Compile Include="Auditing\Audit.cs" />