Renamed Umbraco.Abstractions to Umbraco.Core

src/Umbraco.Core/SystemLock.cs

@@ -0,0 +1,196 @@
+using System;
+using System.Runtime.ConstrainedExecution;
+using System.Runtime.InteropServices;
+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
+    //
+    // using a named Semaphore here and not a Mutex because mutexes have thread
+    // affinity which does not work with async situations
+    //
+    // it is important that managed code properly release the Semaphore before
+    // going down else it will maintain the lock - however note that when the
+    // whole process (w3wp.exe) goes down and all handles to the Semaphore have
+    // been closed, the Semaphore system object is destroyed - so in any case
+    // an iisreset should clean up everything
+    //
+    public class SystemLock
+    {
+        private readonly SemaphoreSlim _semaphore;
+        private readonly Semaphore _semaphore2;
+        private readonly IDisposable _releaser;
+        private readonly Task<IDisposable> _releaserTask;
+
+        public SystemLock()
+            : this(null)
+        { }
+
+        public SystemLock(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)
+                : new NamedSemaphoreReleaser(_semaphore2);
+        }
+
+        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;
+            }
+
+            #region IDisposable Support
+
+            // This code added to correctly implement the disposable pattern.
+
+            private bool disposedValue = false; // To detect redundant calls
+
+            public void Dispose()
+            {
+                Dispose(true);
+                GC.SuppressFinalize(this); // finalize will not run
+            }
+
+            private void Dispose(bool disposing)
+            {
+                if (!disposedValue)
+                {
+                    try
+                    {
+                        _semaphore.Release();
+                    }
+                    finally
+                    {
+                        try
+                        {
+                            _semaphore.Dispose();
+                        }
+                        catch { }
+                    }
+                    disposedValue = true;
+                }
+            }
+
+            // we WANT to release the semaphore because it's a system object, ie a critical
+            // non-managed resource - and if it is not released then noone else can acquire
+            // the lock - so we inherit from CriticalFinalizerObject which means that the
+            // finalizer "should" run in all situations - there is always a chance that it
+            // does not run and the semaphore remains "acquired" but then chances are the
+            // whole process (w3wp.exe...) is going down, at which point the semaphore will
+            // be destroyed by Windows.
+
+            // however, the semaphore is a managed object, and so when the finalizer runs it
+            // might have been finalized already, and then we get a, ObjectDisposedException
+            // in the finalizer - which is bad.
+
+            // in order to prevent this we do two things
+            // - use a GCHandler to ensure the semaphore is still there when the finalizer
+            //   runs, so we can actually release it
+            // - wrap the finalizer code in a try...catch to make sure it never throws
+
+            ~NamedSemaphoreReleaser()
+            {
+                try
+                {
+                    Dispose(false);
+                }
+                catch
+                {
+                    // we do NOT want the finalizer to throw - never ever
+                }
+            }
+
+            #endregion
+
+        }
+
+        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);
+            }
+        }
+    }
+}