// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.

using System;
using System.Diagnostics.CodeAnalysis;
using System.Web.Mvc.Async;

namespace Microsoft.Web.Mvc
{
    public static class AsyncManagerExtensions
    {
        [SuppressMessage("Microsoft.Design", "CA1031:DoNotCatchGeneralExceptionTypes", Justification = "An unhandled exception here will bring down the worker process.")]
        [SuppressMessage("StyleCop.CSharp.MaintainabilityRules", "SA1409:RemoveUnnecessaryCode",
            Justification = "The empty lock statement is required for synchronization.")]
        public static void RegisterTask(this AsyncManager asyncManager, Func<AsyncCallback, IAsyncResult> beginDelegate, Action<IAsyncResult> endDelegate)
        {
            if (asyncManager == null)
            {
                throw new ArgumentNullException("asyncManager");
            }
            if (beginDelegate == null)
            {
                throw new ArgumentNullException("beginDelegate");
            }
            if (endDelegate == null)
            {
                throw new ArgumentNullException("endDelegate");
            }

            // need to wait to execute the callback until after BeginXxx() has completed
            object delegateExecutingLockObj = new object();

            AsyncCallback callback = ar =>
            {
                lock (delegateExecutingLockObj)
                {
                    // this empty lock is required to synchronized with the beginDelegate call
                }
                if (!ar.CompletedSynchronously)
                {
                    try
                    {
                        asyncManager.Sync(() => endDelegate(ar)); // called on different thread, so have to take application lock
                    }
                    catch
                    {
                        // Need to swallow exceptions, as otherwise unhandled exceptions on a ThreadPool thread
                        // can bring down the entire worker process.
                    }
                    finally
                    {
                        asyncManager.OutstandingOperations.Decrement();
                    }
                }
            };

            IAsyncResult asyncResult;
            asyncManager.OutstandingOperations.Increment();
            try
            {
                lock (delegateExecutingLockObj)
                {
                    asyncResult = beginDelegate(callback);
                }
            }
            catch
            {
                asyncManager.OutstandingOperations.Decrement();
                throw;
            }

            if (asyncResult.CompletedSynchronously)
            {
                try
                {
                    endDelegate(asyncResult); // call on same thread
                }
                finally
                {
                    asyncManager.OutstandingOperations.Decrement();
                }
            }
        }
    }
}
