Use a copy on write list for items in Resource Filters

We allocate a separate list for model-binding related objects when we
create the resource filter contexts, and these lists then live the
lifetime of the request. These *may* be modified by user code in
filters as a supported feature, but rarely are changed in practice.

This change adds a simple CopyOnWriteList implementation to reduce the
amount of copying that's actually done.

src/Microsoft.AspNet.Mvc.Core/FilterActionInvoker.cs

@@ -7,6 +7,7 @@ using System.Diagnostics;
 using System.Linq;
 using System.Runtime.ExceptionServices;
 using System.Threading.Tasks;
+using Microsoft.AspNet.Mvc.Internal;
 using Microsoft.AspNet.Mvc.ModelBinding;
 using Microsoft.AspNet.Mvc.ModelBinding.Validation;
 using Microsoft.Framework.Internal;
@@ -241,11 +242,11 @@ namespace Microsoft.AspNet.Mvc.Core
 
             var context = new ResourceExecutingContext(ActionContext, _filters);
 
-            context.InputFormatters = new List<IInputFormatter>(_inputFormatters);
-            context.OutputFormatters = new List<IOutputFormatter>(_outputFormatters);
-            context.ModelBinders = new List<IModelBinder>(_modelBinders);
-            context.ValidatorProviders = new List<IModelValidatorProvider>(_modelValidatorProviders);
-            context.ValueProviderFactories = new List<IValueProviderFactory>(_valueProviderFactories);
+            context.InputFormatters = new CopyOnWriteList<IInputFormatter>(_inputFormatters);
+            context.OutputFormatters = new CopyOnWriteList<IOutputFormatter>(_outputFormatters);
+            context.ModelBinders = new CopyOnWriteList<IModelBinder>(_modelBinders);
+            context.ValidatorProviders = new CopyOnWriteList<IModelValidatorProvider>(_modelValidatorProviders);
+            context.ValueProviderFactories = new CopyOnWriteList<IValueProviderFactory>(_valueProviderFactories);
 
             _resourceExecutingContext = context;
             await InvokeResourceFilterAsync();

src/Microsoft.AspNet.Mvc.Core/Internal/CopyOnWriteList.cs

@@ -0,0 +1,140 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Microsoft.AspNet.Mvc.Internal
+{
+    public class CopyOnWriteList<T> : IList<T>
+    {
+        private readonly IReadOnlyList<T> _source;
+        private List<T> _copy;
+
+        public CopyOnWriteList(IReadOnlyList<T> source)
+        {
+            _source = source;
+        }
+
+        protected IReadOnlyList<T> Readable
+        {
+            get
+            {
+                return _copy ?? _source;
+            }
+        }
+
+        protected List<T> Writable
+        {
+            get
+            {
+                if (_copy == null)
+                {
+                    _copy = new List<T>(_source);
+                }
+
+                return _copy;
+            }
+        }
+
+        public T this[int index]
+        {
+            get
+            {
+                return Readable[index];
+            }
+            set
+            {
+                Writable[index] = value;
+            }
+        }
+
+        public int Count
+        {
+            get
+            {
+                return Readable.Count;
+            }
+        }
+
+        public bool IsReadOnly
+        {
+            get
+            {
+                return false;
+            }
+        }
+
+        public void Add(T item)
+        {
+            Writable.Add(item);
+        }
+
+        public void Clear()
+        {
+            Writable.Clear();
+        }
+
+        public bool Contains(T item)
+        {
+            foreach (var obj in Readable)
+            {
+                if (object.Equals(obj, item))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        public void CopyTo(T[] array, int arrayIndex)
+        {
+            foreach (var item in Readable)
+            {
+                array[arrayIndex++] = item;
+            }
+        }
+
+        public IEnumerator<T> GetEnumerator()
+        {
+            return Readable.GetEnumerator();
+        }
+
+        public int IndexOf(T item)
+        {
+            var i = 0;
+            foreach (var obj in Readable)
+            {
+                if (object.Equals(obj, item))
+                {
+                    return i;
+                }
+
+                i++;
+            }
+
+            return -1;
+        }
+
+        public void Insert(int index, T item)
+        {
+            Writable.Insert(index, item);
+        }
+
+        public bool Remove(T item)
+        {
+            return Writable.Remove(item);
+        }
+
+        public void RemoveAt(int index)
+        {
+            Writable.RemoveAt(index);
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+    }
+}