Pool `TagHelperExecutionContext`s within `TagHelperScopeManager`.

- Currently the `TagHelperScopeManager` creates a new `TagHelperExecutionContext` per `TagHelper` on a given page. With this change the max number of `TagHelperExecutionContext`s per page is the number of nested levels that exist.
- Added two tests validating that specific pieces of `TagHelperExecutionContext` are updated as expected.

#674

src/Microsoft.AspNetCore.Razor.Runtime/Runtime/TagHelpers/TagHelperExecutionContext.cs

@@ -14,14 +14,14 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
     /// </summary>
     public class TagHelperExecutionContext
     {
-        private readonly string _tagName;
-        private readonly string _uniqueId;
-        private readonly TagMode _tagMode;
         private readonly List<ITagHelper> _tagHelpers;
-        private readonly Func<Task> _executeChildContentAsync;
         private readonly Action<HtmlEncoder> _startTagHelperWritingScope;
         private readonly Func<TagHelperContent> _endTagHelperWritingScope;
         private TagHelperContent _childContent;
+        private string _tagName;
+        private string _uniqueId;
+        private TagMode _tagMode;
+        private Func<Task> _executeChildContentAsync;
         private Dictionary<HtmlEncoder, TagHelperContent> _perEncoderChildContent;
         private TagHelperAttributeList _htmlAttributes;
         private TagHelperAttributeList _allAttributes;
@@ -63,26 +63,6 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
             Action<HtmlEncoder> startTagHelperWritingScope,
             Func<TagHelperContent> endTagHelperWritingScope)
         {
-            if (tagName == null)
-            {
-                throw new ArgumentNullException(nameof(tagName));
-            }
-
-            if (items == null)
-            {
-                throw new ArgumentNullException(nameof(items));
-            }
-
-            if (uniqueId == null)
-            {
-                throw new ArgumentNullException(nameof(uniqueId));
-            }
-
-            if (executeChildContentAsync == null)
-            {
-                throw new ArgumentNullException(nameof(executeChildContentAsync));
-            }
-
             if (startTagHelperWritingScope == null)
             {
                 throw new ArgumentNullException(nameof(startTagHelperWritingScope));
@@ -94,14 +74,11 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
             }
 
             _tagHelpers = new List<ITagHelper>();
-            _executeChildContentAsync = executeChildContentAsync;
+
+            Reinitialize(tagName, tagMode, items, uniqueId, executeChildContentAsync);
+
             _startTagHelperWritingScope = startTagHelperWritingScope;
             _endTagHelperWritingScope = endTagHelperWritingScope;
-
-            _tagMode = tagMode;
-            _tagName = tagName;
-            Items = items;
-            _uniqueId = uniqueId;
         }
 
         /// <summary>
@@ -118,7 +95,7 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
         /// <summary>
         /// Gets the collection of items used to communicate with other <see cref="ITagHelper"/>s.
         /// </summary>
-        public IDictionary<object, object> Items { get; }
+        public IDictionary<object, object> Items { get; private set; }
 
         /// <summary>
         /// <see cref="ITagHelper"/>s that should be run.
@@ -214,6 +191,53 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
             _allAttributes.Add(name, value);
         }
 
+        /// <summary>
+        /// Clears the <see cref="TagHelperExecutionContext"/> and updates its state with the provided values.
+        /// </summary>
+        /// <param name="tagName">The tag name to use.</param>
+        /// <param name="tagMode">The <see cref="TagMode"/> to use.</param>
+        /// <param name="items">The <see cref="IDictionary{Object, Object}"/> to use.</param>
+        /// <param name="uniqueId">The unique id to use.</param>
+        /// <param name="executeChildContentAsync">The <see cref="Func{Task}"/> to use.</param>
+        public void Reinitialize(
+            string tagName,
+            TagMode tagMode,
+            IDictionary<object, object> items,
+            string uniqueId,
+            Func<Task> executeChildContentAsync)
+        {
+            if (tagName == null)
+            {
+                throw new ArgumentNullException(nameof(tagName));
+            }
+
+            if (items == null)
+            {
+                throw new ArgumentNullException(nameof(items));
+            }
+
+            if (uniqueId == null)
+            {
+                throw new ArgumentNullException(nameof(uniqueId));
+            }
+
+            if (executeChildContentAsync == null)
+            {
+                throw new ArgumentNullException(nameof(executeChildContentAsync));
+            }
+
+            _tagName = tagName;
+            _tagMode = tagMode;
+            Items = items;
+            _uniqueId = uniqueId;
+            _executeChildContentAsync = executeChildContentAsync;
+            _tagHelpers.Clear();
+            _perEncoderChildContent?.Clear();
+            _htmlAttributes = null;
+            _allAttributes = null;
+            _childContent = null;
+        }
+
         // Internal for testing.
         internal async Task<TagHelperContent> GetChildContentAsync(bool useCachedResult, HtmlEncoder encoder)
         {

src/Microsoft.AspNetCore.Razor.Runtime/Runtime/TagHelpers/TagHelperScopeManager.cs

@@ -15,14 +15,14 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
     /// </summary>
     public class TagHelperScopeManager
     {
-        private readonly Stack<TagHelperExecutionContext> _executionScopes;
+        private readonly ExecutionContextPool _executionContextPool;
 
         /// <summary>
         /// Instantiates a new <see cref="TagHelperScopeManager"/>.
         /// </summary>
         public TagHelperScopeManager()
         {
-            _executionScopes = new Stack<TagHelperExecutionContext>();
+            _executionContextPool = new ExecutionContextPool();
         }
 
         /// <summary>
@@ -72,12 +72,13 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
             }
 
             IDictionary<object, object> items;
+            var parentExecutionContext = _executionContextPool.Current;
 
             // If we're not wrapped by another TagHelper, then there will not be a parentExecutionContext.
-            if (_executionScopes.Count > 0)
+            if (parentExecutionContext != null)
             {
                 items = new CopyOnWriteDictionary<object, object>(
-                    _executionScopes.Peek().Items,
+                    parentExecutionContext.Items,
                     comparer: EqualityComparer<object>.Default);
             }
             else
@@ -85,7 +86,7 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
                 items = new Dictionary<object, object>();
             }
 
-            var executionContext = new TagHelperExecutionContext(
+            var executionContext = _executionContextPool.Rent(
                 tagName,
                 tagMode,
                 items,
@@ -94,8 +95,6 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
                 startTagHelperWritingScope,
                 endTagHelperWritingScope);
 
-            _executionScopes.Push(executionContext);
-
             return executionContext;
         }
 
@@ -106,7 +105,7 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
         /// <c>null</c> otherwise.</returns>
         public TagHelperExecutionContext End()
         {
-            if (_executionScopes.Count == 0)
+            if (_executionContextPool.Current == null)
             {
                 throw new InvalidOperationException(
                     Resources.FormatScopeManager_EndCannotBeCalledWithoutACallToBegin(
@@ -115,14 +114,61 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
                         nameof(TagHelperScopeManager)));
             }
 
-            _executionScopes.Pop();
+            _executionContextPool.ReturnCurrent();
 
-            if (_executionScopes.Count != 0)
+            var parentExecutionContext = _executionContextPool.Current;
+
+            return parentExecutionContext;
+        }
+
+        private class ExecutionContextPool
+        {
+            private readonly List<TagHelperExecutionContext> _executionContexts;
+            private int _nextIndex;
+
+            public ExecutionContextPool()
             {
-                return _executionScopes.Peek();
+                _executionContexts = new List<TagHelperExecutionContext>();
             }
 
-            return null;
+            public TagHelperExecutionContext Current => _nextIndex > 0 ? _executionContexts[_nextIndex - 1] : null;
+
+            public TagHelperExecutionContext Rent(
+                string tagName,
+                TagMode tagMode,
+                IDictionary<object, object> items,
+                string uniqueId,
+                Func<Task> executeChildContentAsync,
+                Action<HtmlEncoder> startTagHelperWritingScope,
+                Func<TagHelperContent> endTagHelperWritingScope)
+            {
+                TagHelperExecutionContext tagHelperExecutionContext;
+
+                if (_nextIndex == _executionContexts.Count)
+                {
+                    tagHelperExecutionContext = new TagHelperExecutionContext(
+                        tagName,
+                        tagMode,
+                        items,
+                        uniqueId,
+                        executeChildContentAsync,
+                        startTagHelperWritingScope,
+                        endTagHelperWritingScope);
+
+                    _executionContexts.Add(tagHelperExecutionContext);
+                }
+                else
+                {
+                    tagHelperExecutionContext = _executionContexts[_nextIndex];
+                    tagHelperExecutionContext.Reinitialize(tagName, tagMode, items, uniqueId, executeChildContentAsync);
+                }
+
+                _nextIndex++;
+
+                return tagHelperExecutionContext;
+            }
+
+            public void ReturnCurrent() => _nextIndex--;
         }
     }
 }

test/Microsoft.AspNetCore.Razor.Runtime.Test/Runtime/TagHelpers/TagHelperExecutionContextTest.cs

@@ -1,6 +1,7 @@
 // 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;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text.Encodings.Web;
@@ -13,6 +14,106 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
 {
     public class TagHelperExecutionContextTest
     {
+        [Fact]
+        public async Task ExecutionContext_Reinitialize_UpdatesTagHelperOutputAsExpected()
+        {
+            // Arrange
+            var tagName = "div";
+            var tagMode = TagMode.StartTagOnly;
+            var callCount = 0;
+            Func<Task> executeChildContentAsync = () =>
+            {
+                callCount++;
+                return Task.FromResult(true);
+            };
+            Action<HtmlEncoder> startTagHelperWritingScope = _ => { };
+            Func<TagHelperContent> endTagHelperWritingScope = () => null;
+            var executionContext = new TagHelperExecutionContext(
+                tagName,
+                tagMode,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: executeChildContentAsync,
+                startTagHelperWritingScope: startTagHelperWritingScope,
+                endTagHelperWritingScope: endTagHelperWritingScope);
+            var updatedTagName = "p";
+            var updatedTagMode = TagMode.SelfClosing;
+            var updatedCallCount = 0;
+            Func<Task> updatedExecuteChildContentAsync = () =>
+            {
+                updatedCallCount++;
+                return Task.FromResult(true);
+            };
+            executionContext.AddMinimizedHtmlAttribute("something");
+
+            // Act - 1
+            executionContext.Reinitialize(
+                updatedTagName,
+                updatedTagMode,
+                items: new Dictionary<object, object>(),
+                uniqueId: string.Empty,
+                executeChildContentAsync: updatedExecuteChildContentAsync);
+            executionContext.AddMinimizedHtmlAttribute("Another attribute");
+
+            // Assert - 1
+            var output = executionContext.CreateTagHelperOutput();
+            Assert.Equal(updatedTagName, output.TagName);
+            Assert.Equal(updatedTagMode, output.TagMode);
+            var attribute = Assert.Single(output.Attributes);
+            Assert.Equal("Another attribute", attribute.Name);
+
+            // Act - 2
+            await output.GetChildContentAsync();
+
+            // Assert - 2
+            Assert.Equal(callCount, 0);
+            Assert.Equal(updatedCallCount, 1);
+        }
+
+        [Fact]
+        public void ExecutionContext_Reinitialize_UpdatesTagHelperContextAsExpected()
+        {
+            // Arrange
+            var tagName = "div";
+            var tagMode = TagMode.StartTagOnly;
+            var items = new Dictionary<object, object>();
+            var uniqueId = "some unique id";
+            var callCount = 0;
+            Func<Task> executeChildContentAsync = () =>
+            {
+                callCount++;
+                return Task.FromResult(true);
+            };
+            Action<HtmlEncoder> startWritingScope = _ => { };
+            Func<TagHelperContent> endWritingScope = () => null;
+            var executionContext = new TagHelperExecutionContext(
+                tagName,
+                tagMode,
+                items,
+                uniqueId,
+                executeChildContentAsync,
+                startWritingScope,
+                endWritingScope);
+            var updatedItems = new Dictionary<object, object>();
+            var updatedUniqueId = "another unique id";
+            executionContext.AddMinimizedHtmlAttribute("something");
+
+            // Act
+            executionContext.Reinitialize(
+                tagName,
+                tagMode,
+                updatedItems,
+                updatedUniqueId,
+                executeChildContentAsync);
+            executionContext.AddMinimizedHtmlAttribute("Another attribute");
+
+            // Assert
+            var context = executionContext.CreateTagHelperContext();
+            var attribute = Assert.Single(context.AllAttributes);
+            Assert.Equal(attribute.Name, "Another attribute");
+            Assert.Equal(updatedUniqueId, context.UniqueId);
+            Assert.Same(updatedItems, context.Items);
+        }
 
         [Theory]
         [InlineData(TagMode.SelfClosing)]