Make SyntaxTree mutations not leak.

- Prior to this when the partial parser would successfully parse a change we'd mutate the returned syntax tree so any data inquired about the tree would then be wrong. We now isolate mutations to copied versions of the syntax tree.
- Added copy tests to ensure that we were appropriately copying all the various syntax node types.

#1793

src/Microsoft.AspNetCore.Razor.Language/Legacy/Block.cs

@@ -214,6 +214,20 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
             visitor.VisitBlock(this);
         }
 
+        public override SyntaxTreeNode Clone()
+        {
+            var blockBuilder = new BlockBuilder(this);
+
+            blockBuilder.Children.Clear();
+            for (var i = 0; i < Children.Count; i++)
+            {
+                var clonedChild = Children[i].Clone();
+                blockBuilder.Children.Add(clonedChild);
+            }
+
+            return blockBuilder.Build();
+        }
+
         private class EquivalenceComparer : IEqualityComparer<SyntaxTreeNode>
         {
             public static readonly EquivalenceComparer Default = new EquivalenceComparer();

src/Microsoft.AspNetCore.Razor.Language/Legacy/Span.cs

@@ -151,5 +151,11 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         {
             visitor.VisitSpan(this);
         }
+
+        public override SyntaxTreeNode Clone()
+        {
+            var spanBuilder = new SpanBuilder(this);
+            return spanBuilder.Build();
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/Legacy/SyntaxTreeNode.cs

@@ -43,5 +43,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         public abstract int GetEquivalenceHash();
 
         public abstract void Accept(ParserVisitor visitor);
+
+        public abstract SyntaxTreeNode Clone();
     }
 }

src/Microsoft.AspNetCore.Razor.Language/Legacy/TagHelperBlock.cs

@@ -159,6 +159,41 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
             return null;
         }
 
+        public override SyntaxTreeNode Clone()
+        {
+            var tagHelperBlockBuilder = new TagHelperBlockBuilder(this);
+
+            tagHelperBlockBuilder.Children.Clear();
+            for (var i = 0; i < Children.Count; i++)
+            {
+                var clonedChild = Children[i].Clone();
+                tagHelperBlockBuilder.Children.Add(clonedChild);
+            }
+
+            tagHelperBlockBuilder.Attributes.Clear();
+            for (var i = 0; i < Attributes.Count; i++)
+            {
+                var existingAttribute = Attributes[i];
+                var clonedValue = existingAttribute.Value != null ? existingAttribute.Value.Clone() : null;
+                tagHelperBlockBuilder.Attributes.Add(
+                    new TagHelperAttributeNode(existingAttribute.Name, clonedValue, existingAttribute.AttributeStructure));
+            }
+
+            if (SourceStartTag != null)
+            {
+                var clonedStartTag = (Block)SourceStartTag.Clone();
+                tagHelperBlockBuilder.SourceStartTag = clonedStartTag;
+            }
+
+            if (SourceEndTag != null)
+            {
+                var clonedEndTag = (Block)SourceEndTag.Clone();
+                tagHelperBlockBuilder.SourceEndTag = clonedEndTag;
+            }
+
+            return tagHelperBlockBuilder.Build();
+        }
+
         /// <inheritdoc />
         public override string ToString()
         {

src/Microsoft.AspNetCore.Razor.Language/Legacy/TagHelperBlockBuilder.cs

@@ -18,9 +18,12 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         public TagHelperBlockBuilder(TagHelperBlock original)
             : base(original)
         {
-            TagName = original.TagName;
+            SourceStartTag = original.SourceStartTag;
+            SourceEndTag = original.SourceEndTag;
+            TagMode = original.TagMode;
             BindingResult = original.Binding;
             Attributes = new List<TagHelperAttributeNode>(original.Attributes);
+            TagName = original.TagName;
         }
 
         /// <summary>

src/Microsoft.VisualStudio.Editor.Razor/DefaultVisualStudioRazorParser.cs

@@ -25,6 +25,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
         internal Timer _idleTimer;
         internal BackgroundParser _parser;
         internal ChangeReference _latestChangeReference;
+        internal RazorSyntaxTreePartialParser _partialParser;
 
         private readonly object IdleLock = new object();
         private readonly VisualStudioCompletionBroker _completionBroker;
@@ -32,7 +33,6 @@ namespace Microsoft.VisualStudio.Editor.Razor
         private readonly ForegroundDispatcher _dispatcher;
         private readonly RazorTemplateEngineFactoryService _templateEngineFactory;
         private readonly ErrorReporter _errorReporter;
-        private RazorSyntaxTreePartialParser _partialParser;
         private RazorTemplateEngine _templateEngine;
         private RazorCodeDocument _codeDocument;
         private ITextSnapshot _snapshot;

src/Microsoft.VisualStudio.Editor.Razor/RazorSyntaxTreePartialParser.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 Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Span = Microsoft.AspNetCore.Razor.Language.Legacy.Span;
@@ -9,15 +10,24 @@ namespace Microsoft.VisualStudio.Editor.Razor
 {
     internal class RazorSyntaxTreePartialParser
     {
-        private readonly RazorSyntaxTree _syntaxTree;
         private Span _lastChangeOwner;
         private bool _lastResultProvisional;
 
         public RazorSyntaxTreePartialParser(RazorSyntaxTree syntaxTree)
         {
-            _syntaxTree = syntaxTree;
+            if (syntaxTree == null)
+            {
+                throw new ArgumentNullException(nameof(syntaxTree));
+            }
+
+            // We mutate the existing syntax tree so we need to clone the one passed in so our mutations don't
+            // impact external state.
+            SyntaxTreeRoot = (Block)syntaxTree.Root.Clone();
         }
 
+        // Internal for testing
+        internal Block SyntaxTreeRoot { get; }
+
         public PartialParseResultInternal Parse(SourceChange change)
         {
             var result = GetPartialParseResult(change);
@@ -46,7 +56,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
             }
 
             // Locate the span responsible for this change
-            _lastChangeOwner = _syntaxTree.Root.LocateOwner(change);
+            _lastChangeOwner = SyntaxTreeRoot.LocateOwner(change);
 
             if (_lastResultProvisional)
             {

test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/BlockTest.cs

@@ -8,6 +8,26 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
 {
     public class BlockTest
     {
+        [Fact]
+        public void Clone_ClonesBlock()
+        {
+            // Arrange
+            var blockBuilder = new BlockBuilder()
+            {
+                ChunkGenerator = new DynamicAttributeBlockChunkGenerator(new LocationTagged<string>("class=\"", SourceLocation.Zero), 0, 0, 0),
+                Type = BlockKindInternal.Expression,
+            };
+            blockBuilder.Children.Add(new SpanBuilder(new SourceLocation(1, 2, 3)).Build());
+            var block = blockBuilder.Build();
+
+            // Act
+            var copy = (Block)block.Clone();
+
+            // Assert
+            ParserTestBase.EvaluateParseTree(copy, block);
+            Assert.NotSame(block, copy);
+        }
+
         [Fact]
         public void ConstructorWithBlockBuilderSetsParent()
         {

test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/SpanTest.cs

@@ -0,0 +1,31 @@
+// 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 Xunit;
+
+namespace Microsoft.AspNetCore.Razor.Language.Legacy
+{
+    public class SpanTest
+    {
+        [Fact]
+        public void Clone_ClonesSpan()
+        {
+            // Arrange
+            var spanBuilder = new SpanBuilder(new SourceLocation(1, 2, 3))
+            {
+                EditHandler = new SpanEditHandler(CSharpLanguageCharacteristics.Instance.TokenizeString),
+                Kind = SpanKindInternal.Transition,
+                ChunkGenerator = new ExpressionChunkGenerator(),
+            };
+            spanBuilder.Accept(new CSharpSymbol("@", CSharpSymbolType.Transition));
+            var span = spanBuilder.Build();
+
+            // Act
+            var copy = (Span)span.Clone();
+
+            // Assert
+            Assert.Equal(span, copy);
+            Assert.NotSame(span, copy);
+        }
+    }
+}

test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/TagHelperBlockTest.cs

@@ -1,12 +1,114 @@
 // 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.Generic;
+using System.Linq;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language.Legacy
 {
     public class TagHelperBlockTest
     {
+        [Fact]
+        public void Clone_ClonesTagHelperChildren()
+        {
+            // Arrange
+            var tagHelper = new TagHelperBlockBuilder(
+                "p",
+                TagMode.StartTagAndEndTag,
+                attributes: new List<TagHelperAttributeNode>(),
+                children: new[]
+            {
+                new SpanBuilder(SourceLocation.Zero).Build(),
+                new SpanBuilder(new SourceLocation(0, 1, 2)).Build(),
+            }).Build();
+
+            // Act
+            var copy = (TagHelperBlock)tagHelper.Clone();
+
+            // Assert
+            ParserTestBase.EvaluateParseTree(copy, tagHelper);
+            Assert.Collection(
+                copy.Children,
+                child => Assert.NotSame(tagHelper.Children[0], child),
+                child => Assert.NotSame(tagHelper.Children[1], child));
+        }
+
+        [Fact]
+        public void Clone_ClonesTagHelperAttributes()
+        {
+            // Arrange
+            var tagHelper = (TagHelperBlock)new TagHelperBlockBuilder(
+                "p",
+                TagMode.StartTagAndEndTag,
+                attributes: new List<TagHelperAttributeNode>()
+                {
+                    new TagHelperAttributeNode("class", new SpanBuilder(SourceLocation.Zero).Build(), AttributeStructure.NoQuotes),
+                    new TagHelperAttributeNode("checked", new SpanBuilder(SourceLocation.Undefined).Build(), AttributeStructure.NoQuotes)
+                },
+                children: Enumerable.Empty<SyntaxTreeNode>()).Build();
+
+            // Act
+            var copy = (TagHelperBlock)tagHelper.Clone();
+
+            // Assert
+            ParserTestBase.EvaluateParseTree(copy, tagHelper);
+            Assert.Collection(
+                copy.Attributes,
+                attribute => Assert.NotSame(tagHelper.Attributes[0], attribute),
+                attribute => Assert.NotSame(tagHelper.Attributes[1], attribute));
+        }
+
+        [Fact]
+        public void Clone_ClonesTagHelperSourceStartTag()
+        {
+            // Arrange
+            var tagHelper = (TagHelperBlock)new TagHelperBlockBuilder(
+                "p",
+                TagMode.StartTagAndEndTag,
+                attributes: new List<TagHelperAttributeNode>(),
+                children: Enumerable.Empty<SyntaxTreeNode>())
+            {
+                SourceStartTag = new BlockBuilder()
+                {
+                    Type = BlockKindInternal.Comment,
+                    ChunkGenerator = new RazorCommentChunkGenerator()
+                }.Build()
+            }.Build();
+
+            // Act
+            var copy = (TagHelperBlock)tagHelper.Clone();
+
+            // Assert
+            ParserTestBase.EvaluateParseTree(copy, tagHelper);
+            Assert.NotSame(tagHelper.SourceStartTag, copy.SourceStartTag);
+        }
+
+        [Fact]
+        public void Clone_ClonesTagHelperSourceEndTag()
+        {
+            // Arrange
+            var tagHelper = (TagHelperBlock)new TagHelperBlockBuilder(
+                "p",
+                TagMode.StartTagAndEndTag,
+                attributes: new List<TagHelperAttributeNode>(),
+                children: Enumerable.Empty<SyntaxTreeNode>())
+            {
+                SourceEndTag = new BlockBuilder()
+                {
+                    Type = BlockKindInternal.Comment,
+                    ChunkGenerator = new RazorCommentChunkGenerator()
+                }.Build()
+            }.Build();
+
+            // Act
+            var copy = (TagHelperBlock)tagHelper.Clone();
+
+            // Assert
+            ParserTestBase.EvaluateParseTree(copy, tagHelper);
+            Assert.NotSame(tagHelper.SourceEndTag, copy.SourceEndTag);
+        }
+
         [Fact]
         public void FlattenFlattensSelfClosingTagHelpers()
         {

test/Microsoft.VisualStudio.Editor.Razor.Test/DefaultVisualStudioRazorParserIntegrationTest.cs

@@ -97,7 +97,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 {
                     manager.ApplyEdit(testEdit);
                     Assert.Equal(1, manager.ParseCount);
-                    ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root, new MarkupBlock(
+                    ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot, new MarkupBlock(
                         factory.EmptyHtml(),
                         new StatementBlock(
                             factory.CodeTransition(),
@@ -156,7 +156,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 {
                     manager.ApplyEdit(testEdit);
                     Assert.Equal(1, manager.ParseCount);
-                    ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root, new MarkupBlock(
+                    ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot, new MarkupBlock(
                         factory.EmptyHtml(),
                         new StatementBlock(
                             factory.CodeTransition(),
@@ -203,7 +203,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                     manager.ApplyEdit(testEdit);
                     Assert.Equal(1, manager.ParseCount);
 
-                    ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root, new MarkupBlock(
+                    ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot, new MarkupBlock(
                         factory.Markup("foo "),
                         new ExpressionBlock(
                             factory.CodeTransition(),
@@ -249,7 +249,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                     manager.ApplyEdit(testEdit);
                     Assert.Equal(1, manager.ParseCount);
 
-                    ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root, new MarkupBlock(
+                    ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot, new MarkupBlock(
                         factory.Markup("foo "),
                         new ExpressionBlock(
                             factory.CodeTransition(),
@@ -301,7 +301,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                     applyEdit();
                     Assert.Equal(1, manager.ParseCount);
 
-                    ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root, new MarkupBlock(
+                    ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot, new MarkupBlock(
                         factory.Markup("foo "),
                         new ExpressionBlock(
                             factory.CodeTransition(),
@@ -369,7 +369,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
                 // Assert
                 Assert.Equal(2, manager.ParseCount);
-                ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root,
+                ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot,
                     new MarkupBlock(
                         factory.Markup("foo "),
                         new ExpressionBlock(
@@ -406,7 +406,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
                 // Assert
                 Assert.Equal(1, manager.ParseCount);
-                ParserTestBase.EvaluateParseTree(manager.CurrentSyntaxTree.Root,
+                ParserTestBase.EvaluateParseTree(manager.PartialParsingSyntaxTreeRoot,
                     new MarkupBlock(
                         factory.Markup("foo "),
                         new ExpressionBlock(
@@ -642,6 +642,8 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
             public RazorSyntaxTree CurrentSyntaxTree { get; private set; }
 
+            public Block PartialParsingSyntaxTreeRoot => _parser._partialParser.SyntaxTreeRoot;
+
             public async Task InitializeWithDocumentAsync(ITextSnapshot snapshot)
             {
                 var old = new StringTextSnapshot(string.Empty);

test/Microsoft.VisualStudio.Editor.Razor.Test/DefaultVisualStudioRazorParserTest.cs

@@ -117,10 +117,12 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 var latestChange = new SourceChange(0, 0, string.Empty);
                 var latestSnapshot = documentTracker.TextBuffer.CurrentSnapshot;
                 parser._latestChangeReference = new DefaultVisualStudioRazorParser.ChangeReference(latestChange, latestSnapshot);
+                var codeDocument = TestRazorCodeDocument.CreateEmpty();
+                codeDocument.SetSyntaxTree(RazorSyntaxTree.Parse(TestRazorSourceDocument.Create()));
                 var args = new DocumentStructureChangedEventArgs(
                     latestChange,
                     latestSnapshot,
-                    TestRazorCodeDocument.CreateEmpty());
+                    codeDocument);
 
                 // Act
                 parser.OnDocumentStructureChanged(args);

test/Microsoft.VisualStudio.Editor.Razor.Test/RazorSyntaxTreePartialParserTest.cs

@@ -3,12 +3,14 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.AspNetCore.Mvc.Razor.Extensions;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Microsoft.VisualStudio.Test;
 using Microsoft.VisualStudio.Text;
 using Xunit;
+using Span = Microsoft.AspNetCore.Razor.Language.Legacy.Span;
 
 namespace Microsoft.VisualStudio.Editor.Razor
 {
@@ -566,7 +568,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
             var result = parser.Parse(edit.Change);
             Assert.Equal(PartialParseResultInternal.Accepted | additionalFlags, result);
-            ParserTestBase.EvaluateParseTree(expectedTree, syntaxTree.Root);
+            ParserTestBase.EvaluateParseTree(parser.SyntaxTreeRoot, expectedTree);
         }
 
         private static TestEdit CreateInsertionChange(string initialText, int insertionLocation, string insertionText)