Made the Visual Studio parser smarter about overlapping reparse requests.

- Prior to this the parser would think that a non-latest reparse request was the latest because the only way we would check to see if a change reference was "latest" would be to do an equality check on the snapshot and `SourceChange`; the issue here was that `SourceChange` was null but the snapshots were the same. Problems could arise with this due to project context changes.
- Added tests to validate the new reparse behavior.
- Renamed `Edit` in the `BackgroundParser` to `ChangeReference` also refactored all the "edit" text in `BackgroundParser` to be `ChangerReference` like.
- Added a new event args to be the DTO between the internal and external parser implementations. This is how we could pas additional information in order to determine "latest" change reference.

#2336

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

@@ -30,7 +30,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
         /// <summary>
         /// Fired on the main thread.
         /// </summary>
-        public event EventHandler<DocumentStructureChangedEventArgs> ResultsReady;
+        public event EventHandler<BackgroundParserResultsReadyEventArgs> ResultsReady;
 
         public bool IsIdle
         {
@@ -47,10 +47,11 @@ namespace Microsoft.VisualStudio.Editor.Razor
             _main.Cancel();
         }
 
-        public void QueueChange(SourceChange change, ITextSnapshot snapshot)
+        public ChangeReference QueueChange(SourceChange change, ITextSnapshot snapshot)
         {
-            var edit = new Edit(change, snapshot);
+            var changeReference = new ChangeReference(change, snapshot);
-            _main.QueueChange(edit);
+            _main.QueueChange(changeReference);
+            return changeReference;
         }
 
         public void Dispose()
@@ -63,7 +64,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
             return _main.Lock();
         }
 
-        protected virtual void OnResultsReady(DocumentStructureChangedEventArgs args)
+        protected virtual void OnResultsReady(BackgroundParserResultsReadyEventArgs args)
         {
             using (SynchronizeMainThreadState())
             {
@@ -115,7 +116,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
             private string _fileName;
             private readonly object _stateLock = new object();
-            private IList<Edit> _changes = new List<Edit>();
+            private IList<ChangeReference> _changes = new List<ChangeReference>();
 
             public MainThreadState(string fileName)
             {
@@ -124,7 +125,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 SetThreadId(Thread.CurrentThread.ManagedThreadId);
             }
 
-            public event EventHandler<DocumentStructureChangedEventArgs> ResultsReady;
+            public event EventHandler<BackgroundParserResultsReadyEventArgs> ResultsReady;
 
             public CancellationToken CancelToken
             {
@@ -154,7 +155,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 return new DisposableAction(() => Monitor.Exit(_stateLock));
             }
 
-            public void QueueChange(Edit edit)
+            public void QueueChange(ChangeReference change)
             {
                 // Any thread can queue a change.
 
@@ -166,7 +167,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
                         _currentParcelCancelSource.Cancel();
                     }
 
-                    _changes.Add(edit);
+                    _changes.Add(change);
                     _hasParcel.Set();
                 }
             }
@@ -182,12 +183,12 @@ namespace Microsoft.VisualStudio.Editor.Razor
                     _currentParcelCancelSource = new CancellationTokenSource();
 
                     var changes = _changes;
-                    _changes = new List<Edit>();
+                    _changes = new List<ChangeReference>();
                     return new WorkParcel(changes, _currentParcelCancelSource.Token);
                 }
             }
 
-            public void ReturnParcel(DocumentStructureChangedEventArgs args)
+            public void ReturnParcel(BackgroundParserResultsReadyEventArgs args)
             {
                 lock (_stateLock)
                 {
@@ -242,7 +243,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
             private CancellationToken _shutdownToken;
             private RazorProjectEngine _projectEngine;
             private RazorSyntaxTree _currentSyntaxTree;
-            private IList<Edit> _previouslyDiscarded = new List<Edit>();
+            private IList<ChangeReference> _previouslyDiscarded = new List<ChangeReference>();
 
             public BackgroundThread(MainThreadState main, RazorProjectEngine projectEngine, string filePath, string projectDirectory)
             {
@@ -274,30 +275,30 @@ namespace Microsoft.VisualStudio.Editor.Razor
                     {
                         // Grab the parcel of work to do
                         var parcel = _main.GetParcel();
-                        if (parcel.Edits.Any())
+                        if (parcel.Changes.Any())
                         {
                             try
                             {
-                                DocumentStructureChangedEventArgs args = null;
+                                BackgroundParserResultsReadyEventArgs args = null;
                                 using (var linkedCancel = CancellationTokenSource.CreateLinkedTokenSource(_shutdownToken, parcel.CancelToken))
                                 {
                                     if (!linkedCancel.IsCancellationRequested)
                                     {
                                         // Collect ALL changes
-                                        List<Edit> allEdits;
+                                        List<ChangeReference> allChanges;
 
                                         if (_previouslyDiscarded != null)
                                         {
-                                            allEdits = Enumerable.Concat(_previouslyDiscarded, parcel.Edits).ToList();
+                                            allChanges = Enumerable.Concat(_previouslyDiscarded, parcel.Changes).ToList();
                                         }
                                         else
                                         {
-                                            allEdits = parcel.Edits.ToList();
+                                            allChanges = parcel.Changes.ToList();
                                         }
 
-                                        var finalEdit = allEdits.Last();
+                                        var finalChange = allChanges.Last();
 
-                                        var results = ParseChange(finalEdit.Snapshot, linkedCancel.Token);
+                                        var results = ParseChange(finalChange.Snapshot, linkedCancel.Token);
 
                                         if (results != null && !linkedCancel.IsCancellationRequested)
                                         {
@@ -307,15 +308,12 @@ namespace Microsoft.VisualStudio.Editor.Razor
                                             _currentSyntaxTree = results.GetSyntaxTree();
 
                                             // Build Arguments
-                                            args = new DocumentStructureChangedEventArgs(
+                                            args = new BackgroundParserResultsReadyEventArgs(finalChange, results);
-                                                finalEdit.Change,
-                                                finalEdit.Snapshot,
-                                                results);
                                         }
                                         else
                                         {
                                             // Parse completed but we were cancelled in the mean time. Add these to the discarded changes set
-                                            _previouslyDiscarded = allEdits;
+                                            _previouslyDiscarded = allChanges;
                                         }
                                     }
                                 }
@@ -378,20 +376,20 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
         private class WorkParcel
         {
-            public WorkParcel(IList<Edit> changes, CancellationToken cancelToken)
+            public WorkParcel(IList<ChangeReference> changes, CancellationToken cancelToken)
             {
-                Edits = changes;
+                Changes = changes;
                 CancelToken = cancelToken;
             }
 
             public CancellationToken CancelToken { get; }
 
-            public IList<Edit> Edits { get; }
+            public IList<ChangeReference> Changes { get; }
         }
 
-        private class Edit
+        internal class ChangeReference
         {
-            public Edit(SourceChange change, ITextSnapshot snapshot)
+            public ChangeReference(SourceChange change, ITextSnapshot snapshot)
             {
                 Change = change;
                 Snapshot = snapshot;
@@ -399,7 +397,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
 
             public SourceChange Change { get; }
 
-            public ITextSnapshot Snapshot { get; set; }
+            public ITextSnapshot Snapshot { get; }
         }
     }
 }

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

@@ -0,0 +1,22 @@
+// 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 static Microsoft.VisualStudio.Editor.Razor.BackgroundParser;
+
+namespace Microsoft.VisualStudio.Editor.Razor
+{
+    internal class BackgroundParserResultsReadyEventArgs : EventArgs
+    {
+        public BackgroundParserResultsReadyEventArgs(ChangeReference edit, RazorCodeDocument codeDocument)
+        {
+            ChangeReference = edit;
+            CodeDocument = codeDocument;
+        }
+
+        public ChangeReference ChangeReference { get; }
+
+        public RazorCodeDocument CodeDocument { get; }
+    }
+}

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

@@ -12,6 +12,7 @@ using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Microsoft.CodeAnalysis.Razor;
 using Microsoft.CodeAnalysis.Razor.Editor;
 using Microsoft.VisualStudio.Text;
+using static Microsoft.VisualStudio.Editor.Razor.BackgroundParser;
 using ITextBuffer = Microsoft.VisualStudio.Text.ITextBuffer;
 using Timer = System.Threading.Timer;
 
@@ -317,8 +318,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
         {
             _dispatcher.AssertForegroundThread();
 
-            _latestChangeReference = new ChangeReference(change, snapshot);
+            _latestChangeReference = _parser.QueueChange(change, snapshot);
-            _parser.QueueChange(change, snapshot);
         }
 
         private void OnNotifyForegroundIdle()
@@ -357,7 +357,7 @@ namespace Microsoft.VisualStudio.Editor.Razor
             }
         }
 
-        private void OnResultsReady(object sender, DocumentStructureChangedEventArgs args)
+        private void OnResultsReady(object sender, BackgroundParserResultsReadyEventArgs args)
         {
             _dispatcher.AssertBackgroundThread();
 
@@ -375,21 +375,25 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 return;
             }
 
-            var args = (DocumentStructureChangedEventArgs)state;
+            var backgroundParserArgs = (BackgroundParserResultsReadyEventArgs)state;
             if (_latestChangeReference == null || // extra hardening
-                !_latestChangeReference.IsAssociatedWith(args) ||
+                _latestChangeReference != backgroundParserArgs.ChangeReference ||
-                args.Snapshot != TextBuffer.CurrentSnapshot)
+                backgroundParserArgs.ChangeReference.Snapshot != TextBuffer.CurrentSnapshot)
             {
                 // In the middle of parsing a newer change or about to parse a newer change.
                 return;
             }
 
             _latestChangeReference = null;
-            _codeDocument = args.CodeDocument;
+            _codeDocument = backgroundParserArgs.CodeDocument;
-            _snapshot = args.Snapshot;
+            _snapshot = backgroundParserArgs.ChangeReference.Snapshot;
             _partialParser = new RazorSyntaxTreePartialParser(CodeDocument.GetSyntaxTree());
 
-            DocumentStructureChanged?.Invoke(this, args);
+            var documentStructureChangedArgs = new DocumentStructureChangedEventArgs(
+                backgroundParserArgs.ChangeReference.Change, 
+                backgroundParserArgs.ChangeReference.Snapshot, 
+                backgroundParserArgs.CodeDocument);
+            DocumentStructureChanged?.Invoke(this, documentStructureChangedArgs);
         }
 
         private void ConfigureProjectEngine(RazorProjectEngineBuilder builder)
@@ -432,25 +436,5 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 return _tagHelpers;
             }
         }
-
-        // Internal for testing
-        internal class ChangeReference
-        {
-            public ChangeReference(SourceChange change, ITextSnapshot snapshot)
-            {
-                Change = change;
-                Snapshot = snapshot;
-            }
-
-            public SourceChange Change { get; }
-
-            public ITextSnapshot Snapshot { get; }
-
-            public bool IsAssociatedWith(DocumentStructureChangedEventArgs other)
-            {
-                return Change == other.SourceChange &&
-                    Snapshot == other.Snapshot;
-            }
-        }
     }
 }

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

@@ -108,10 +108,9 @@ namespace Microsoft.VisualStudio.Editor.Razor
             {
                 var called = false;
                 parser.DocumentStructureChanged += (sender, e) => called = true;
-                parser._latestChangeReference = new DefaultVisualStudioRazorParser.ChangeReference(null, new StringTextSnapshot(string.Empty));
+                parser._latestChangeReference = new BackgroundParser.ChangeReference(null, new StringTextSnapshot(string.Empty));
-                var args = new DocumentStructureChangedEventArgs(
+                var args = new BackgroundParserResultsReadyEventArgs(
-                    new SourceChange(0, 0, string.Empty),
+                    new BackgroundParser.ChangeReference(new SourceChange(0, 0, string.Empty), new StringTextSnapshot(string.Empty)),
-                    new StringTextSnapshot(string.Empty),
                     TestRazorCodeDocument.CreateEmpty());
 
                 // Act
@@ -138,12 +137,11 @@ namespace Microsoft.VisualStudio.Editor.Razor
                 parser.DocumentStructureChanged += (sender, e) => called = true;
                 var latestChange = new SourceChange(0, 0, string.Empty);
                 var latestSnapshot = documentTracker.TextBuffer.CurrentSnapshot;
-                parser._latestChangeReference = new DefaultVisualStudioRazorParser.ChangeReference(latestChange, latestSnapshot);
+                parser._latestChangeReference = new BackgroundParser.ChangeReference(latestChange, latestSnapshot);
                 var codeDocument = TestRazorCodeDocument.CreateEmpty();
                 codeDocument.SetSyntaxTree(RazorSyntaxTree.Parse(TestRazorSourceDocument.Create()));
-                var args = new DocumentStructureChangedEventArgs(
+                var args = new BackgroundParserResultsReadyEventArgs(
-                    latestChange,
+                    parser._latestChangeReference,
-                    latestSnapshot,
                     codeDocument);
 
                 // Act
@@ -154,6 +152,46 @@ namespace Microsoft.VisualStudio.Editor.Razor
             }
         }
 
+        [ForegroundFact]
+        public void OnDocumentStructureChanged_FiresForOnlyLatestTextBufferReparseEdit()
+        {
+            // Arrange
+            var documentTracker = CreateDocumentTracker();
+            using (var parser = new DefaultVisualStudioRazorParser(
+                Dispatcher,
+                documentTracker,
+                ProjectEngineFactory,
+                new DefaultErrorReporter(),
+                Mock.Of<VisualStudioCompletionBroker>()))
+            {
+                var called = false;
+                parser.DocumentStructureChanged += (sender, e) => called = true;
+                var latestSnapshot = documentTracker.TextBuffer.CurrentSnapshot;
+                parser._latestChangeReference = new BackgroundParser.ChangeReference(null, latestSnapshot);
+                var codeDocument = TestRazorCodeDocument.CreateEmpty();
+                codeDocument.SetSyntaxTree(RazorSyntaxTree.Parse(TestRazorSourceDocument.Create()));
+                var badArgs = new BackgroundParserResultsReadyEventArgs(
+                    // This is a different reparse edit, shouldn't be fired for this call
+                    new BackgroundParser.ChangeReference(null, latestSnapshot),
+                    codeDocument);
+                var goodArgs = new BackgroundParserResultsReadyEventArgs(
+                    parser._latestChangeReference,
+                    codeDocument);
+
+                // Act - 1
+                parser.OnDocumentStructureChanged(badArgs);
+
+                // Assert - 1
+                Assert.False(called);
+
+                // Act - 2
+                parser.OnDocumentStructureChanged(goodArgs);
+
+                // Assert - 2
+                Assert.True(called);
+            }
+        }
+
         [ForegroundFact]
         public void StartIdleTimer_DoesNotRestartTimerWhenAlreadyRunning()
         {