Fix NullReferenceException during debugging

When opening a file during a debugging session, VS sets its buffer read-only when it is not editable. To do so, VS asks its document to the workspace but in some cases, the workspace returns nothing, causing NRE and VS Crash. Issue dotnet#1202 is tracking to fix this issue directly (by enforcing the workspace to load projects before EnC). In this PR, we defensively add a null check and log it to make a further investigation easier.

src/VisualStudio/Core/Def/Implementation/EditAndContinue/VsReadOnlyDocumentTracker.cs

@@ -4,6 +4,7 @@
 using System.Diagnostics;
 using Microsoft.CodeAnalysis.EditAndContinue;
 using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
+using Microsoft.CodeAnalysis.Internal.Log;
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.VisualStudio.Editor;
 using Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem;
@@ -20,7 +21,9 @@ internal sealed class VsReadOnlyDocumentTracker : ForegroundThreadAffinitizedObj
         private readonly Workspace _workspace;
         private readonly AbstractProject _vsProject;
 
-        private bool _isDisposed;        
+        private bool _isDisposed;
+
+        internal static readonly TraceLog log = new TraceLog(2048, "VsReadOnlyDocumentTracker");
 
         public VsReadOnlyDocumentTracker(IEditAndContinueWorkspaceService encService, IVsEditorAdaptersFactoryService adapters, AbstractProject vsProject)
             : base(assertIsForeground: true)
@@ -142,6 +145,12 @@ private void SetReadOnlyFlag(ITextBuffer buffer, bool value)
         private static ITextBuffer GetTextBuffer(Workspace workspace, DocumentId documentId)
         {
             var doc = workspace.CurrentSolution.GetDocument(documentId);
+            if (doc == null)
+            {
+                log.Write($"GetTextBuffer: document not found for '{documentId?.GetDebuggerDisplay()}'");
+                return null;
+            }
+
             SourceText text;
             if (!doc.TryGetText(out text))
             {

src/VisualStudio/Core/Test/EditAndContinue/VsReadOnlyDocumentTrackerTests.vb

@@ -1,6 +1,7 @@
 ' Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 Imports System.Collections.Immutable
+Imports System.IO
 Imports Microsoft.CodeAnalysis
 Imports Microsoft.CodeAnalysis.Diagnostics
 Imports Microsoft.CodeAnalysis.EditAndContinue
@@ -115,6 +116,61 @@ Namespace Microsoft.VisualStudio.LanguageServices.UnitTests.EditAndContinue
             Assert.Equal(Of UInteger)(0, mockVsBuffer._oldFlags) ' Editable
         End Sub
 
+        <WorkItem(1147868, "DevDiv")>
+        <Fact>
+        Public Sub InvalidDocumentTest1()
+            Dim diagnosticService As IDiagnosticAnalyzerService = New EditAndContinueTestHelper.TestDiagnosticAnalyzerService()
+            Dim encService As IEditAndContinueWorkspaceService = New EditAndContinueWorkspaceService(diagnosticService)
+            Dim workspace = EditAndContinueTestHelper.CreateTestWorkspace()
+            Dim currentSolution = workspace.CurrentSolution
+            Dim project = currentSolution.Projects(0)
+
+            Dim mockVsBuffer = New VsTextBufferMock()
+            Assert.Equal(Of UInteger)(0, mockVsBuffer._oldFlags)
+
+            Dim mockEditorAdaptersFactoryService = New VsEditorAdaptersFactoryServiceMock(mockVsBuffer)
+            Dim readOnlyDocumentTracker As VsReadOnlyDocumentTracker
+
+            ' start debugging & readOnlyDocumentTracker
+            encService.StartDebuggingSession(workspace.CurrentSolution)
+            readOnlyDocumentTracker = New VsReadOnlyDocumentTracker(encService, mockEditorAdaptersFactoryService, Nothing)
+
+            ' valid document
+            readOnlyDocumentTracker.SetReadOnly(project.DocumentIds.First(), False)
+            Assert.Equal(Of UInteger)(0, mockVsBuffer._oldFlags) ' Editable
+
+            ' invlid documentId
+            readOnlyDocumentTracker.SetReadOnly(Nothing, False) ' Check no NRE
+        End Sub
+
+        <WorkItem(1147868, "DevDiv")>
+        <Fact>
+        Public Sub InvalidDocumentTest2()
+            Dim diagnosticService As IDiagnosticAnalyzerService = New EditAndContinueTestHelper.TestDiagnosticAnalyzerService()
+            Dim encService As IEditAndContinueWorkspaceService = New EditAndContinueWorkspaceService(diagnosticService)
+            Dim workspace = EditAndContinueTestHelper.CreateTestWorkspace()
+            Dim currentSolution = workspace.CurrentSolution
+            Dim project = currentSolution.Projects(0)
+
+            Dim mockVsBuffer = New VsTextBufferMock()
+            Assert.Equal(Of UInteger)(0, mockVsBuffer._oldFlags)
+
+            Dim mockEditorAdaptersFactoryService = New VsEditorAdaptersFactoryServiceMock(mockVsBuffer)
+            Dim readOnlyDocumentTracker As VsReadOnlyDocumentTracker
+
+            ' start debugging & readOnlyDocumentTracker
+            encService.StartDebuggingSession(workspace.CurrentSolution)
+            readOnlyDocumentTracker = New VsReadOnlyDocumentTracker(encService, mockEditorAdaptersFactoryService, Nothing)
+
+            ' valid document
+            readOnlyDocumentTracker.SetReadOnly(project.DocumentIds.First(), False)
+            Assert.Equal(Of UInteger)(0, mockVsBuffer._oldFlags) ' Editable
+
+            ' the given project does not contain this document
+            Dim newDocumentId = New DocumentId(New ProjectId(New Guid(), "TestProject"), New Guid(), "TestDoc")
+            readOnlyDocumentTracker.SetReadOnly(newDocumentId, False) ' Check no NRE
+        End Sub
+
 #Region "Helper Methods"
 
         Public Class VsTextBufferMock