Merge pull request #32577 from dotnet/merges/dev16.0-preview2-to-master

Merge dev16.0-preview2 to master

src/Workspaces/Core/Portable/Workspace/WorkspaceRegistration.cs

@@ -6,13 +6,14 @@ namespace Microsoft.CodeAnalysis
 {
     public sealed class WorkspaceRegistration
     {
-        private Workspace _registeredWorkspace;
+        private readonly object _gate = new object();
 
         internal WorkspaceRegistration()
         {
         }
 
-        public Workspace Workspace => _registeredWorkspace;
+        public Workspace Workspace { get; private set; }
+
         public event EventHandler WorkspaceChanged;
 
         internal void SetWorkspaceAndRaiseEvents(Workspace workspace)
@@ -23,12 +24,30 @@ internal void SetWorkspaceAndRaiseEvents(Workspace workspace)
 
         internal void SetWorkspace(Workspace workspace)
         {
-            _registeredWorkspace = workspace;
+            Workspace = workspace;
         }
 
         internal void RaiseEvents()
         {
-            WorkspaceChanged?.Invoke(this, EventArgs.Empty);
+            lock (_gate)
+            {
+                // this is a workaround for https://devdiv.visualstudio.com/DevDiv/_workitems/edit/744145
+                // for preview 2.
+                //
+                // it is a workaround since we are calling event handler under a lock to make sure
+                // we don't raise event concurrently
+                //
+                // we have this issue to track proper fix for preview 3 or later
+                // https://github.com/dotnet/roslyn/issues/32551
+                //
+                // issue we are working around is the fact this event can happen concurrently
+                // if RegisteryText happens and then UnregisterText happens in perfect timing, 
+                // RegisterText got slightly delayed since it is async event, and UnregisterText happens 
+                // at the same time since it is a synchronous event, and they my happens in 2 different threads, 
+                // cause this event to be raised concurrently.
+                // that can cause some listener to mess up its internal state like the issue linked above
+                WorkspaceChanged?.Invoke(this, EventArgs.Empty);
+            }
         }
     }
 }