Fix a race in the production of compilation events.

Fixes #9759

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1698,34 +1698,23 @@ private AliasSymbol CreateGlobalNamespaceAlias()
 
         private void CompleteTree(SyntaxTree tree)
         {
-            bool completedCompilationUnit = false;
-            bool completedCompilation = false;
-
             if (_lazyCompilationUnitCompletedTrees == null) Interlocked.CompareExchange(ref _lazyCompilationUnitCompletedTrees, new HashSet<SyntaxTree>(), null);
             lock (_lazyCompilationUnitCompletedTrees)
             {
                 if (_lazyCompilationUnitCompletedTrees.Add(tree))
                 {
-                    completedCompilationUnit = true;
+                    // signal the end of the compilation unit
+                    EventQueue.TryEnqueue(new CompilationUnitCompletedEvent(this, tree));
+
                     if (_lazyCompilationUnitCompletedTrees.Count == this.SyntaxTrees.Length)
                     {
-                        completedCompilation = true;
+                        // if that was the last tree, signal the end of compilation
+                        EventQueue.TryEnqueue(new CompilationCompletedEvent(this));
+                        EventQueue.PromiseNotToEnqueue();
+                        EventQueue.TryComplete();
                     }
                 }
             }
-
-            if (completedCompilationUnit)
-            {
-                EventQueue.TryEnqueue(new CompilationUnitCompletedEvent(this, tree));
-            }
-
-            if (completedCompilation)
-            {
-                // signal the end of compilation events
-                EventQueue.TryEnqueue(new CompilationCompletedEvent(this));
-                EventQueue.PromiseNotToEnqueue();
-                EventQueue.TryComplete();
-            }
         }
 
         internal void ReportUnusedImports(DiagnosticBag diagnostics, CancellationToken cancellationToken, SyntaxTree filterTree = null)

src/Compilers/VisualBasic/Portable/Compilation/VisualBasicCompilation.vb

@@ -1632,8 +1632,6 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End If
 
             Debug.Assert(AllSyntaxTrees.Contains(tree))
-            Dim completedCompilationUnit As Boolean = False
-            Dim completedCompilation As Boolean = False
 
             If _lazyCompilationUnitCompletedTrees Is Nothing Then
                 Interlocked.CompareExchange(_lazyCompilationUnitCompletedTrees, New HashSet(Of SyntaxTree)(), Nothing)
@@ -1641,22 +1639,17 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
             SyncLock _lazyCompilationUnitCompletedTrees
                 If _lazyCompilationUnitCompletedTrees.Add(tree) Then
-                    completedCompilationUnit = True
+                    ' signal the end of the compilation unit
+                    EventQueue.TryEnqueue(New CompilationUnitCompletedEvent(Me, tree))
+
                     If _lazyCompilationUnitCompletedTrees.Count = SyntaxTrees.Length Then
-                        completedCompilation = True
+                        ' if that was the last tree, signal the end of compilation
+                        EventQueue.TryEnqueue(New CompilationCompletedEvent(Me))
+                        EventQueue.PromiseNotToEnqueue()
+                        EventQueue.TryComplete()
                     End If
                 End If
             End SyncLock
-
-            If completedCompilationUnit Then
-                EventQueue.TryEnqueue(New CompilationUnitCompletedEvent(Me, tree))
-            End If
-
-            If completedCompilation Then
-                EventQueue.TryEnqueue(New CompilationCompletedEvent(Me))
-                EventQueue.PromiseNotToEnqueue()
-                EventQueue.TryComplete() ' signal the End Of compilation events
-            End If
         End Sub
 
         Friend Function ShouldAddEvent(symbol As Symbol) As Boolean