fixup! Fix race condition in RequestExecutionQueue

Author: sharwell

src/Features/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/RequestExecutionQueue.cs

@@ -233,7 +233,7 @@ private async Task ProcessQueueAsync()
                             await Task.WhenAll(concurrentlyExecutingTasksArray.Select(kvp => kvp.Key)).NoThrowAwaitable(captureContext: false);
                         }
 
-                        Debug.Assert(!concurrentlyExecutingTasks.Any(), "The tasks should have all been drained before continuing");
+                        Debug.Assert(!concurrentlyExecutingTasks.Any(t => !t.Key.IsCompleted), "The tasks should have all been drained before continuing");
                         // Mutating requests block other requests from starting to ensure an up to date snapshot is used.
                         // Since we're explicitly awaiting exceptions to mutating requests will bubble up here.
                         await WrapStartRequestTaskAsync(work.StartRequestAsync(context, cancellationToken), rethrowExceptions: true).ConfigureAwait(false);
@@ -254,21 +254,20 @@ private async Task ProcessQueueAsync()
                                 throw new InvalidOperationException($"unexpected null value for {nameof(currentWorkCts)}");
                             }
 
-                            // Only mark the task complete when it has removed itself from the executing task list.
-                            currentWorkTask = currentWorkTask.ContinueWith(t =>
+                            if (!concurrentlyExecutingTasks.TryAdd(currentWorkTask, currentWorkCts))
+                            {
+                                throw new InvalidOperationException($"unable to add {nameof(currentWorkTask)} into {nameof(concurrentlyExecutingTasks)}");
+                            }
+
+                            _ = currentWorkTask.ContinueWith(t =>
                             {
-                                if (!concurrentlyExecutingTasks.TryRemove(currentWorkTask, out var concurrentlyExecutingTaskCts))
+                                if (!concurrentlyExecutingTasks.TryRemove(t, out var concurrentlyExecutingTaskCts))
                                 {
                                     throw new InvalidOperationException($"unexpected failure to remove task from {nameof(concurrentlyExecutingTasks)}");
                                 }
 
                                 concurrentlyExecutingTaskCts.Dispose();
                             }, CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
-
-                            if (!concurrentlyExecutingTasks.TryAdd(currentWorkTask, currentWorkCts))
-                            {
-                                throw new InvalidOperationException($"unable to add {nameof(currentWorkTask)} into {nameof(concurrentlyExecutingTasks)}");
-                            }
                         }
                     }
                 }