Do not stop flutter_tester if microtasks are still pending (flutter/engine#56432)

Flutter_tester has a task observer that checks whether the test's Dart code has finished execution.  If Dart no longer has live ports but does have pending microtasks, then flutter_tester should continue running and force a drain of the microtask queue.

Fixes https://github.com/flutter/flutter/issues/158129

engine/src/flutter/lib/ui/ui_dart_state.cc

@@ -169,6 +169,10 @@ void UIDartState::FlushMicrotasksNow() {
   microtask_queue_.RunMicrotasks();
 }
 
+bool UIDartState::HasPendingMicrotasks() {
+  return microtask_queue_.HasMicrotasks();
+}
+
 void UIDartState::AddOrRemoveTaskObserver(bool add) {
   auto task_runner = context_.task_runners.GetUITaskRunner();
   if (!task_runner) {

engine/src/flutter/lib/ui/ui_dart_state.h

@@ -130,6 +130,8 @@ class UIDartState : public tonic::DartState {
 
   void FlushMicrotasksNow();
 
+  bool HasPendingMicrotasks();
+
   fml::WeakPtr<IOManager> GetIOManager() const;
 
   fml::RefPtr<flutter::SkiaUnrefQueue> GetSkiaUnrefQueue() const;

engine/src/flutter/runtime/runtime_controller.cc

@@ -511,6 +511,14 @@ bool RuntimeController::HasLivePorts() {
   return Dart_HasLivePorts();
 }
 
+bool RuntimeController::HasPendingMicrotasks() {
+  std::shared_ptr<DartIsolate> root_isolate = root_isolate_.lock();
+  if (!root_isolate) {
+    return false;
+  }
+  return root_isolate->HasPendingMicrotasks();
+}
+
 tonic::DartErrorHandleType RuntimeController::GetLastError() {
   std::shared_ptr<DartIsolate> root_isolate = root_isolate_.lock();
   return root_isolate ? root_isolate->GetLastError() : tonic::kNoError;

engine/src/flutter/runtime/runtime_controller.h

@@ -523,6 +523,15 @@ class RuntimeController : public PlatformConfigurationClient,
   ///
   bool HasLivePorts();
 
+  //----------------------------------------------------------------------------
+  /// @brief      Returns if the root isolate has any pending microtasks.
+  ///
+  /// @return     True if there are microtasks that have been queued but not
+  ///             run, False otherwise. Return False if the root isolate is not
+  ///             running as well.
+  ///
+  bool HasPendingMicrotasks();
+
   //----------------------------------------------------------------------------
   /// @brief      Get the last error encountered by the microtask queue.
   ///

engine/src/flutter/shell/common/engine.cc

@@ -293,6 +293,10 @@ bool Engine::UIIsolateHasLivePorts() {
   return runtime_controller_->HasLivePorts();
 }
 
+bool Engine::UIIsolateHasPendingMicrotasks() {
+  return runtime_controller_->HasPendingMicrotasks();
+}
+
 tonic::DartErrorHandleType Engine::GetUIIsolateLastError() {
   return runtime_controller_->GetLastError();
 }

engine/src/flutter/shell/common/engine.h

@@ -682,6 +682,15 @@ class Engine final : public RuntimeDelegate, PointerDataDispatcher::Delegate {
   ///
   bool UIIsolateHasLivePorts();
 
+  /// @brief      Another signal of liveness is the presence of microtasks that
+  ///             have been queued by the application but have not yet been
+  ///             executed.  Embedders may want to check for pending microtasks
+  ///             and ensure that the microtask queue has been drained before
+  ///             the embedder terminates.
+  ///
+  /// @return     Check if the root isolate has any pending microtasks.
+  bool UIIsolateHasPendingMicrotasks();
+
   //----------------------------------------------------------------------------
   /// @brief      Errors that are unhandled on the Dart message loop are kept
   ///             for further inspection till the next unhandled error comes

engine/src/flutter/shell/common/shell.cc

@@ -710,6 +710,17 @@ bool Shell::EngineHasLivePorts() const {
   return weak_engine_->UIIsolateHasLivePorts();
 }
 
+bool Shell::EngineHasPendingMicrotasks() const {
+  FML_DCHECK(is_set_up_);
+  FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());
+
+  if (!weak_engine_) {
+    return false;
+  }
+
+  return weak_engine_->UIIsolateHasPendingMicrotasks();
+}
+
 bool Shell::IsSetup() const {
   return is_set_up_;
 }

engine/src/flutter/shell/common/shell.h

@@ -358,6 +358,17 @@ class Shell final : public PlatformView::Delegate,
   ///
   bool EngineHasLivePorts() const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Used by embedders to check if the Engine is running and has
+  ///             any microtasks that have been queued but have not yet run.
+  ///             The Flutter tester uses this as a signal that a test is still
+  ///             running.
+  ///
+  /// @return     Returns if the shell has an engine and the engine has pending
+  ///             microtasks.
+  ///
+  bool EngineHasPendingMicrotasks() const;
+
   //----------------------------------------------------------------------------
   /// @brief     Accessor for the disable GPU SyncSwitch.
   // |Rasterizer::Delegate|

engine/src/flutter/shell/testing/tester_main.cc

@@ -267,9 +267,11 @@ class ScriptCompletionTaskObserver {
  public:
   ScriptCompletionTaskObserver(Shell& shell,
                                fml::RefPtr<fml::TaskRunner> main_task_runner,
+                               fml::RefPtr<fml::TaskRunner> ui_task_runner,
                                bool run_forever)
       : shell_(shell),
         main_task_runner_(std::move(main_task_runner)),
+        ui_task_runner_(std::move(ui_task_runner)),
         run_forever_(run_forever) {}
 
   int GetExitCodeForLastError() const {
@@ -283,6 +285,12 @@ class ScriptCompletionTaskObserver {
       // just yet.
       return;
     }
+    if (shell_.EngineHasPendingMicrotasks()) {
+      // Post an empty task to force a run of the engine task observer that
+      // drains the microtask queue.
+      ui_task_runner_->PostTask([] {});
+      return;
+    }
 
     if (run_forever_) {
       // We need this script to run forever. We have already recorded the last
@@ -302,6 +310,7 @@ class ScriptCompletionTaskObserver {
  private:
   Shell& shell_;
   fml::RefPtr<fml::TaskRunner> main_task_runner_;
+  fml::RefPtr<fml::TaskRunner> ui_task_runner_;
   bool run_forever_ = false;
   std::optional<DartErrorCode> last_error_;
   bool has_terminated_ = false;
@@ -456,6 +465,7 @@ int RunTester(const flutter::Settings& settings,
       *shell,  // a valid shell
       fml::MessageLoop::GetCurrent()
           .GetTaskRunner(),  // the message loop to terminate
+      ui_task_runner,        // runner for Dart microtasks
       run_forever            // should the exit be ignored
   );