Do not call Dart_NotifyIdle when in Dart_PerformanceMode_Latency (flutter/engine#37499)

engine/src/flutter/lib/ui/window/platform_configuration.cc

@@ -371,9 +371,17 @@ void PlatformConfigurationNativeApi::SetIsolateDebugName(
   UIDartState::Current()->SetDebugName(name);
 }
 
+Dart_PerformanceMode PlatformConfigurationNativeApi::current_performace_mode_ =
+    Dart_PerformanceMode_Default;
+
+Dart_PerformanceMode PlatformConfigurationNativeApi::GetDartPerformanceMode() {
+  return current_performace_mode_;
+}
+
 int PlatformConfigurationNativeApi::RequestDartPerformanceMode(int mode) {
   UIDartState::ThrowIfUIOperationsProhibited();
-  return Dart_SetPerformanceMode(static_cast<Dart_PerformanceMode>(mode));
+  current_performace_mode_ = static_cast<Dart_PerformanceMode>(mode);
+  return Dart_SetPerformanceMode(current_performace_mode_);
 }
 
 Dart_Handle PlatformConfigurationNativeApi::GetPersistentIsolateData() {

engine/src/flutter/lib/ui/window/platform_configuration.h

@@ -515,9 +515,18 @@ class PlatformConfigurationNativeApi {
   ///
   static int RequestDartPerformanceMode(int mode);
 
+  //--------------------------------------------------------------------------
+  /// @brief      Returns the current performance mode of the Dart VM. Defaults
+  /// to `Dart_PerformanceMode_Default` if no prior requests to change the
+  /// performance mode have been made.
+  static Dart_PerformanceMode GetDartPerformanceMode();
+
   static int64_t GetRootIsolateToken();
 
   static void RegisterBackgroundIsolate(int64_t root_isolate_token);
+
+ private:
+  static Dart_PerformanceMode current_performace_mode_;
 };
 
 }  // namespace flutter

engine/src/flutter/runtime/runtime_controller.cc

@@ -219,6 +219,12 @@ bool RuntimeController::NotifyIdle(fml::TimePoint deadline) {
 
   tonic::DartState::Scope scope(root_isolate);
 
+  Dart_PerformanceMode performance_mode =
+      PlatformConfigurationNativeApi::GetDartPerformanceMode();
+  if (performance_mode == Dart_PerformanceMode::Dart_PerformanceMode_Latency) {
+    return false;
+  }
+
   Dart_NotifyIdle(deadline.ToEpochDelta().ToMicroseconds());
 
   // Idle notifications being in isolate scope are part of the contract.

engine/src/flutter/shell/common/fixtures/shell_test.dart

@@ -204,6 +204,14 @@ void canAccessIsolateLaunchData() {
   );
 }
 
+@pragma('vm:entry-point')
+void performanceModeImpactsNotifyIdle() {
+  notifyNativeBool(false);
+  PlatformDispatcher.instance.requestDartPerformanceMode(DartPerformanceMode.latency);
+  notifyNativeBool(true);
+  PlatformDispatcher.instance.requestDartPerformanceMode(DartPerformanceMode.balanced);
+}
+
 @pragma('vm:external-name', 'NotifyMessage')
 external void notifyMessage(String string);
 

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

@@ -3891,6 +3891,47 @@ TEST_F(ShellTest, PluginUtilitiesCallbackHandleErrorHandling) {
   DestroyShell(std::move(shell));
 }
 
+TEST_F(ShellTest, NotifyIdleNotCalledInLatencyMode) {
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+  Settings settings = CreateSettingsForFixture();
+  ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
+                         ThreadHost::Type::Platform | ThreadHost::UI |
+                             ThreadHost::IO | ThreadHost::RASTER);
+  auto platform_task_runner = thread_host.platform_thread->GetTaskRunner();
+  TaskRunners task_runners("test", thread_host.platform_thread->GetTaskRunner(),
+                           thread_host.raster_thread->GetTaskRunner(),
+                           thread_host.ui_thread->GetTaskRunner(),
+                           thread_host.io_thread->GetTaskRunner());
+  auto shell = CreateShell(settings, task_runners);
+  ASSERT_TRUE(DartVMRef::IsInstanceRunning());
+  ASSERT_TRUE(ValidateShell(shell.get()));
+
+  // we start off in balanced mode, where we expect idle notifications to
+  // succeed. After the first `NotifyNativeBool` we expect to be in latency
+  // mode, where we expect idle notifications to fail.
+  fml::CountDownLatch latch(2);
+  AddNativeCallback(
+      "NotifyNativeBool", CREATE_NATIVE_ENTRY([&](auto args) {
+        Dart_Handle exception = nullptr;
+        bool is_in_latency_mode =
+            tonic::DartConverter<bool>::FromArguments(args, 0, exception);
+        auto runtime_controller = const_cast<RuntimeController*>(
+            shell->GetEngine()->GetRuntimeController());
+        bool success = runtime_controller->NotifyIdle(fml::TimePoint::Now());
+        EXPECT_EQ(success, !is_in_latency_mode);
+        latch.CountDown();
+      }));
+
+  auto configuration = RunConfiguration::InferFromSettings(settings);
+  configuration.SetEntrypoint("performanceModeImpactsNotifyIdle");
+  RunEngine(shell.get(), std::move(configuration));
+
+  latch.Wait();
+
+  DestroyShell(std::move(shell), task_runners);
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+}
+
 }  // namespace testing
 }  // namespace flutter