[engine] dispatch platform channel messages through event loop, except navigation on start. (flutter#55027)

This is a re-land of flutter#55006, except that we special case the navigation channel to immediately dispatch its message if the isolate is not yet running.

This preserves the existing behavior relied upon by several iOS add2app tests, as well as the still used embedder v1 - and potentially undicovered future embedders.

Author: Jonah Williams

runtime/runtime_controller.cc

@@ -100,7 +100,7 @@ RuntimeController::~RuntimeController() {
   }
 }
 
-bool RuntimeController::IsRootIsolateRunning() {
+bool RuntimeController::IsRootIsolateRunning() const {
   std::shared_ptr<DartIsolate> root_isolate = root_isolate_.lock();
   if (root_isolate) {
     return root_isolate->GetPhase() == DartIsolate::Phase::Running;

runtime/runtime_controller.h

@@ -452,7 +452,7 @@ class RuntimeController : public PlatformConfigurationClient,
   ///
   /// @return     True if root isolate running, False otherwise.
   ///
-  virtual bool IsRootIsolateRunning();
+  virtual bool IsRootIsolateRunning() const;
 
   //----------------------------------------------------------------------------
   /// @brief      Dispatch the specified platform message to running root

shell/common/engine_unittests.cc

@@ -144,7 +144,7 @@ class MockRuntimeController : public RuntimeController {
   MockRuntimeController(RuntimeDelegate& client,
                         const TaskRunners& p_task_runners)
       : RuntimeController(client, p_task_runners) {}
-  MOCK_METHOD(bool, IsRootIsolateRunning, (), (override));
+  MOCK_METHOD(bool, IsRootIsolateRunning, (), (override, const));
   MOCK_METHOD(bool,
               DispatchPlatformMessage,
               (std::unique_ptr<PlatformMessage>),

shell/common/shell.cc

@@ -1074,16 +1074,38 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
   }
 #endif  // FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG
 
-  // The static leak checker gets confused by the use of fml::MakeCopyable.
-  // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
-  fml::TaskRunner::RunNowOrPostTask(
-      task_runners_.GetUITaskRunner(),
-      fml::MakeCopyable([engine = engine_->GetWeakPtr(),
-                         message = std::move(message)]() mutable {
-        if (engine) {
-          engine->DispatchPlatformMessage(std::move(message));
-        }
-      }));
+  // If the root isolate is not yet running this may be the navigation
+  // channel initial route and must be dispatched immediately so that
+  // it can be set before isolate creation.
+  static constexpr char kNavigationChannel[] = "flutter/navigation";
+  if (!engine_->GetRuntimeController()->IsRootIsolateRunning() &&
+      message->channel() == kNavigationChannel) {
+    // The static leak checker gets confused by the use of fml::MakeCopyable.
+    // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
+    fml::TaskRunner::RunNowOrPostTask(
+        task_runners_.GetUITaskRunner(),
+        fml::MakeCopyable([engine = engine_->GetWeakPtr(),
+                           message = std::move(message)]() mutable {
+          if (engine) {
+            engine->DispatchPlatformMessage(std::move(message));
+          }
+        }));
+  } else {
+    // In all other cases, the message must be dispatched via a new task so
+    // that the completion of the platform channel response future is guaranteed
+    // to wake up the Dart event loop, even in cases where the platform and UI
+    // threads are the same.
+
+    // The static leak checker gets confused by the use of fml::MakeCopyable.
+    // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
+    task_runners_.GetUITaskRunner()->PostTask(
+        fml::MakeCopyable([engine = engine_->GetWeakPtr(),
+                           message = std::move(message)]() mutable {
+          if (engine) {
+            engine->DispatchPlatformMessage(std::move(message));
+          }
+        }));
+  }
 }
 
 // |PlatformView::Delegate|

shell/common/shell_unittests.cc

@@ -68,6 +68,33 @@ using ::testing::_;
 using ::testing::Return;
 
 namespace {
+
+std::unique_ptr<PlatformMessage> MakePlatformMessage(
+    const std::string& channel,
+    const std::map<std::string, std::string>& values,
+    const fml::RefPtr<PlatformMessageResponse>& response) {
+  rapidjson::Document document;
+  auto& allocator = document.GetAllocator();
+  document.SetObject();
+
+  for (const auto& pair : values) {
+    rapidjson::Value key(pair.first.c_str(), strlen(pair.first.c_str()),
+                         allocator);
+    rapidjson::Value value(pair.second.c_str(), strlen(pair.second.c_str()),
+                           allocator);
+    document.AddMember(key, value, allocator);
+  }
+
+  rapidjson::StringBuffer buffer;
+  rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
+  document.Accept(writer);
+  const uint8_t* data = reinterpret_cast<const uint8_t*>(buffer.GetString());
+
+  std::unique_ptr<PlatformMessage> message = std::make_unique<PlatformMessage>(
+      channel, fml::MallocMapping::Copy(data, buffer.GetSize()), response);
+  return message;
+}
+
 class MockPlatformViewDelegate : public PlatformView::Delegate {
   MOCK_METHOD(void,
               OnPlatformViewCreated,
@@ -4260,6 +4287,31 @@ TEST_F(ShellTest, PrintsErrorWhenPlatformMessageSentFromWrongThread) {
 #endif
 }
 
+TEST_F(ShellTest, NavigationMessageDispachedImmediately) {
+  Settings settings = CreateSettingsForFixture();
+  ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
+                         ThreadHost::Type::kPlatform);
+  auto task_runner = thread_host.platform_thread->GetTaskRunner();
+  TaskRunners task_runners("test", task_runner, task_runner, task_runner,
+                           task_runner);
+  auto shell = CreateShell(settings, task_runners);
+
+  auto latch = std::make_shared<fml::CountDownLatch>(1u);
+  task_runner->PostTask([&]() {
+    auto message = MakePlatformMessage(
+        "flutter/navigation",
+        {{"method", "setInitialRoute"}, {"args", "/testo"}}, nullptr);
+    SendPlatformMessage(shell.get(), std::move(message));
+    EXPECT_EQ(shell->GetEngine()->InitialRoute(), "/testo");
+
+    latch->CountDown();
+  });
+  latch->Wait();
+
+  DestroyShell(std::move(shell), task_runners);
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+}
+
 TEST_F(ShellTest, DiesIfSoftwareRenderingAndImpellerAreEnabledDeathTest) {
 #if defined(OS_FUCHSIA)
   GTEST_SKIP() << "Fuchsia";