Engine::Run returns enum: success, failure, or isolate already running (flutter/engine#6324)

* If isolate is already running, return true

* Use shell::Engine::RunStatus as result of Engine::Run

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

@@ -103,18 +103,23 @@ bool Engine::Restart(RunConfiguration configuration) {
   delegate_.OnPreEngineRestart();
   runtime_controller_ = runtime_controller_->Clone();
   UpdateAssetManager(nullptr);
-  return Run(std::move(configuration));
+  return Run(std::move(configuration)) == Engine::RunStatus::Success;
 }
 
-bool Engine::Run(RunConfiguration configuration) {
+Engine::RunStatus Engine::Run(RunConfiguration configuration) {
   if (!configuration.IsValid()) {
     FML_LOG(ERROR) << "Engine run configuration was invalid.";
-    return false;
+    return RunStatus::Failure;
   }
 
-  if (!PrepareAndLaunchIsolate(std::move(configuration))) {
+  auto isolate_launch_status =
+      PrepareAndLaunchIsolate(std::move(configuration));
+  if (isolate_launch_status == Engine::RunStatus::Failure) {
     FML_LOG(ERROR) << "Engine not prepare and launch isolate.";
-    return false;
+    return isolate_launch_status;
+  } else if (isolate_launch_status ==
+             Engine::RunStatus::FailureAlreadyRunning) {
+    return isolate_launch_status;
   }
 
   std::shared_ptr<blink::DartIsolate> isolate =
@@ -136,10 +141,12 @@ bool Engine::Run(RunConfiguration configuration) {
     }
   }
 
-  return isolate_running;
+  return isolate_running ? Engine::RunStatus::Success
+                         : Engine::RunStatus::Failure;
 }
 
-bool Engine::PrepareAndLaunchIsolate(RunConfiguration configuration) {
+shell::Engine::RunStatus Engine::PrepareAndLaunchIsolate(
+    RunConfiguration configuration) {
   TRACE_EVENT0("flutter", "Engine::PrepareAndLaunchIsolate");
 
   UpdateAssetManager(configuration.GetAssetManager());
@@ -150,28 +157,35 @@ bool Engine::PrepareAndLaunchIsolate(RunConfiguration configuration) {
       runtime_controller_->GetRootIsolate().lock();
 
   if (!isolate) {
-    return false;
+    return RunStatus::Failure;
+  }
+
+  // This can happen on iOS after a plugin shows a native window and returns to
+  // the Flutter ViewController.
+  if (isolate->GetPhase() == blink::DartIsolate::Phase::Running) {
+    FML_DLOG(WARNING) << "Isolate was already running!";
+    return RunStatus::FailureAlreadyRunning;
   }
 
   if (!isolate_configuration->PrepareIsolate(*isolate)) {
     FML_LOG(ERROR) << "Could not prepare to run the isolate.";
-    return false;
+    return RunStatus::Failure;
   }
 
   if (configuration.GetEntrypointLibrary().empty()) {
     if (!isolate->Run(configuration.GetEntrypoint())) {
       FML_LOG(ERROR) << "Could not run the isolate.";
-      return false;
+      return RunStatus::Failure;
     }
   } else {
     if (!isolate->RunFromLibrary(configuration.GetEntrypointLibrary(),
                                  configuration.GetEntrypoint())) {
       FML_LOG(ERROR) << "Could not run the isolate.";
-      return false;
+      return RunStatus::Failure;
     }
   }
 
-  return true;
+  return RunStatus::Success;
 }
 
 void Engine::BeginFrame(fml::TimePoint frame_time) {

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

@@ -29,6 +29,13 @@ namespace shell {
 
 class Engine final : public blink::RuntimeDelegate {
  public:
+  // Used by Engine::Run
+  enum class RunStatus {
+    Success,                // Successful call to Run()
+    FailureAlreadyRunning,  // Isolate was already running; may not be considered a failure by callers
+    Failure,                // Isolate could not be started or other unspecified failure
+  };
+
   class Delegate {
    public:
     virtual void OnEngineUpdateSemantics(
@@ -56,7 +63,7 @@ class Engine final : public blink::RuntimeDelegate {
   fml::WeakPtr<Engine> GetWeakPtr() const;
 
   FML_WARN_UNUSED_RESULT
-  bool Run(RunConfiguration configuration);
+  RunStatus Run(RunConfiguration configuration);
 
   // Used to "cold reload" a running application where the shell (along with the
   // platform view and its rasterizer bindings) remains the same but the root
@@ -149,7 +156,7 @@ class Engine final : public blink::RuntimeDelegate {
 
   bool GetAssetAsBuffer(const std::string& name, std::vector<uint8_t>* data);
 
-  bool PrepareAndLaunchIsolate(RunConfiguration configuration);
+  RunStatus PrepareAndLaunchIsolate(RunConfiguration configuration);
 
   FML_DISALLOW_COPY_AND_ASSIGN(Engine);
 };

engine/src/flutter/shell/platform/android/android_shell_holder.cc

@@ -162,7 +162,7 @@ void AndroidShellHolder::Launch(RunConfiguration config) {
                          config = std::move(config)     //
   ]() mutable {
         FML_LOG(INFO) << "Attempting to launch engine configuration...";
-        if (!engine || !engine->Run(std::move(config))) {
+        if (!engine || engine->Run(std::move(config)) == shell::Engine::RunStatus::Failure) {
           FML_LOG(ERROR) << "Could not launch engine in configuration.";
         } else {
           FML_LOG(INFO) << "Isolate for engine configuration successfully "

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterHeadlessDartRunner.mm

@@ -101,7 +101,7 @@ static std::string CreateShellLabel() {
       fml::MakeCopyable([engine = _shell->GetEngine(), config = std::move(config)]() mutable {
         BOOL success = NO;
         FML_LOG(INFO) << "Attempting to launch background engine configuration...";
-        if (!engine || !engine->Run(std::move(config))) {
+        if (!engine || engine->Run(std::move(config)) == shell::Engine::RunStatus::Failure) {
           FML_LOG(ERROR) << "Could not launch engine with configuration.";
         } else {
           FML_LOG(INFO) << "Background Isolate successfully started and run.";

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -458,7 +458,7 @@
   ]() mutable {
         if (engine) {
           auto result = engine->Run(std::move(config));
-          if (!result) {
+          if (result == shell::Engine::RunStatus::Failure) {
             FML_LOG(ERROR) << "Could not launch engine with configuration.";
           }
         }

engine/src/flutter/shell/platform/embedder/embedder_engine.cc

@@ -61,7 +61,7 @@ bool EmbedderEngine::Run(RunConfiguration run_configuration) {
   ]() mutable {
         if (engine) {
           auto result = engine->Run(std::move(config));
-          if (!result) {
+          if (result == shell::Engine::RunStatus::Failure) {
             FML_LOG(ERROR) << "Could not launch the engine with configuration.";
           }
         }

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

@@ -171,7 +171,7 @@ int RunTester(const blink::Settings& settings, bool run_forever) {
         fml::MessageLoop::GetCurrent().AddTaskObserver(
             reinterpret_cast<intptr_t>(&completion_observer),
             [&completion_observer]() { completion_observer.DidProcessTask(); });
-        if (engine->Run(std::move(config))) {
+        if (engine->Run(std::move(config)) != shell::Engine::RunStatus::Failure) {
           engine_did_run = true;
 
           blink::ViewportMetrics metrics;