Switch the MacOS Desktop embedder to using a thread configuration where the platform and render task runners are the same. (flutter/engine#13300)

Also creates a new test harness for the desktop embedder framework target and adds a test that launches a headless engine in this new thread configuration.

Fixes https://github.com/flutter/flutter/issues/17579

engine/src/flutter/BUILD.gn

@@ -58,8 +58,10 @@ group("flutter") {
   # If on the host, compile all unittests targets.
   if (current_toolchain == host_toolchain) {
     if (is_mac) {
-      public_deps +=
-          [ "$flutter_root/shell/platform/darwin:flutter_channels_unittests" ]
+      public_deps += [
+        "$flutter_root/shell/platform/darwin:flutter_channels_unittests",
+        "$flutter_root/shell/platform/darwin/macos:flutter_desktop_darwin_unittests",
+      ]
     }
 
     public_deps += [

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -824,6 +824,7 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDe
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject_Internal.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterEngineUnittests.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterExternalTextureGL.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterExternalTextureGL.mm
@@ -835,6 +836,7 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterView.
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterView.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/fixtures/flutter_desktop_test.dart
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/module.modulemap
 FILE: ../../../flutter/shell/platform/embedder/assets/EmbedderInfo.plist
 FILE: ../../../flutter/shell/platform/embedder/assets/embedder.modulemap

engine/src/flutter/shell/platform/darwin/macos/BUILD.gn

@@ -8,6 +8,7 @@ import("//build/config/mac/mac_sdk.gni")
 import("$flutter_root/common/config.gni")
 import("$flutter_root/shell/platform/darwin/common/framework_shared.gni")
 import("$flutter_root/shell/platform/glfw/config.gni")
+import("$flutter_root/testing/testing.gni")
 
 group("macos") {
   deps = [
@@ -41,11 +42,9 @@ _flutter_framework_headers = [
 _flutter_framework_headers_copy_dir =
     "$_flutter_framework_dir/Versions/A/Headers"
 
-shared_library("create_flutter_framework_dylib") {
+source_set("flutter_framework_source") {
   visibility = [ ":*" ]
 
-  output_name = "$_flutter_framework_name"
-
   sources = [
     "framework/Source/FlutterAppDelegate.mm",
     "framework/Source/FlutterDartProject.mm",
@@ -83,6 +82,43 @@ shared_library("create_flutter_framework_dylib") {
   ]
 }
 
+shared_library("flutter_framework_dylib") {
+  visibility = [ ":*" ]
+
+  output_name = "$_flutter_framework_name"
+
+  deps = [
+    ":flutter_framework_source",
+  ]
+}
+
+test_fixtures("flutter_desktop_darwin_fixtures") {
+  dart_main = "framework/Source/fixtures/flutter_desktop_test.dart"
+
+  fixtures = [ "//third_party/icu/common/icudtl.dat" ]
+}
+
+executable("flutter_desktop_darwin_unittests") {
+  testonly = true
+
+  sources = [
+    "framework/Source/FlutterEngineUnittests.mm",
+  ]
+
+  cflags_objcc = [ "-fobjc-arc" ]
+
+  deps = [
+    ":flutter_desktop_darwin_fixtures",
+    ":flutter_framework_source",
+    "$flutter_root/shell/platform/darwin/common:framework_shared",
+    "$flutter_root/shell/platform/embedder:embedder_with_symbol_prefix",
+    "$flutter_root/testing",
+    "$flutter_root/testing:dart",
+    "$flutter_root/testing:skia",
+    "$flutter_root/testing:testing_lib",
+  ]
+}
+
 action("copy_dylib_and_update_framework_install_name") {
   visibility = [ ":*" ]
   script = "$flutter_root/sky/tools/change_install_name.py"
@@ -108,7 +144,7 @@ action("copy_dylib_and_update_framework_install_name") {
   ]
 
   deps = [
-    ":create_flutter_framework_dylib",
+    ":flutter_framework_dylib",
   ]
 }
 

engine/src/flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h

@@ -72,6 +72,13 @@ FLUTTER_EXPORT
  */
 @property(nonatomic, nonnull, readonly) id<FlutterBinaryMessenger> binaryMessenger;
 
+/**
+ * Shuts the Flutter engine if it is running. The FlutterEngine instance must always be shutdown
+ * before it may be collected. Not shutting down the FlutterEngine instance before releasing it will
+ * result in the leak of that engine instance.
+ */
+- (void)shutDownEngine;
+
 @end
 
 #endif  // FLUTTER_FLUTTERENGINE_H_

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm

@@ -12,6 +12,8 @@ static NSString* const kAppBundleIdentifier = @"io.flutter.flutter.app";
 
 @implementation FlutterDartProject {
   NSBundle* _dartBundle;
+  NSString* _assetsPath;
+  NSString* _ICUDataPath;
 }
 
 - (instancetype)init {
@@ -26,7 +28,19 @@ static NSString* const kAppBundleIdentifier = @"io.flutter.flutter.app";
   return self;
 }
 
+- (instancetype)initWithAssetsPath:(NSString*)assets ICUDataPath:(NSString*)icuPath {
+  self = [super init];
+  NSAssert(self, @"Super init cannot be nil");
+  _assetsPath = assets;
+  _ICUDataPath = icuPath;
+  return self;
+}
+
 - (NSString*)assetsPath {
+  if (_assetsPath) {
+    return _assetsPath;
+  }
+
   // If there's no App.framework, fall back to checking the main bundle for assets.
   NSBundle* assetBundle = _dartBundle ?: [NSBundle mainBundle];
   NSString* flutterAssetsName = [assetBundle objectForInfoDictionaryKey:@"FLTAssetsPath"];
@@ -41,6 +55,10 @@ static NSString* const kAppBundleIdentifier = @"io.flutter.flutter.app";
 }
 
 - (NSString*)ICUDataPath {
+  if (_ICUDataPath) {
+    return _ICUDataPath;
+  }
+
   NSString* path = [[NSBundle bundleForClass:[self class]] pathForResource:kICUBundlePath
                                                                     ofType:nil];
   if (!path) {

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject_Internal.h

@@ -33,6 +33,13 @@
  */
 @property(nonatomic, readonly) std::vector<const char*> argv;
 
+/**
+ * Instead of looking up the assets and ICU data path in the application bundle, this initializer
+ * allows callers to create a Dart project with custom locations specified for the both.
+ */
+- (nonnull instancetype)initWithAssetsPath:(nonnull NSString*)assets
+                               ICUDataPath:(nonnull NSString*)icuPath NS_DESIGNATED_INITIALIZER;
+
 @end
 
 #endif  // SHELL_PLATFORM_DARWIN_MACOS_FRAMEWORK_SOURCE_FLUTTERDARTPROJECT_INTERNAL_H_

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -42,17 +42,18 @@
  */
 - (void)engineCallbackOnPlatformMessage:(const FlutterPlatformMessage*)message;
 
-/**
- * Shuts the Flutter engine if it is running.
- */
-- (void)shutDownEngine;
-
 /**
  * Forwards texture copy request to the corresponding texture via |textureID|.
  */
 - (BOOL)populateTextureWithIdentifier:(int64_t)textureID
                         openGLTexture:(FlutterOpenGLTexture*)openGLTexture;
 
+/**
+ * Requests that the task be posted back the to the Flutter engine at the target time. The target
+ * time is in the clock used by the Flutter engine.
+ */
+- (void)postMainThreadTask:(FlutterTask)task targetTimeInNanoseconds:(uint64_t)targetTime;
+
 @end
 
 #pragma mark -
@@ -218,6 +219,26 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
   flutterArguments.command_line_argv = &arguments[0];
   flutterArguments.platform_message_callback = (FlutterPlatformMessageCallback)OnPlatformMessage;
   flutterArguments.custom_dart_entrypoint = entrypoint.UTF8String;
+  static size_t sTaskRunnerIdentifiers = 0;
+  const FlutterTaskRunnerDescription cocoa_task_runner_description = {
+      .struct_size = sizeof(FlutterTaskRunnerDescription),
+      .user_data = (void*)CFBridgingRetain(self),
+      .runs_task_on_current_thread_callback = [](void* user_data) -> bool {
+        return [[NSThread currentThread] isMainThread];
+      },
+      .post_task_callback = [](FlutterTask task, uint64_t target_time_nanos,
+                               void* user_data) -> void {
+        [((__bridge FlutterEngine*)(user_data)) postMainThreadTask:task
+                                           targetTimeInNanoseconds:target_time_nanos];
+      },
+      .identifier = ++sTaskRunnerIdentifiers,
+  };
+  const FlutterCustomTaskRunners custom_task_runners = {
+      .struct_size = sizeof(FlutterCustomTaskRunners),
+      .platform_task_runner = &cocoa_task_runner_description,
+      .render_task_runner = &cocoa_task_runner_description,
+  };
+  flutterArguments.custom_task_runners = &custom_task_runners;
 
   FlutterEngineResult result = FlutterEngineInitialize(
       FLUTTER_ENGINE_VERSION, &rendererConfig, &flutterArguments, (__bridge void*)(self), &_engine);
@@ -358,6 +379,9 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
     NSLog(@"Could not de-initialize the Flutter engine: error %d", result);
   }
 
+  // Balancing release for the retain in the task runner dispatch table.
+  CFRelease((CFTypeRef)self);
+
   result = FlutterEngineShutdown(_engine);
   if (result != kSuccess) {
     NSLog(@"Failed to shut down Flutter engine: error %d", result);
@@ -459,4 +483,29 @@ static bool OnAcquireExternalTexture(FlutterEngine* engine,
   [_textures removeObjectForKey:@(textureID)];
 }
 
+#pragma mark - Task runner integration
+
+- (void)postMainThreadTask:(FlutterTask)task targetTimeInNanoseconds:(uint64_t)targetTime {
+  const auto engine_time = FlutterEngineGetCurrentTime();
+
+  __weak FlutterEngine* weak_self = self;
+  auto worker = ^{
+    FlutterEngine* strong_self = weak_self;
+    if (strong_self && strong_self->_engine) {
+      auto result = FlutterEngineRunTask(strong_self->_engine, &task);
+      if (result != kSuccess) {
+        NSLog(@"Could not post a task to the Flutter engine.");
+      }
+    }
+  };
+
+  if (targetTime <= engine_time) {
+    dispatch_async(dispatch_get_main_queue(), worker);
+
+  } else {
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, targetTime - engine_time),
+                   dispatch_get_main_queue(), worker);
+  }
+}
+
 @end

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngineUnittests.mm

@@ -0,0 +1,25 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h"
+#include "flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject_Internal.h"
+#include "flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h"
+#include "flutter/testing/testing.h"
+
+namespace flutter::testing {
+
+TEST(FlutterEngineTest, FlutterEngineCanLaunch) {
+  NSString* fixtures = @(testing::GetFixturesPath());
+  FlutterDartProject* project = [[FlutterDartProject alloc]
+      initWithAssetsPath:fixtures
+             ICUDataPath:[fixtures stringByAppendingString:@"/icudtl.dat"]];
+  FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"test"
+                                                      project:project
+                                       allowHeadlessExecution:true];
+  ASSERT_TRUE([engine runWithEntrypoint:@"main"]);
+  ASSERT_TRUE(engine.running);
+  [engine shutDownEngine];
+}
+
+}  // flutter::testing

engine/src/flutter/shell/platform/darwin/macos/framework/Source/fixtures/flutter_desktop_test.dart

@@ -0,0 +1,7 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+void main() {
+
+}