Provide a resource context in the GLFW shell (#8895)

Uses an invisible GLFW window to provide a background resource context
for the Flutter engine to upload textures.

Fixes #31941

shell/platform/glfw/flutter_glfw.cc

@@ -34,14 +34,20 @@
 #define GLFW_FALSE 0
 #endif
 
+using UniqueGLFWwindowPtr = std::unique_ptr<GLFWwindow, void (*)(GLFWwindow*)>;
+
 static_assert(FLUTTER_ENGINE_VERSION == 1, "");
 
 static constexpr double kDpPerInch = 160.0;
 
 // Struct for storing state within an instance of the GLFW Window.
 struct FlutterDesktopWindowControllerState {
-  // The GLFW window that owns this state object.
-  GLFWwindow* window;
+  // The GLFW window that is bound to this state object.
+  UniqueGLFWwindowPtr window = UniqueGLFWwindowPtr(nullptr, glfwDestroyWindow);
+
+  // The invisible GLFW window used to upload resources in the background.
+  UniqueGLFWwindowPtr resource_window =
+      UniqueGLFWwindowPtr(nullptr, glfwDestroyWindow);
 
   // The handle to the Flutter engine instance.
   FlutterEngine engine;
@@ -116,6 +122,16 @@ static FlutterDesktopWindowControllerState* GetSavedWindowState(
       glfwGetWindowUserPointer(window));
 }
 
+// Creates and returns an invisible GLFW window that shares |window|'s resource
+// context.
+static UniqueGLFWwindowPtr CreateShareWindowForWindow(GLFWwindow* window) {
+  glfwWindowHint(GLFW_DECORATED, GLFW_FALSE);
+  glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);
+  GLFWwindow* share_window = glfwCreateWindow(1, 1, "", NULL, window);
+  glfwDefaultWindowHints();
+  return UniqueGLFWwindowPtr(share_window, glfwDestroyWindow);
+}
+
 // Converts a FlutterPlatformMessage to an equivalent FlutterDesktopMessage.
 static FlutterDesktopMessage ConvertToDesktopMessage(
     const FlutterPlatformMessage& engine_message) {
@@ -379,6 +395,12 @@ static bool GLFWMakeContextCurrent(void* user_data) {
   return true;
 }
 
+static bool GLFWMakeResourceContextCurrent(void* user_data) {
+  GLFWwindow* window = reinterpret_cast<GLFWwindow*>(user_data);
+  glfwMakeContextCurrent(GetSavedWindowState(window)->resource_window.get());
+  return true;
+}
+
 static bool GLFWClearContext(void* user_data) {
   glfwMakeContextCurrent(nullptr);
   return true;
@@ -458,6 +480,7 @@ static FlutterEngine RunFlutterEngine(GLFWwindow* window,
     config.open_gl.clear_current = GLFWClearContext;
     config.open_gl.present = GLFWPresent;
     config.open_gl.fbo_callback = GLFWGetActiveFbo;
+    config.open_gl.make_resource_current = GLFWMakeResourceContextCurrent;
     config.open_gl.gl_proc_resolver = GLFWProcResolver;
   }
   FlutterProjectArgs args = {};
@@ -499,34 +522,37 @@ FlutterDesktopWindowControllerRef FlutterDesktopCreateWindow(
 #ifdef __linux__
   gtk_init(0, nullptr);
 #endif
-  // Create the window.
-  auto* window =
-      glfwCreateWindow(initial_width, initial_height, title, NULL, NULL);
+
+  auto state = std::make_unique<FlutterDesktopWindowControllerState>();
+
+  // Create the window, and set the state as its user data.
+  state->window = UniqueGLFWwindowPtr(
+      glfwCreateWindow(initial_width, initial_height, title, NULL, NULL),
+      glfwDestroyWindow);
+  GLFWwindow* window = state->window.get();
   if (window == nullptr) {
     return nullptr;
   }
   GLFWClearCanvas(window);
+  glfwSetWindowUserPointer(window, state.get());
+
+  // Create the share window before starting the engine, since it may call
+  // GLFWMakeResourceContextCurrent immediately.
+  state->resource_window = CreateShareWindowForWindow(window);
 
   // Start the engine.
-  auto engine = RunFlutterEngine(window, assets_path, icu_data_path, arguments,
-                                 argument_count);
-  if (engine == nullptr) {
-    glfwDestroyWindow(window);
+  state->engine = RunFlutterEngine(window, assets_path, icu_data_path,
+                                   arguments, argument_count);
+  if (state->engine == nullptr) {
     return nullptr;
   }
 
-  // Create a state object attached to the window.
-  auto* state = new FlutterDesktopWindowControllerState();
-  state->window = window;
-  glfwSetWindowUserPointer(window, state);
-  state->engine = engine;
-
   // TODO: Restructure the internals to follow the structure of the C++ API, so
   // that this isn't a tangle of references.
   auto messenger = std::make_unique<FlutterDesktopMessenger>();
   state->message_dispatcher =
       std::make_unique<flutter::IncomingMessageDispatcher>(messenger.get());
-  messenger->engine = engine;
+  messenger->engine = state->engine;
   messenger->dispatcher = state->message_dispatcher.get();
 
   state->window_wrapper = std::make_unique<FlutterDesktopWindow>();
@@ -557,12 +583,11 @@ FlutterDesktopWindowControllerRef FlutterDesktopCreateWindow(
   glfwSetFramebufferSizeCallback(window, GLFWFramebufferSizeCallback);
   GLFWAssignEventCallbacks(window);
 
-  return state;
+  return state.release();
 }
 
 void FlutterDesktopDestroyWindow(FlutterDesktopWindowControllerRef controller) {
   FlutterEngineShutdown(controller->engine);
-  glfwDestroyWindow(controller->window);
   delete controller;
 }
 
@@ -633,7 +658,7 @@ double FlutterDesktopWindowGetScaleFactor(
 }
 
 void FlutterDesktopRunWindowLoop(FlutterDesktopWindowControllerRef controller) {
-  GLFWwindow* window = controller->window;
+  GLFWwindow* window = controller->window.get();
 #ifdef __linux__
   // Necessary for GTK thread safety.
   XInitThreads();