[embedder] Ensure FlutterMetalTexture cleanup call (flutter/engine#38038)

This ensures FlutterMetalTexture.destruction_callback gets called.

FlutterRendererConfig.get_next_drawable_callback holds a callback used by the embedder API to request a drawable; in the case of Metal, this drawable is a FlutterMetalTexture.

FlutterMetalTexture.destruction_callback should be called when it's safe to release resources associated with the FlutterMetalTexture. This callback is not currently invoked for textures returned via FlutterRendererConfig.get_next_drawable_callback; instead we unpack the returned struct and pass it on.

In the compositor codepath, we do create an SkSurface that triggers the destruction callback, here:
bbdb5d6a3e/shell/platform/embedder/embedder.cc (L868-L881)

Issue: https://github.com/flutter/flutter/issues/116381

engine/src/flutter/shell/gpu/gpu_surface_metal_delegate.h

@@ -26,9 +26,13 @@ typedef void* GPUCAMetalLayerHandle;
 // expected to be id<MTLTexture>
 typedef const void* GPUMTLTextureHandle;
 
+typedef void (*GPUMTLDestructionCallback)(void* /* destruction_context */);
+
 struct GPUMTLTextureInfo {
   int64_t texture_id;
   GPUMTLTextureHandle texture;
+  GPUMTLDestructionCallback destruction_callback;
+  void* destruction_context;
 };
 
 enum class MTLRenderTargetType { kMTLTexture, kCAMetalLayer };

engine/src/flutter/shell/gpu/gpu_surface_metal_skia.mm

@@ -38,13 +38,15 @@ sk_sp<SkSurface> CreateSurfaceFromMetalTexture(GrDirectContext* context,
                                                MsaaSampleCount sample_cnt,
                                                SkColorType color_type,
                                                sk_sp<SkColorSpace> color_space,
-                                               const SkSurfaceProps* props) {
+                                               const SkSurfaceProps* props,
+                                               SkSurface::TextureReleaseProc release_proc,
+                                               SkSurface::ReleaseContext release_context) {
   GrMtlTextureInfo info;
   info.fTexture.reset([texture retain]);
   GrBackendTexture backend_texture(texture.width, texture.height, GrMipmapped::kNo, info);
-  return SkSurface::MakeFromBackendTexture(context, backend_texture, origin,
-                                           static_cast<int>(sample_cnt), color_type,
-                                           std::move(color_space), props);
+  return SkSurface::MakeFromBackendTexture(
+      context, backend_texture, origin, static_cast<int>(sample_cnt), color_type,
+      std::move(color_space), props, release_proc, release_context);
 }
 }  // namespace
 
@@ -137,7 +139,9 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalSkia::AcquireFrameFromCAMetalLayer(
                                                msaa_samples_,             // sample count
                                                kBGRA_8888_SkColorType,    // color type
                                                nullptr,                   // colorspace
-                                               nullptr                    // surface properties
+                                               nullptr,                   // surface properties
+                                               nullptr,                   // release proc
+                                               nullptr                    // release context
   );
 
   if (!surface) {
@@ -203,9 +207,10 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalSkia::AcquireFrameFromMTLTexture(
     return nullptr;
   }
 
-  sk_sp<SkSurface> surface =
-      CreateSurfaceFromMetalTexture(context_.get(), mtl_texture, kTopLeft_GrSurfaceOrigin,
-                                    msaa_samples_, kBGRA_8888_SkColorType, nullptr, nullptr);
+  sk_sp<SkSurface> surface = CreateSurfaceFromMetalTexture(
+      context_.get(), mtl_texture, kTopLeft_GrSurfaceOrigin, msaa_samples_, kBGRA_8888_SkColorType,
+      nullptr, nullptr, static_cast<SkSurface::TextureReleaseProc>(texture.destruction_callback),
+      texture.destruction_context);
 
   if (!surface) {
     FML_LOG(ERROR) << "Could not create the SkSurface from the metal texture.";

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

@@ -477,6 +477,8 @@ InferMetalPlatformViewCreationCallback(
     FlutterMetalTexture metal_texture = ptr(user_data, &frame_info);
     texture_info.texture_id = metal_texture.texture_id;
     texture_info.texture = metal_texture.texture;
+    texture_info.destruction_callback = metal_texture.destruction_callback;
+    texture_info.destruction_context = metal_texture.user_data;
     return texture_info;
   };
 

engine/src/flutter/shell/platform/embedder/fixtures/main.dart

@@ -498,6 +498,25 @@ void can_composite_platform_views_with_platform_layer_on_bottom() {
   PlatformDispatcher.instance.scheduleFrame();
 }
 
+@pragma('vm:external-name', 'SignalBeginFrame')
+external void signalBeginFrame();
+
+@pragma('vm:entry-point')
+void texture_destruction_callback_called_without_custom_compositor() async {
+  PlatformDispatcher.instance.onBeginFrame = (Duration duration) {
+    Color red = Color.fromARGB(127, 255, 0, 0);
+    Size size = Size(50.0, 150.0);
+    SceneBuilder builder = SceneBuilder();
+    builder.pushOffset(0.0, 0.0);
+    builder.addPicture(
+        Offset(10.0, 10.0), CreateColoredBox(red, size)); // red - flutter
+    builder.pop();
+    PlatformDispatcher.instance.views.first.render(builder.build());
+  };
+  PlatformDispatcher.instance.scheduleFrame();
+}
+
+
 @pragma('vm:entry-point')
 void can_render_scene_without_custom_compositor() {
   PlatformDispatcher.instance.onBeginFrame = (Duration duration) {

engine/src/flutter/shell/platform/embedder/tests/embedder_test_context_metal.cc

@@ -71,10 +71,22 @@ bool EmbedderTestContextMetal::PopulateExternalTexture(
   }
 }
 
+TestMetalContext::TextureInfo EmbedderTestContextMetal::GetTextureInfo() {
+  return metal_surface_->GetTextureInfo();
+}
+
+void EmbedderTestContextMetal::SetNextDrawableCallback(
+    NextDrawableCallback next_drawable_callback) {
+  next_drawable_callback_ = std::move(next_drawable_callback);
+}
+
 FlutterMetalTexture EmbedderTestContextMetal::GetNextDrawable(
     const FlutterFrameInfo* frame_info) {
-  auto texture_info = metal_surface_->GetTextureInfo();
+  if (next_drawable_callback_ != nullptr) {
+    return next_drawable_callback_(frame_info);
+  }
 
+  auto texture_info = metal_surface_->GetTextureInfo();
   FlutterMetalTexture texture;
   texture.struct_size = sizeof(FlutterMetalTexture);
   texture.texture_id = texture_info.texture_id;

engine/src/flutter/shell/platform/embedder/tests/embedder_test_context_metal.h

@@ -20,6 +20,9 @@ class EmbedderTestContextMetal : public EmbedderTestContext {
                          size_t h,
                          FlutterMetalExternalTexture* output)>;
 
+  using NextDrawableCallback =
+      std::function<FlutterMetalTexture(const FlutterFrameInfo* frame_info)>;
+
   explicit EmbedderTestContextMetal(std::string assets_path = "");
 
   ~EmbedderTestContextMetal() override;
@@ -45,6 +48,12 @@ class EmbedderTestContextMetal : public EmbedderTestContext {
 
   TestMetalContext* GetTestMetalContext();
 
+  // Returns the TextureInfo for the test Metal surface.
+  TestMetalContext::TextureInfo GetTextureInfo();
+
+  // Override the default handling for GetNextDrawable.
+  void SetNextDrawableCallback(NextDrawableCallback next_drawable_callback);
+
   FlutterMetalTexture GetNextDrawable(const FlutterFrameInfo* frame_info);
 
  private:
@@ -56,6 +65,7 @@ class EmbedderTestContextMetal : public EmbedderTestContext {
   std::unique_ptr<TestMetalContext> metal_context_;
   std::unique_ptr<TestMetalSurface> metal_surface_;
   size_t present_count_ = 0;
+  NextDrawableCallback next_drawable_callback_ = nullptr;
 
   void SetupSurface(SkISize surface_size) override;
 

engine/src/flutter/shell/platform/embedder/tests/embedder_unittests_metal.mm

@@ -227,6 +227,49 @@ TEST_F(EmbedderTest, CanRenderSceneWithoutCustomCompositorMetal) {
   ASSERT_TRUE(ImageMatchesFixture("scene_without_custom_compositor.png", rendered_scene));
 }
 
+TEST_F(EmbedderTest, TextureDestructionCallbackCalledWithoutCustomCompositorMetal) {
+  EmbedderTestContextMetal& context = reinterpret_cast<EmbedderTestContextMetal&>(
+      GetEmbedderContext(EmbedderTestContextType::kMetalContext));
+  EmbedderConfigBuilder builder(context);
+  builder.SetMetalRendererConfig(SkISize::Make(800, 600));
+  builder.SetDartEntrypoint("texture_destruction_callback_called_without_custom_compositor");
+
+  struct CollectContext {
+    int collect_count = 0;
+    fml::AutoResetWaitableEvent latch;
+  };
+
+  auto collect_context = std::make_unique<CollectContext>();
+  context.SetNextDrawableCallback([&context, &collect_context](const FlutterFrameInfo* frame_info) {
+    auto texture_info = context.GetTextureInfo();
+    FlutterMetalTexture texture;
+    texture.struct_size = sizeof(FlutterMetalTexture);
+    texture.texture_id = texture_info.texture_id;
+    texture.texture = reinterpret_cast<FlutterMetalTextureHandle>(texture_info.texture);
+    texture.user_data = collect_context.get();
+    texture.destruction_callback = [](void* user_data) {
+      CollectContext* callback_collect_context = reinterpret_cast<CollectContext*>(user_data);
+      callback_collect_context->collect_count++;
+      callback_collect_context->latch.Signal();
+    };
+    return texture;
+  });
+
+  auto engine = builder.LaunchEngine();
+
+  // Send a window metrics events so frames may be scheduled.
+  FlutterWindowMetricsEvent event = {};
+  event.struct_size = sizeof(event);
+  event.width = 800;
+  event.height = 600;
+  event.pixel_ratio = 1.0;
+  ASSERT_EQ(FlutterEngineSendWindowMetricsEvent(engine.get(), &event), kSuccess);
+  ASSERT_TRUE(engine.is_valid());
+
+  collect_context->latch.Wait();
+  EXPECT_EQ(collect_context->collect_count, 1);
+}
+
 TEST_F(EmbedderTest, CompositorMustBeAbleToRenderKnownSceneMetal) {
   auto& context = GetEmbedderContext(EmbedderTestContextType::kMetalContext);