[Impeller] set stencil attachment descriptor for runtime effect. (flutter/engine#42054)

This fixes the correctness issues observed in https://github.com/flutter/flutter/issues/126701 . The missing stencil descriptor meant that the clips were always ignored.

Have not yet investigated the reported debug performance issues.

engine/src/flutter/impeller/aiks/aiks_unittests.cc

@@ -2263,5 +2263,39 @@ TEST_P(AiksTest, CanRenderBackdropBlur) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+// Regression test for https://github.com/flutter/flutter/issues/126701 .
+TEST_P(AiksTest, CanRenderClippedRuntimeEffects) {
+  if (GetParam() != PlaygroundBackend::kMetal) {
+    GTEST_SKIP_("This backend doesn't support runtime effects.");
+  }
+
+  auto runtime_stage =
+      OpenAssetAsRuntimeStage("runtime_stage_example.frag.iplr");
+  ASSERT_TRUE(runtime_stage->IsDirty());
+
+  struct FragUniforms {
+    Vector2 iResolution;
+    Scalar iTime;
+  } frag_uniforms = {.iResolution = Vector2(400, 400), .iTime = 100.0};
+  auto uniform_data = std::make_shared<std::vector<uint8_t>>();
+  uniform_data->resize(sizeof(FragUniforms));
+  memcpy(uniform_data->data(), &frag_uniforms, sizeof(FragUniforms));
+
+  std::vector<RuntimeEffectContents::TextureInput> texture_inputs;
+
+  Paint paint;
+  paint.color_source = ColorSource::MakeRuntimeEffect(
+      runtime_stage, uniform_data, texture_inputs);
+
+  Canvas canvas;
+  canvas.Save();
+  canvas.ClipRRect(Rect{0, 0, 400, 400}, 10.0,
+                   Entity::ClipOperation::kIntersect);
+  canvas.DrawRect(Rect{0, 0, 400, 400}, paint);
+  canvas.Restore();
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

engine/src/flutter/impeller/entity/contents/runtime_effect_contents.cc

@@ -124,7 +124,10 @@ bool RuntimeEffectContents::Render(const ContentContext& renderer,
   desc.SetVertexDescriptor(std::move(vertex_descriptor));
   desc.SetColorAttachmentDescriptor(
       0u, {.format = color_attachment_format, .blending_enabled = true});
-  desc.SetStencilAttachmentDescriptors({});
+
+  StencilAttachmentDescriptor stencil0;
+  stencil0.stencil_compare = CompareFunction::kEqual;
+  desc.SetStencilAttachmentDescriptors(stencil0);
   desc.SetStencilPixelFormat(stencil_attachment_format);
 
   auto options = OptionsFromPassAndEntity(pass, entity);

engine/src/flutter/impeller/golden_tests/golden_playground_test.h

@@ -36,6 +36,9 @@ class GoldenPlaygroundTest
       const char* fixture_name,
       bool enable_mipmapping = false) const;
 
+  std::shared_ptr<RuntimeStage> OpenAssetAsRuntimeStage(
+      const char* asset_name) const;
+
   std::shared_ptr<Context> GetContext() const;
 
   Point GetContentScale() const;

engine/src/flutter/impeller/golden_tests/golden_playground_test_mac.cc

@@ -147,6 +147,19 @@ std::shared_ptr<Texture> GoldenPlaygroundTest::CreateTextureForFixture(
   return result;
 }
 
+std::shared_ptr<RuntimeStage> GoldenPlaygroundTest::OpenAssetAsRuntimeStage(
+    const char* asset_name) const {
+  auto fixture = flutter::testing::OpenFixtureAsMapping(asset_name);
+  if (!fixture || fixture->GetSize() == 0) {
+    return nullptr;
+  }
+  auto stage = std::make_unique<RuntimeStage>(std::move(fixture));
+  if (!stage->IsValid()) {
+    return nullptr;
+  }
+  return stage;
+}
+
 std::shared_ptr<Context> GoldenPlaygroundTest::GetContext() const {
   return pimpl_->screenshoter->GetContext().GetContext();
 }

engine/src/flutter/impeller/golden_tests/golden_playground_test_stub.cc

@@ -33,6 +33,11 @@ std::shared_ptr<Texture> GoldenPlaygroundTest::CreateTextureForFixture(
   return nullptr;
 }
 
+std::shared_ptr<RuntimeStage> GoldenPlaygroundTest::OpenAssetAsRuntimeStage(
+    const char* asset_name) const {
+  return nullptr;
+}
+
 std::shared_ptr<Context> GoldenPlaygroundTest::GetContext() const {
   return nullptr;
 }