Ensure we use the base CompositorContext's AcquireFrame method when screenshotting (#13934)

This ensures we rasterize into the canvas passed in as subclasses may reimplement AcquireFrame in different ways that don't utilize the canvas object passed in (such as Fuchsia's flutter_runner::CompositorContext).

ci/licenses_golden/licenses_flutter

@@ -503,6 +503,7 @@ FILE: ../../../flutter/shell/common/canvas_spy_unittests.cc
 FILE: ../../../flutter/shell/common/engine.cc
 FILE: ../../../flutter/shell/common/engine.h
 FILE: ../../../flutter/shell/common/fixtures/shell_test.dart
+FILE: ../../../flutter/shell/common/fixtures/shelltest_screenshot.png
 FILE: ../../../flutter/shell/common/input_events_unittests.cc
 FILE: ../../../flutter/shell/common/isolate_configuration.cc
 FILE: ../../../flutter/shell/common/isolate_configuration.h

shell/common/BUILD.gn

@@ -153,6 +153,8 @@ if (current_toolchain == host_toolchain) {
 
   test_fixtures("shell_unittests_fixtures") {
     dart_main = "fixtures/shell_test.dart"
+
+    fixtures = [ "fixtures/shelltest_screenshot.png" ]
   }
 
   shell_host_executable("shell_unittests") {

shell/common/rasterizer.cc

@@ -399,9 +399,14 @@ static sk_sp<SkData> ScreenshotLayerTreeAsImage(
   SkMatrix root_surface_transformation;
   root_surface_transformation.reset();
 
-  auto frame = compositor_context.AcquireFrame(surface_context, canvas, nullptr,
-                                               root_surface_transformation,
-                                               false, nullptr);
+  // We want to ensure we call the base method for
+  // CompositorContext::AcquireFrame instead of the platform-specific method.
+  // Specifically, Fuchsia's CompositorContext handles the rendering surface
+  // itself which means that we will still continue to render to the onscreen
+  // surface if we don't call the base method.
+  auto frame = compositor_context.flutter::CompositorContext::AcquireFrame(
+      surface_context, canvas, nullptr, root_surface_transformation, false,
+      nullptr);
   canvas->clear(SK_ColorTRANSPARENT);
   frame->Raster(*tree, true);
   canvas->flush();

shell/common/shell_unittests.cc

@@ -10,6 +10,7 @@
 #include <memory>
 
 #include "flutter/flow/layers/layer_tree.h"
+#include "flutter/flow/layers/picture_layer.h"
 #include "flutter/flow/layers/transform_layer.h"
 #include "flutter/fml/command_line.h"
 #include "flutter/fml/make_copyable.h"
@@ -952,5 +953,66 @@ TEST_F(ShellTest, IsolateCanAccessPersistentIsolateData) {
   DestroyShell(std::move(shell), std::move(task_runners));
 }
 
+TEST_F(ShellTest, Screenshot) {
+  auto settings = CreateSettingsForFixture();
+  fml::AutoResetWaitableEvent firstFrameLatch;
+  settings.frame_rasterized_callback =
+      [&firstFrameLatch](const FrameTiming& t) { firstFrameLatch.Signal(); };
+
+  std::unique_ptr<Shell> shell = CreateShell(settings);
+
+  // Create the surface needed by rasterizer
+  PlatformViewNotifyCreated(shell.get());
+
+  auto configuration = RunConfiguration::InferFromSettings(settings);
+  configuration.SetEntrypoint("emptyMain");
+
+  RunEngine(shell.get(), std::move(configuration));
+
+  LayerTreeBuilder builder = [&](std::shared_ptr<ContainerLayer> root) {
+    SkPictureRecorder recorder;
+    SkCanvas* recording_canvas =
+        recorder.beginRecording(SkRect::MakeXYWH(0, 0, 80, 80));
+    recording_canvas->drawRect(SkRect::MakeXYWH(0, 0, 80, 80),
+                               SkPaint(SkColor4f::FromColor(SK_ColorRED)));
+    auto sk_picture = recorder.finishRecordingAsPicture();
+    fml::RefPtr<SkiaUnrefQueue> queue = fml::MakeRefCounted<SkiaUnrefQueue>(
+        this->GetCurrentTaskRunner(), fml::TimeDelta::FromSeconds(0));
+    auto picture_layer = std::make_shared<PictureLayer>(
+        SkPoint::Make(10, 10),
+        flutter::SkiaGPUObject<SkPicture>({sk_picture, queue}), false, false);
+    root->Add(picture_layer);
+  };
+
+  PumpOneFrame(shell.get(), 100, 100, builder);
+  firstFrameLatch.Wait();
+
+  std::promise<Rasterizer::Screenshot> screenshot_promise;
+  auto screenshot_future = screenshot_promise.get_future();
+
+  fml::TaskRunner::RunNowOrPostTask(
+      shell->GetTaskRunners().GetGPUTaskRunner(),
+      [&screenshot_promise, &shell]() {
+        auto rasterizer = shell->GetRasterizer();
+        screenshot_promise.set_value(rasterizer->ScreenshotLastLayerTree(
+            Rasterizer::ScreenshotType::CompressedImage, false));
+      });
+
+  auto fixtures_dir =
+      fml::OpenDirectory(GetFixturesPath(), false, fml::FilePermission::kRead);
+
+  auto reference_png = fml::FileMapping::CreateReadOnly(
+      fixtures_dir, "shelltest_screenshot.png");
+
+  // Use MakeWithoutCopy instead of MakeWithCString because we don't want to
+  // encode the null sentinel
+  sk_sp<SkData> reference_data = SkData::MakeWithoutCopy(
+      reference_png->GetMapping(), reference_png->GetSize());
+
+  ASSERT_TRUE(reference_data->equals(screenshot_future.get().data.get()));
+
+  DestroyShell(std::move(shell));
+}
+
 }  // namespace testing
 }  // namespace flutter