Reland "Lift restriction that embedders may not trample the render thread OpenGL context in composition callbacks." (flutter#16711)

This reverts commit ef9e7b1 with the following changes to accommodate an embedder for whom the original optimizations caused issues:

* Ensure stable order in the backing stores presented to the embedder. This is a pessimization that will be reverted when the embedder migrates. Tracked in  flutter#51228
* Forego the optimization where the unused layers would be collected before allocation of new layers needs to happen. This is a pessimization that will be reverted when the embedder migrates. Tracked in  flutter#51229

More context in b/146142979.

Author: chinmaygarde

ci/licenses_golden/licenses_flutter

@@ -126,6 +126,8 @@ FILE: ../../../flutter/fml/file_unittest.cc
 FILE: ../../../flutter/fml/gpu_thread_merger.cc
 FILE: ../../../flutter/fml/gpu_thread_merger.h
 FILE: ../../../flutter/fml/gpu_thread_merger_unittests.cc
+FILE: ../../../flutter/fml/hash_combine.h
+FILE: ../../../flutter/fml/hash_combine_unittests.cc
 FILE: ../../../flutter/fml/icu_util.cc
 FILE: ../../../flutter/fml/icu_util.h
 FILE: ../../../flutter/fml/log_level.h
@@ -912,15 +914,20 @@ FILE: ../../../flutter/shell/platform/embedder/embedder_engine.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_engine.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_external_texture_gl.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_external_texture_gl.h
+FILE: ../../../flutter/shell/platform/embedder/embedder_external_view.cc
+FILE: ../../../flutter/shell/platform/embedder/embedder_external_view.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_external_view_embedder.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_external_view_embedder.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_include.c
+FILE: ../../../flutter/shell/platform/embedder/embedder_include2.c
 FILE: ../../../flutter/shell/platform/embedder/embedder_layers.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_layers.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_platform_message_response.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_platform_message_response.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_render_target.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_render_target.h
+FILE: ../../../flutter/shell/platform/embedder/embedder_render_target_cache.cc
+FILE: ../../../flutter/shell/platform/embedder/embedder_render_target_cache.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_safe_access.h
 FILE: ../../../flutter/shell/platform/embedder/embedder_surface.cc
 FILE: ../../../flutter/shell/platform/embedder/embedder_surface.h

fml/BUILD.gn

@@ -26,6 +26,7 @@ source_set("fml") {
     "file.h",
     "gpu_thread_merger.cc",
     "gpu_thread_merger.h",
+    "hash_combine.h",
     "icu_util.cc",
     "icu_util.h",
     "log_level.h",
@@ -232,6 +233,7 @@ executable("fml_unittests") {
     "command_line_unittest.cc",
     "file_unittest.cc",
     "gpu_thread_merger_unittests.cc",
+    "hash_combine_unittests.cc",
     "memory/ref_counted_unittest.cc",
     "memory/weak_ptr_unittest.cc",
     "message_loop_task_queues_merge_unmerge_unittests.cc",

fml/hash_combine.h

@@ -0,0 +1,38 @@
+// 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.
+
+#ifndef FLUTTER_FML_HASH_COMBINE_H_
+#define FLUTTER_FML_HASH_COMBINE_H_
+
+#include <functional>
+
+namespace fml {
+
+template <class Type>
+constexpr void HashCombineSeed(std::size_t& seed, Type arg) {
+  seed ^= std::hash<Type>{}(arg) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+}
+
+template <class Type, class... Rest>
+constexpr void HashCombineSeed(std::size_t& seed,
+                               Type arg,
+                               Rest... other_args) {
+  HashCombineSeed(seed, arg);
+  HashCombineSeed(seed, other_args...);
+}
+
+constexpr std::size_t HashCombine() {
+  return 0xdabbad00;
+}
+
+template <class... Type>
+constexpr std::size_t HashCombine(Type... args) {
+  std::size_t seed = HashCombine();
+  HashCombineSeed(seed, args...);
+  return seed;
+}
+
+}  // namespace fml
+
+#endif  // FLUTTER_FML_HASH_COMBINE_H_

fml/hash_combine_unittests.cc

@@ -0,0 +1,23 @@
+// 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/fml/hash_combine.h"
+#include "flutter/testing/testing.h"
+
+namespace fml {
+namespace testing {
+
+TEST(HashCombineTest, CanHash) {
+  ASSERT_EQ(HashCombine(), HashCombine());
+  ASSERT_EQ(HashCombine("Hello"), HashCombine("Hello"));
+  ASSERT_NE(HashCombine("Hello"), HashCombine("World"));
+  ASSERT_EQ(HashCombine("Hello", "World"), HashCombine("Hello", "World"));
+  ASSERT_NE(HashCombine("World", "Hello"), HashCombine("Hello", "World"));
+  ASSERT_EQ(HashCombine(12u), HashCombine(12u));
+  ASSERT_NE(HashCombine(12u), HashCombine(12.0f));
+  ASSERT_EQ(HashCombine('a'), HashCombine('a'));
+}
+
+}  // namespace testing
+}  // namespace fml

shell/common/canvas_spy.h

@@ -26,16 +26,16 @@ class CanvasSpy {
  public:
   CanvasSpy(SkCanvas* target_canvas);
 
-  //------------------------------------------------------------------------------
-  /// @brief      Returns true if any non trasnparent content has been drawn
+  //----------------------------------------------------------------------------
+  /// @brief      Returns true if any non transparent content has been drawn
   /// into
   ///             the spying canvas. Note that this class does tries to detect
   ///             empty canvases but in some cases may return true even for
   ///             empty canvases (e.g when a transparent image is drawn into the
   ///             canvas).
   bool DidDrawIntoCanvas();
 
-  //------------------------------------------------------------------------------
+  //----------------------------------------------------------------------------
   /// @brief      The returned canvas delegate all operations to the target
   /// canvas
   ///             while spying on them.

shell/platform/embedder/BUILD.gn

@@ -29,15 +29,20 @@ template("embedder_source_set") {
       "embedder_engine.h",
       "embedder_external_texture_gl.cc",
       "embedder_external_texture_gl.h",
+      "embedder_external_view.cc",
+      "embedder_external_view.h",
       "embedder_external_view_embedder.cc",
       "embedder_external_view_embedder.h",
       "embedder_include.c",
+      "embedder_include2.c",
       "embedder_layers.cc",
       "embedder_layers.h",
       "embedder_platform_message_response.cc",
       "embedder_platform_message_response.h",
       "embedder_render_target.cc",
       "embedder_render_target.h",
+      "embedder_render_target_cache.cc",
+      "embedder_render_target_cache.h",
       "embedder_safe_access.h",
       "embedder_surface.cc",
       "embedder_surface.h",

shell/platform/embedder/embedder_external_view.cc

@@ -0,0 +1,107 @@
+// 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/embedder/embedder_external_view.h"
+#include "flutter/fml/trace_event.h"
+#include "flutter/shell/common/canvas_spy.h"
+
+namespace flutter {
+
+static SkISize TransformedSurfaceSize(const SkISize& size,
+                                      const SkMatrix& transformation) {
+  const auto source_rect = SkRect::MakeWH(size.width(), size.height());
+  const auto transformed_rect = transformation.mapRect(source_rect);
+  return SkISize::Make(transformed_rect.width(), transformed_rect.height());
+}
+
+EmbedderExternalView::EmbedderExternalView(
+    const SkISize& frame_size,
+    const SkMatrix& surface_transformation)
+    : EmbedderExternalView(frame_size, surface_transformation, {}, nullptr) {}
+
+EmbedderExternalView::EmbedderExternalView(
+    const SkISize& frame_size,
+    const SkMatrix& surface_transformation,
+    ViewIdentifier view_identifier,
+    std::unique_ptr<EmbeddedViewParams> params)
+    : render_surface_size_(
+          TransformedSurfaceSize(frame_size, surface_transformation)),
+      surface_transformation_(surface_transformation),
+      view_identifier_(view_identifier),
+      embedded_view_params_(std::move(params)),
+      recorder_(std::make_unique<SkPictureRecorder>()),
+      canvas_spy_(std::make_unique<CanvasSpy>(
+          recorder_->beginRecording(frame_size.width(), frame_size.height()))) {
+}
+
+EmbedderExternalView::~EmbedderExternalView() = default;
+
+EmbedderExternalView::RenderTargetDescriptor
+EmbedderExternalView::CreateRenderTargetDescriptor() const {
+  return {view_identifier_, render_surface_size_};
+}
+
+SkCanvas* EmbedderExternalView::GetCanvas() const {
+  return canvas_spy_->GetSpyingCanvas();
+}
+
+SkISize EmbedderExternalView::GetRenderSurfaceSize() const {
+  return render_surface_size_;
+}
+
+bool EmbedderExternalView::IsRootView() const {
+  return !HasPlatformView();
+}
+
+bool EmbedderExternalView::HasPlatformView() const {
+  return view_identifier_.platform_view_id.has_value();
+}
+
+bool EmbedderExternalView::HasEngineRenderedContents() const {
+  return canvas_spy_->DidDrawIntoCanvas();
+}
+
+EmbedderExternalView::ViewIdentifier EmbedderExternalView::GetViewIdentifier()
+    const {
+  return view_identifier_;
+}
+
+const EmbeddedViewParams* EmbedderExternalView::GetEmbeddedViewParams() const {
+  return embedded_view_params_.get();
+}
+
+bool EmbedderExternalView::Render(const EmbedderRenderTarget& render_target) {
+  TRACE_EVENT0("flutter", "EmbedderExternalView::Render");
+
+  FML_DCHECK(HasEngineRenderedContents())
+      << "Unnecessarily asked to render into a render target when there was "
+         "nothing to render.";
+
+  auto picture = recorder_->finishRecordingAsPicture();
+  if (!picture) {
+    return false;
+  }
+
+  auto surface = render_target.GetRenderSurface();
+  if (!surface) {
+    return false;
+  }
+
+  FML_DCHECK(SkISize::Make(surface->width(), surface->height()) ==
+             render_surface_size_);
+
+  auto canvas = surface->getCanvas();
+  if (!canvas) {
+    return false;
+  }
+
+  canvas->setMatrix(surface_transformation_);
+  canvas->clear(SK_ColorTRANSPARENT);
+  canvas->drawPicture(picture);
+  canvas->flush();
+
+  return true;
+}
+
+}  // namespace flutter

shell/platform/embedder/embedder_external_view.h

@@ -0,0 +1,126 @@
+// 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.
+
+#ifndef FLUTTER_SHELL_PLATFORM_EMBEDDER_EMBEDDER_EXTERNAL_VIEW_H_
+#define FLUTTER_SHELL_PLATFORM_EMBEDDER_EMBEDDER_EXTERNAL_VIEW_H_
+
+#include <optional>
+#include <unordered_map>
+#include <unordered_set>
+
+#include "flutter/flow/embedded_views.h"
+#include "flutter/fml/hash_combine.h"
+#include "flutter/fml/macros.h"
+#include "flutter/shell/common/canvas_spy.h"
+#include "flutter/shell/platform/embedder/embedder_render_target.h"
+#include "third_party/skia/include/core/SkPictureRecorder.h"
+
+namespace flutter {
+
+class EmbedderExternalView {
+ public:
+  using PlatformViewID = int64_t;
+  struct ViewIdentifier {
+    std::optional<PlatformViewID> platform_view_id;
+
+    ViewIdentifier() {}
+
+    ViewIdentifier(PlatformViewID view_id) : platform_view_id(view_id) {}
+
+    struct Hash {
+      constexpr std::size_t operator()(const ViewIdentifier& desc) const {
+        if (!desc.platform_view_id.has_value()) {
+          return fml::HashCombine();
+        }
+
+        return fml::HashCombine(desc.platform_view_id.value());
+      }
+    };
+
+    struct Equal {
+      constexpr bool operator()(const ViewIdentifier& lhs,
+                                const ViewIdentifier& rhs) const {
+        return lhs.platform_view_id == rhs.platform_view_id;
+      }
+    };
+  };
+
+  struct RenderTargetDescriptor {
+    ViewIdentifier view_identifier;
+    SkISize surface_size;
+
+    RenderTargetDescriptor(ViewIdentifier p_view_identifier,
+                           SkISize p_surface_size)
+        : view_identifier(p_view_identifier), surface_size(p_surface_size) {}
+
+    struct Hash {
+      constexpr std::size_t operator()(
+          const RenderTargetDescriptor& desc) const {
+        return fml::HashCombine(desc.surface_size.width(),
+                                desc.surface_size.height(),
+                                ViewIdentifier::Hash{}(desc.view_identifier));
+      }
+    };
+
+    struct Equal {
+      bool operator()(const RenderTargetDescriptor& lhs,
+                      const RenderTargetDescriptor& rhs) const {
+        return lhs.surface_size == rhs.surface_size &&
+               ViewIdentifier::Equal{}(lhs.view_identifier,
+                                       rhs.view_identifier);
+      }
+    };
+  };
+
+  using ViewIdentifierSet = std::unordered_set<ViewIdentifier,
+                                               ViewIdentifier::Hash,
+                                               ViewIdentifier::Equal>;
+
+  using PendingViews = std::unordered_map<ViewIdentifier,
+                                          std::unique_ptr<EmbedderExternalView>,
+                                          ViewIdentifier::Hash,
+                                          ViewIdentifier::Equal>;
+
+  EmbedderExternalView(const SkISize& frame_size,
+                       const SkMatrix& surface_transformation);
+
+  EmbedderExternalView(const SkISize& frame_size,
+                       const SkMatrix& surface_transformation,
+                       ViewIdentifier view_identifier,
+                       std::unique_ptr<EmbeddedViewParams> params);
+
+  ~EmbedderExternalView();
+
+  bool IsRootView() const;
+
+  bool HasPlatformView() const;
+
+  bool HasEngineRenderedContents() const;
+
+  ViewIdentifier GetViewIdentifier() const;
+
+  const EmbeddedViewParams* GetEmbeddedViewParams() const;
+
+  RenderTargetDescriptor CreateRenderTargetDescriptor() const;
+
+  SkCanvas* GetCanvas() const;
+
+  SkISize GetRenderSurfaceSize() const;
+
+  bool Render(const EmbedderRenderTarget& render_target);
+
+ private:
+  const SkISize render_surface_size_;
+  const SkMatrix surface_transformation_;
+  ViewIdentifier view_identifier_;
+  std::unique_ptr<EmbeddedViewParams> embedded_view_params_;
+  std::unique_ptr<SkPictureRecorder> recorder_;
+  std::unique_ptr<CanvasSpy> canvas_spy_;
+
+  FML_DISALLOW_COPY_AND_ASSIGN(EmbedderExternalView);
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_EMBEDDER_EMBEDDER_EXTERNAL_VIEW_H_