cc: Always keep the PictureLayerImpl::twin_layer_ pointer valid.

cc/trees/layer_tree_host_impl_unittest.cc

 // Copyright 2011 The Chromium 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 "cc/trees/layer_tree_host_impl.h"
 
 #include <cmath>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 7411 matching lines @@
   host_impl_->Animate(start_time + base::TimeDelta::FromMilliseconds(250));
   host_impl_->UpdateAnimationState(true);
 
   EXPECT_VECTOR_EQ(gfx::ScrollOffset(0, 100),
                    scrolling_layer->TotalScrollOffset());
   EXPECT_EQ(NULL, host_impl_->CurrentlyScrollingLayer());
 }
 
 TEST_F(LayerTreeHostImplTest, GetPictureLayerImplPairs) {
   host_impl_->CreatePendingTree();
-  host_impl_->ActivateSyncTree();
-  host_impl_->CreatePendingTree();
+  host_impl_->pending_tree()->SetRootLayer(
+      PictureLayerImpl::Create(host_impl_->pending_tree(), 10));
 
-  LayerTreeImpl* active_tree = host_impl_->active_tree();
   LayerTreeImpl* pending_tree = host_impl_->pending_tree();
-  EXPECT_NE(active_tree, pending_tree);
-
-  scoped_ptr<FakePictureLayerImpl> active_layer =
-      FakePictureLayerImpl::Create(active_tree, 10);
-  scoped_ptr<FakePictureLayerImpl> pending_layer =
-      FakePictureLayerImpl::Create(pending_tree, 10);
+  LayerImpl* pending_layer = pending_tree->root_layer();
 
   std::vector<PictureLayerImpl::Pair> layer_pairs;
   host_impl_->GetPictureLayerImplPairs(&layer_pairs);
+  EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(pending_layer, layer_pairs[0].pending);
+  EXPECT_EQ(nullptr, layer_pairs[0].active);
 
-  EXPECT_EQ(2u, layer_pairs.size());

[vmpstr, 2014/10/24 16:47:05]

Can you add another test here that tests active layer and pending layer that are
not related, so that we get 2 layer pairs one has active one has pending?

[danakj, 2014/10/24 16:53:30]

Yep!

-  if (layer_pairs[0].active) {
-    EXPECT_EQ(active_layer.get(), layer_pairs[0].active);
-    EXPECT_EQ(NULL, layer_pairs[0].pending);
-  } else {
-    EXPECT_EQ(pending_layer.get(), layer_pairs[0].pending);
-    EXPECT_EQ(NULL, layer_pairs[0].active);
-  }
+  host_impl_->ActivateSyncTree();
 
-  if (layer_pairs[1].active) {
-    EXPECT_EQ(active_layer.get(), layer_pairs[1].active);
-    EXPECT_EQ(NULL, layer_pairs[1].pending);
-  } else {
-    EXPECT_EQ(pending_layer.get(), layer_pairs[1].pending);
-    EXPECT_EQ(NULL, layer_pairs[1].active);
-  }
+  LayerTreeImpl* active_tree = host_impl_->active_tree();
+  LayerImpl* active_layer = active_tree->root_layer();
+  EXPECT_NE(active_tree, pending_tree);
+  EXPECT_NE(active_layer, pending_layer);
+  EXPECT_NE(nullptr, active_tree);
+  EXPECT_NE(nullptr, active_layer);
 
-  active_layer->set_twin_layer(pending_layer.get());
-  pending_layer->set_twin_layer(active_layer.get());
+  host_impl_->CreatePendingTree();
 
   layer_pairs.clear();
   host_impl_->GetPictureLayerImplPairs(&layer_pairs);
   EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(active_layer, layer_pairs[0].active);
+  EXPECT_EQ(pending_layer, layer_pairs[0].pending);
 
-  EXPECT_EQ(active_layer.get(), layer_pairs[0].active);
-  EXPECT_EQ(pending_layer.get(), layer_pairs[0].pending);
+  // Activate, the active layer has no twin now.
+  host_impl_->ActivateSyncTree();
+
+  layer_pairs.clear();
+  host_impl_->GetPictureLayerImplPairs(&layer_pairs);
+  EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(active_layer, layer_pairs[0].active);
+  EXPECT_EQ(nullptr, layer_pairs[0].pending);
 }
 
 TEST_F(LayerTreeHostImplTest, DidBecomeActive) {
   host_impl_->CreatePendingTree();
   host_impl_->ActivateSyncTree();
   host_impl_->CreatePendingTree();
 
   LayerTreeImpl* pending_tree = host_impl_->pending_tree();
 
   scoped_ptr<FakePictureLayerImpl> pending_layer =
@@ skipping 55 matching lines @@
   // surface.
   EXPECT_EQ(0, num_lost_surfaces_);
   host_impl_->DidLoseOutputSurface();
   EXPECT_EQ(1, num_lost_surfaces_);
   host_impl_->DidLoseOutputSurface();
   EXPECT_LE(1, num_lost_surfaces_);
 }
 
 }  // namespace
 }  // namespace cc