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

cc/layers/picture_layer_impl.cc

 // Copyright 2012 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/layers/picture_layer_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <set>
 
@@ skipping 72 matching lines @@
       low_res_raster_contents_scale_(0.f),
       raster_source_scale_is_fixed_(false),
       was_screen_space_transform_animating_(false),
       needs_post_commit_initialization_(true),
       should_update_tile_priorities_(false),
       only_used_low_res_last_append_quads_(false) {
   layer_tree_impl()->RegisterPictureLayerImpl(this);
 }
 
 PictureLayerImpl::~PictureLayerImpl() {
+  if (twin_layer_)
+    twin_layer_->twin_layer_ = nullptr;
   layer_tree_impl()->UnregisterPictureLayerImpl(this);
 }
 
 const char* PictureLayerImpl::LayerTypeAsString() const {
   return "cc::PictureLayerImpl";
 }
 
 scoped_ptr<LayerImpl> PictureLayerImpl::CreateLayerImpl(
     LayerTreeImpl* tree_impl) {
   return PictureLayerImpl::Create(tree_impl, id());
 }
 
 void PictureLayerImpl::PushPropertiesTo(LayerImpl* base_layer) {
   // It's possible this layer was never drawn or updated (e.g. because it was
   // a descendant of an opacity 0 layer).
   DoPostCommitInitializationIfNeeded();
   PictureLayerImpl* layer_impl = static_cast<PictureLayerImpl*>(base_layer);
 
   LayerImpl::PushPropertiesTo(base_layer);
 
-  // When the pending tree pushes to the active tree, the pending twin
-  // becomes recycled.
-  layer_impl->twin_layer_ = nullptr;
-  twin_layer_ = nullptr;
+  // Twin relationships should never change once established.
+  DCHECK_IMPLIES(twin_layer_, twin_layer_ == layer_impl);
+  DCHECK_IMPLIES(twin_layer_, layer_impl->twin_layer_ == this);
+  twin_layer_ = layer_impl;

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

Can you make a comment here explaining why we don't need the twin relationship
before this push properties? My understanding is that before twins are
established we're in a situation where the active layer has no tilings/tiles, so
it won't access the twin for anything until we push. Is that correct?

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

Ya, before this the only thing we do is create the layer with tree synchronizer
and it knows nothing of picture-ness.

+  layer_impl->twin_layer_ = this;
 
   layer_impl->pile_ = pile_;
 
   DCHECK(!pile_->is_solid_color() || !tilings_->num_tilings());
   // Tilings would be expensive to push, so we swap.
   layer_impl->tilings_.swap(tilings_);
   layer_impl->tilings_->SetClient(layer_impl);
   if (tilings_)
     tilings_->SetClient(this);
 
@@ skipping 427 matching lines @@
     if (screen_space_transform().GetInverse(&view_to_layer)) {
       // Transform from view space to content space.
       visible_rect_in_content_space =
           gfx::ToEnclosingRect(MathUtil::ProjectClippedRect(
               view_to_layer, viewport_rect_for_tile_priority));
     }
   }
   return visible_rect_in_content_space;
 }
 
-PictureLayerImpl* PictureLayerImpl::GetRecycledTwinLayer() {
-  // TODO(vmpstr): Maintain recycled twin as a member. crbug.com/407418
-  return static_cast<PictureLayerImpl*>(
-      layer_tree_impl()->FindRecycleTreeLayerById(id()));
+PictureLayerImpl* PictureLayerImpl::GetPendingOrActiveTwinLayer() const {
+  if (!twin_layer_ || !twin_layer_->IsOnActiveOrPendingTree())
+    return nullptr;
+  return twin_layer_;
+}
+
+PictureLayerImpl* PictureLayerImpl::GetRecycledTwinLayer() const {
+  if (!twin_layer_ || twin_layer_->IsOnActiveOrPendingTree())
+    return nullptr;
+  return twin_layer_;
 }
 
 void PictureLayerImpl::NotifyTileStateChanged(const Tile* tile) {
   if (layer_tree_impl()->IsActiveTree()) {
     gfx::RectF layer_damage_rect =
         gfx::ScaleRect(tile->content_rect(), 1.f / tile->contents_scale());
     AddDamageRect(layer_damage_rect);
   }
 }
 
@@ skipping 51 matching lines @@
 }
 
 PicturePileImpl* PictureLayerImpl::GetPile() {
   return pile_.get();
 }
 
 const Region* PictureLayerImpl::GetInvalidation() {
   return &invalidation_;
 }
 
 const PictureLayerTiling* PictureLayerImpl::GetTwinTiling(

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

nit: This potentially should be GetActiveOrPendingTwinTiling (or GetXTwinTiling
where X is a single word that captures the fact that it's not recycled). Feel
free to ignore this though, since it already is the way it is.

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

Hm, it's part of the PLTClient API. I can change it though.

     const PictureLayerTiling* tiling) const {
-  if (!twin_layer_)
+  PictureLayerImpl* twin_layer = GetPendingOrActiveTwinLayer();
+  if (!twin_layer)
     return nullptr;
-  return twin_layer_->tilings_->TilingAtScale(tiling->contents_scale());
+  return twin_layer->tilings_->TilingAtScale(tiling->contents_scale());
 }
 
 PictureLayerTiling* PictureLayerImpl::GetRecycledTwinTiling(
     const PictureLayerTiling* tiling) {
   PictureLayerImpl* recycled_twin = GetRecycledTwinLayer();
   if (!recycled_twin || !recycled_twin->tilings_)
     return nullptr;
   return recycled_twin->tilings_->TilingAtScale(tiling->contents_scale());
 }
 
@@ skipping 184 matching lines @@
   *resource_size = iter.texture_size();
 }
 
 void PictureLayerImpl::DoPostCommitInitialization() {
   DCHECK(needs_post_commit_initialization_);
   DCHECK(layer_tree_impl()->IsPendingTree());
 
   if (!tilings_)
     tilings_.reset(new PictureLayerTilingSet(this, bounds()));
 
-  DCHECK(!twin_layer_);
-  twin_layer_ = static_cast<PictureLayerImpl*>(
-      layer_tree_impl()->FindActiveTreeLayerById(id()));
-  if (twin_layer_) {
-    DCHECK(!twin_layer_->twin_layer_);
-    twin_layer_->twin_layer_ = this;
+  PictureLayerImpl* twin_layer = GetPendingOrActiveTwinLayer();
+  if (twin_layer) {
     // If the twin has never been pushed to, do not sync from it.
     // This can happen if this function is called during activation.
-    if (!twin_layer_->needs_post_commit_initialization_)
-      SyncFromActiveLayer(twin_layer_);
+    if (!twin_layer->needs_post_commit_initialization_)
+      SyncFromActiveLayer(twin_layer);
   }
 
   needs_post_commit_initialization_ = false;
 }
 
 PictureLayerTiling* PictureLayerImpl::AddTiling(float contents_scale) {
   DCHECK(CanHaveTilingWithScale(contents_scale)) <<
       "contents_scale: " << contents_scale;
 
   PictureLayerTiling* tiling = tilings_->AddTiling(contents_scale);
 
   DCHECK(pile_->HasRecordings());
 
-  if (twin_layer_)
-    twin_layer_->SyncTiling(tiling);
+  if (PictureLayerImpl* twin_layer = GetPendingOrActiveTwinLayer())
+    twin_layer->SyncTiling(tiling);
 
   return tiling;
 }
 
 void PictureLayerImpl::RemoveTiling(float contents_scale) {
   if (!tilings_ || tilings_->num_tilings() == 0)
     return;
 
   for (size_t i = 0; i < tilings_->num_tilings(); ++i) {
     PictureLayerTiling* tiling = tilings_->tiling_at(i);
@@ skipping 223 matching lines @@
   DCHECK(layer_tree_impl()->IsActiveTree());
   if (tilings_->num_tilings() == 0)
     return;
 
   float min_acceptable_high_res_scale = std::min(
       raster_contents_scale_, ideal_contents_scale_);
   float max_acceptable_high_res_scale = std::max(
       raster_contents_scale_, ideal_contents_scale_);
   float twin_low_res_scale = 0.f;
 
-  PictureLayerImpl* twin = twin_layer_;
+  PictureLayerImpl* twin = GetPendingOrActiveTwinLayer();
   if (twin && twin->CanHaveTilings()) {
     min_acceptable_high_res_scale = std::min(
         min_acceptable_high_res_scale,
         std::min(twin->raster_contents_scale_, twin->ideal_contents_scale_));
     max_acceptable_high_res_scale = std::max(
         max_acceptable_high_res_scale,
         std::max(twin->raster_contents_scale_, twin->ideal_contents_scale_));
 
     for (size_t i = 0; i < twin->tilings_->num_tilings(); ++i) {
       PictureLayerTiling* tiling = twin->tilings_->tiling_at(i);
@@ skipping 549 matching lines @@
       PictureLayerTilingSet::TilingRange tiling_range = CurrentTilingRange();
       size_t current_tiling_range_offset = current_tiling_ - tiling_range.start;
       return tiling_range.end - 1 - current_tiling_range_offset;
     }
   }
   NOTREACHED();
   return 0;
 }
 
 }  // namespace cc