Move the Widget hierarchy to the Oilpan heap.

Source/core/rendering/RenderScrollbar.cpp

 /*
  * Copyright (C) 2008, 2009 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 18 matching lines @@
 #include "core/css/PseudoStyleRequest.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/rendering/RenderPart.h"
 #include "core/rendering/RenderScrollbarPart.h"
 #include "core/rendering/RenderScrollbarTheme.h"
 #include "platform/graphics/GraphicsContext.h"
 
 namespace blink {
 
-PassRefPtr<Scrollbar> RenderScrollbar::createCustomScrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, Node* ownerNode, LocalFrame* owningFrame)
+PassRefPtrWillBeRawPtr<Scrollbar> RenderScrollbar::createCustomScrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, Node* ownerNode, LocalFrame* owningFrame)
 {
-    return adoptRef(new RenderScrollbar(scrollableArea, orientation, ownerNode, owningFrame));
+    return adoptRefWillBeNoop(new RenderScrollbar(scrollableArea, orientation, ownerNode, owningFrame));
 }
 
 RenderScrollbar::RenderScrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, Node* ownerNode, LocalFrame* owningFrame)
     : Scrollbar(scrollableArea, orientation, RegularScrollbar, RenderScrollbarTheme::renderScrollbarTheme())
     , m_owner(ownerNode)
     , m_owningFrame(owningFrame)
 {
     ASSERT(ownerNode || owningFrame);
 
     // FIXME: We need to do this because RenderScrollbar::styleChanged is called as soon as the scrollbar is created.
 
     // Update the scrollbar size.
     int width = 0;
     int height = 0;
     updateScrollbarPart(ScrollbarBGPart);
     if (RenderScrollbarPart* part = m_parts.get(ScrollbarBGPart)) {
         part->layout();
         width = part->width();
         height = part->height();
     } else if (this->orientation() == HorizontalScrollbar)
         width = this->width();
     else
         height = this->height();
 
     setFrameRect(IntRect(0, 0, width, height));
+
+#if ENABLE(OILPAN)
+    ThreadState::current()->registerPreFinalizer(*this);
+#endif
 }
 
 RenderScrollbar::~RenderScrollbar()
 {
-    if (!m_parts.isEmpty()) {
-        // When a scrollbar is detached from its parent (causing all parts removal) and
-        // ready to be destroyed, its destruction can be delayed because of RefPtr
-        // maintained in other classes such as EventHandler (m_lastScrollbarUnderMouse).
-        // Meanwhile, we can have a call to updateScrollbarPart which recreates the
-        // scrollbar part. So, we need to destroy these parts since we don't want them
-        // to call on a destroyed scrollbar. See webkit bug 68009.
-        updateScrollbarParts(true);
-    }
+    // Oilpan: to be able to access the hash map that's
+    // also on the heap, a pre-destruction finalizer is used.
+#if !ENABLE(OILPAN)
+    destroyParts();
+#endif
+}
+
+void RenderScrollbar::destroyParts()
+{
+    if (m_parts.isEmpty())
+        return;
+
+    // When a scrollbar is detached from its parent (causing all parts removal) and
+    // ready to be destroyed, its destruction can be delayed because of RefPtr
+    // maintained in other classes such as EventHandler (m_lastScrollbarUnderMouse).
+    // Meanwhile, we can have a call to updateScrollbarPart which recreates the
+    // scrollbar part. So, we need to destroy these parts since we don't want them
+    // to call on a destroyed scrollbar. See webkit bug 68009.
+    //

[haraken, 2014/10/11 17:33:03]

Unnecessary comment.

[sof, 2014/10/12 08:16:22]

Removed the empty comment line.

+    updateScrollbarParts(true);

[haraken, 2014/10/11 17:33:03]

Are you sure that updateScrollbarParts() doesn't do anything that changes an
object graph (because updateScrollbarParts() is called during a pre-finalizer)?

[sof, 2014/10/12 08:16:22]

Updating the parts heap hash map is handled fine, I'm (now) assuming.

For the destroy() call that this triggers on one or more RenderScrollbarPart
objects, I can only see values being cleared.

[haraken, 2014/10/12 12:56:04]

OK, thanks for the verification. I'm worry about a complicated pre-finalizer
because there is no way to verify that the pre-finalizer isn't doing something
forbidden :)

+}
+
+void RenderScrollbar::trace(Visitor* visitor)
+{
+#if ENABLE(OILPAN)
+    visitor->trace(m_owner);
+    visitor->trace(m_owningFrame);
+    visitor->trace(m_parts);
+#endif
+    Scrollbar::trace(visitor);
 }
 
 RenderBox* RenderScrollbar::owningRenderer() const
 {
     if (m_owningFrame) {
         RenderBox* currentRenderer = m_owningFrame->ownerRenderer();
         return currentRenderer;
     }
     return m_owner && m_owner->renderer() ? m_owner->renderer()->enclosingBox() : 0;
 }
@@ skipping 244 matching lines @@
 int RenderScrollbar::minimumThumbLength()
 {
     RenderScrollbarPart* partRenderer = m_parts.get(ThumbPart);
     if (!partRenderer)
         return 0;
     partRenderer->layout();
     return orientation() == HorizontalScrollbar ? partRenderer->width() : partRenderer->height();
 }
 
 }