Nested <use>s can resolve against an external resource

third_party/WebKit/Source/core/svg/SVGUseElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Nikolas Zimmermann
  * <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) Research In Motion Limited 2009-2010. All rights reserved.
  * Copyright (C) 2011 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012 University of Szeged
  * Copyright (C) 2012 Renata Hodovan <reni@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 280 matching lines @@
 void SVGUseElement::ClearInstanceRoot() {
   target_element_instance_ = nullptr;
 }
 
 void SVGUseElement::ClearResourceReference() {
   UnobserveTarget(target_id_observer_);
   ClearInstanceRoot();
   RemoveAllOutgoingReferences();
 }
 
-Element* SVGUseElement::ResolveTargetElement() {
+Element* SVGUseElement::ResolveTargetElement(ObserveBehavior observe_behavior) {
   if (element_identifier_.IsEmpty())
     return nullptr;
   if (element_identifier_is_local_) {
+    if (observe_behavior == kDontAddObserver)
+      return GetTreeScope().getElementById(element_identifier_);
     return ObserveTarget(target_id_observer_, GetTreeScope(),
                          element_identifier_,
                          WTF::Bind(&SVGUseElement::InvalidateShadowTree,
                                    WrapWeakPersistent(this)));
   }
   if (!ResourceIsValid())
     return nullptr;
   return resource_->GetDocument()->getElementById(element_identifier_);
 }
 
 void SVGUseElement::BuildPendingResource() {
   if (InUseShadowTree())
     return;
   // FIXME: We should try to optimize this, to at least allow partial reclones.
   UseShadowRoot().RemoveChildren(kOmitSubtreeModifiedEvent);
   ClearResourceReference();
   CancelShadowTreeRecreation();
   if (!isConnected())
     return;
-  Element* target = ResolveTargetElement();
+  Element* target = ResolveTargetElement(kAddObserver);
   // TODO(fs): Why would the Element not be "connected" at this point?
   if (target && target->isConnected() && target->IsSVGElement()) {
     BuildShadowAndInstanceTree(ToSVGElement(*target));
     InvalidateDependentShadowTrees();
   }
 
   DCHECK(!needs_shadow_tree_recreation_);
 }
 
 String SVGUseElement::title() const {
@@ skipping 212 matching lines @@
   for (SVGElement& element :
        Traversal<SVGElement>::DescendantsOf(UseShadowRoot())) {
     if (EventTargetData* data =
             element.CorrespondingElement()->GetEventTargetData()) {
       data->event_listener_map.CopyEventListenersNotCreatedFromMarkupToTarget(
           &element);
     }
   }
 }
 
-bool SVGUseElement::HasCycleUseReferencing(const SVGUseElement& use,
+bool SVGUseElement::HasCycleUseReferencing(SVGUseElement& use,
                                            const ContainerNode& target_instance,
                                            SVGElement*& new_target) const {
-  Element* target_element =
-      TargetElementFromIRIString(use.HrefString(), use.GetTreeScope());
+  Element* target_element = use.ResolveTargetElement(kDontAddObserver);

[pdr., 2017/05/13 03:10:50]

Can you help me understand why we don't want to add an observer? Isn't an
observer used to catch changes in the used element? If so, would we fail to get
updates if #rect-external changes?

[fs, 2017/05/13 08:24:06]

We don't want to add an observer, because the element itself (also a <use>) will
have done that already (or will) when it receives its "primary" (non-nested)
expansion (BuildPendingResource gets called on it.)

   new_target = 0;
   if (target_element && target_element->IsSVGElement())
     new_target = ToSVGElement(target_element);
 
   if (!new_target)
     return false;
 
   // Shortcut for self-references
   if (new_target == this)
     return true;
@@ skipping 174 matching lines @@
 
   if (resource_)
     resource_->RemoveClient(this);
 
   resource_ = resource;
   if (resource_)
     resource_->AddClient(this);
 }
 
 }  // namespace blink