Nested <use>s can resolve against an external resource

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

Index: third_party/WebKit/Source/core/svg/SVGUseElement.cpp
diff --git a/third_party/WebKit/Source/core/svg/SVGUseElement.cpp b/third_party/WebKit/Source/core/svg/SVGUseElement.cpp
index 8577d27b879bb731073368583540b5c9d901e0e3..0f377678a831344253600b8021db28192d1dd1dd 100644
--- a/third_party/WebKit/Source/core/svg/SVGUseElement.cpp
+++ b/third_party/WebKit/Source/core/svg/SVGUseElement.cpp
@@ -298,10 +298,12 @@ void SVGUseElement::ClearResourceReference() {
   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,
@@ -321,7 +323,7 @@ void SVGUseElement::BuildPendingResource() {
   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));
@@ -554,11 +556,10 @@ void SVGUseElement::CloneNonMarkupEventListeners() {
   }
 }
 
-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);