Make wrapper preservation (the mechanism that makes the GC use reachability information between certain C++ objects rather than rooting at language boundaries) use an interface (nsIDOMGCParticipant) to get reachability information. Preserve the wrappers for event handlers as long as what they are attached to is reachable (from C++ or JS) to avoid entraining event handler closures in cycles. b=241518 r=mrbkap sr=jst

content/base/src/nsGenericElement.cpp

@@ -1017,6 +1017,37 @@ nsGenericElement::InitHashes()
   return NS_OK;
 }
 
+/**
+ * During the Mark phase of the GC, we need to mark all of the preserved
+ * wrappers that are reachable via DOM APIs.  Since reachability for DOM
+ * nodes is symmetric, if one DOM node is reachable from another via DOM
+ * APIs, then they are in the same strongly connected component.
+ * (Strongly connected components are never reachable from each other
+ * via DOM APIs.)  We can refer to each strongly connected component by
+ * walking up to the top of the parent chain.  This function finds that
+ * root node for any DOM node.
+ */
+nsIDOMGCParticipant*
+nsGenericElement::GetSCCIndex()
+{
+  // This is an optimized way of walking nsIDOMNode::GetParentNode to
+  // the top of the tree.
+  nsIDOMGCParticipant *result = GetCurrentDoc();
+  if (!result) {
+    nsIContent *top = this;
+    while (top->GetParent())
+      top = top->GetParent();
+    result = top;
+  }
+
+  return result;
+}
+
+void
+nsGenericElement::AppendReachableList(nsCOMArray<nsIDOMGCParticipant>& aArray)
+{
+}
+
 NS_IMETHODIMP
 nsGenericElement::GetNodeName(nsAString& aNodeName)
 {
@@ -3694,6 +3725,7 @@ nsGenericElement::RemoveChild(nsIDOMNode *aOldChild, nsIDOMNode **aReturn)
 
 NS_INTERFACE_MAP_BEGIN(nsGenericElement)
   NS_INTERFACE_MAP_ENTRY(nsIContent)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMGCParticipant)
   NS_INTERFACE_MAP_ENTRY_TEAROFF(nsIDOM3Node, new nsNode3Tearoff(this))
   NS_INTERFACE_MAP_ENTRY_TEAROFF(nsIDOMEventReceiver,
                                  nsDOMEventRTTearoff::Create(this))