Introduce phantom weak handles in the API and use them internally for debug info

src/global-handles.cc

Index: src/global-handles.cc
diff --git a/src/global-handles.cc b/src/global-handles.cc
index 282ca2d5d86a63d86dd00b1eb4b43888c3289e6b..44abef7517b2bc6a0a2f1d276acb50441a9f602c 100644
--- a/src/global-handles.cc
+++ b/src/global-handles.cc
@@ -146,6 +146,13 @@ class GlobalHandles::Node {
     flags_ = IsInNewSpaceList::update(flags_, v);
   }
 
+  bool is_zapped_during_weak_callback() {
+    return IsZappedDuringWeakCallback::decode(flags_);
+  }
+  void set_is_zapped_during_weak_callback(bool v) {
+    flags_ = IsZappedDuringWeakCallback::update(flags_, v);
+  }
+
   bool IsNearDeath() const {
     // Check for PENDING to ensure correct answer when processing callbacks.
     return state() == PENDING || state() == NEAR_DEATH;
@@ -204,12 +211,14 @@ class GlobalHandles::Node {
     parameter_or_next_free_.next_free = value;
   }
 
-  void MakeWeak(void* parameter, WeakCallback weak_callback) {
+  void MakeWeak(void* parameter, WeakCallback weak_callback,
+                bool is_zapped_during_weak_callback = false) {
     DCHECK(weak_callback != NULL);
     DCHECK(state() != FREE);
     CHECK(object_ != NULL);
     set_state(WEAK);
     set_parameter(parameter);
+    set_is_zapped_during_weak_callback(is_zapped_during_weak_callback);
     weak_callback_ = weak_callback;
   }
 
@@ -227,7 +236,7 @@ class GlobalHandles::Node {
       Release();
       return false;
     }
-    void* par = parameter();
+    void* param = parameter();
     set_state(NEAR_DEATH);
     set_parameter(NULL);
 
@@ -243,10 +252,12 @@ class GlobalHandles::Node {
       VMState<EXTERNAL> state(isolate);
       HandleScope handle_scope(isolate);
       Handle<Object> handle(*object, isolate);
+      if (is_zapped_during_weak_callback()) {
+        DCHECK(*handle == Smi::FromInt(kPhantomReferenceZap));
+      }
       v8::WeakCallbackData<v8::Value, void> data(
-          reinterpret_cast<v8::Isolate*>(isolate),
-          v8::Utils::ToLocal(handle),
-          par);
+          reinterpret_cast<v8::Isolate*>(isolate), v8::Utils::ToLocal(handle),
+          param);
       weak_callback_(data);
     }
     // Absence of explicit cleanup or revival of weak handle
@@ -277,10 +288,11 @@ class GlobalHandles::Node {
 
   // This stores three flags (independent, partially_dependent and
   // in_new_space_list) and a State.
-  class NodeState:            public BitField<State, 0, 4> {};
-  class IsIndependent:        public BitField<bool,  4, 1> {};
-  class IsPartiallyDependent: public BitField<bool,  5, 1> {};
-  class IsInNewSpaceList:     public BitField<bool,  6, 1> {};
+  class NodeState : public BitField<State, 0, 4> {};
+  class IsIndependent : public BitField<bool, 4, 1> {};
+  class IsPartiallyDependent : public BitField<bool, 5, 1> {};
+  class IsInNewSpaceList : public BitField<bool, 6, 1> {};
+  class IsZappedDuringWeakCallback : public BitField<bool, 7, 1> {};
 
   uint8_t flags_;
 
@@ -475,10 +487,10 @@ void GlobalHandles::Destroy(Object** location) {
 }
 
 
-void GlobalHandles::MakeWeak(Object** location,
-                             void* parameter,
-                             WeakCallback weak_callback) {
-  Node::FromLocation(location)->MakeWeak(parameter, weak_callback);
+void GlobalHandles::MakeWeak(Object** location, void* parameter,
+                             WeakCallback weak_callback, PhantomState phantom) {
+  Node::FromLocation(location)
+      ->MakeWeak(parameter, weak_callback, phantom == Phantom);
 }
 
 
@@ -514,7 +526,14 @@ bool GlobalHandles::IsWeak(Object** location) {
 
 void GlobalHandles::IterateWeakRoots(ObjectVisitor* v) {
   for (NodeIterator it(this); !it.done(); it.Advance()) {
-    if (it.node()->IsWeakRetainer()) v->VisitPointer(it.node()->location());
+    if (it.node()->IsWeakRetainer()) {
+      if (it.node()->state() == Node::PENDING &&
+          it.node()->is_zapped_during_weak_callback()) {
+        *(it.node()->location()) = Smi::FromInt(kPhantomReferenceZap);
+      } else {
+        v->VisitPointer(it.node()->location());
+      }
+    }
   }
 }