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

src/global-handles.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/api.h"
 #include "src/global-handles.h"
 
 #include "src/vm-state-inl.h"
@@ skipping 128 matching lines @@
     flags_ = IsPartiallyDependent::update(flags_, v);
   }
 
   bool is_in_new_space_list() {
     return IsInNewSpaceList::decode(flags_);
   }
   void set_in_new_space_list(bool v) {
     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;
   }
 
   bool IsWeak() const { return state() == WEAK; }
 
   bool IsRetainer() const { return state() != FREE; }
 
   bool IsStrongRetainer() const { return state() == NORMAL; }
@@ skipping 38 matching lines @@
   // Accessors for next free node in the free list.
   Node* next_free() {
     DCHECK(state() == FREE);
     return parameter_or_next_free_.next_free;
   }
   void set_next_free(Node* value) {
     DCHECK(state() == FREE);
     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;
   }
 
   void* ClearWeakness() {
     DCHECK(state() != FREE);
     void* p = parameter();
     set_state(NORMAL);
     set_parameter(NULL);
     return p;
   }
 
   bool PostGarbageCollectionProcessing(Isolate* isolate) {
     if (state() != Node::PENDING) return false;
     if (weak_callback_ == NULL) {
       Release();
       return false;
     }
-    void* par = parameter();
+    void* param = parameter();
     set_state(NEAR_DEATH);
     set_parameter(NULL);
 
     Object** object = location();
     {
       // Check that we are not passing a finalized external string to
       // the callback.
       DCHECK(!object_->IsExternalOneByteString() ||
              ExternalOneByteString::cast(object_)->resource() != NULL);
       DCHECK(!object_->IsExternalTwoByteString() ||
              ExternalTwoByteString::cast(object_)->resource() != NULL);
       // Leaving V8.
-      VMState<EXTERNAL> state(isolate);
+      VMState<EXTERNAL> vmstate(isolate);
       HandleScope handle_scope(isolate);
-      Handle<Object> handle(*object, isolate);
-      v8::WeakCallbackData<v8::Value, void> data(
-          reinterpret_cast<v8::Isolate*>(isolate),
-          v8::Utils::ToLocal(handle),
-          par);
-      weak_callback_(data);
+      if (is_zapped_during_weak_callback()) {
+        // Phantom weak pointer case.
+        DCHECK(*object == Smi::FromInt(kPhantomReferenceZap));
+        // Make data with a null handle.
+        v8::WeakCallbackData<v8::Value, void> data(
+            reinterpret_cast<v8::Isolate*>(isolate), v8::Local<v8::Object>(),
+            param);
+        weak_callback_(data);
+        if (state() != FREE) {
+          // Callback does not have to clear the global handle if it is a
+          // phantom handle.
+          Release();
+        }
+      } else {
+        Handle<Object> handle(*object, isolate);

[Erik Corry, 2014/11/02 18:41:45]

This CL is a reland of https://codereview.chromium.org/649563006/

This line has been changed from

Handle<Object> handle(object).

That caused the new handle to have the same backing global handle slot, which
didn't work with Blink, where some of the bindings reset the GetParameter handle
and then expect to still be able to use the GetValue handle.

I added a new test in test-api.cc, so that V8's own tests can catch this in
future (ResetWeakHandle).

+        v8::WeakCallbackData<v8::Value, void> data(
+            reinterpret_cast<v8::Isolate*>(isolate), v8::Utils::ToLocal(handle),
+            param);
+        weak_callback_(data);
+      }
     }
     // Absence of explicit cleanup or revival of weak handle
     // in most of the cases would lead to memory leak.
     CHECK(state() != NEAR_DEATH);
     return true;
   }
 
   inline GlobalHandles* GetGlobalHandles();
 
  private:
   inline NodeBlock* FindBlock();
   inline void IncreaseBlockUses();
   inline void DecreaseBlockUses();
 
   // Storage for object pointer.
   // Placed first to avoid offset computation.
   Object* object_;
 
   // Next word stores class_id, index, state, and independent.
   // Note: the most aligned fields should go first.
 
   // Wrapper class ID.
   uint16_t class_id_;
 
   // Index in the containing handle block.
   uint8_t index_;
 
   // 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_;
 
   // Handle specific callback - might be a weak reference in disguise.
   WeakCallback weak_callback_;
 
   // Provided data for callback.  In FREE state, this is used for
   // the free list link.
   union {
     void* parameter;
@@ skipping 174 matching lines @@
   DCHECK(location != NULL);
   return Node::FromLocation(location)->GetGlobalHandles()->Create(*location);
 }
 
 
 void GlobalHandles::Destroy(Object** location) {
   if (location != NULL) Node::FromLocation(location)->Release();
 }
 
 
-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);
 }
 
 
 void* GlobalHandles::ClearWeakness(Object** location) {
   return Node::FromLocation(location)->ClearWeakness();
 }
 
 
 void GlobalHandles::MarkIndependent(Object** location) {
   Node::FromLocation(location)->MarkIndependent();
@@ skipping 15 matching lines @@
 }
 
 
 bool GlobalHandles::IsWeak(Object** location) {
   return Node::FromLocation(location)->IsWeak();
 }
 
 
 void GlobalHandles::IterateWeakRoots(ObjectVisitor* v) {
   for (NodeIterator it(this); !it.done(); it.Advance()) {
-    if (it.node()->IsWeakRetainer()) v->VisitPointer(it.node()->location());
+    Node* node = it.node();
+    if (node->IsWeakRetainer()) {
+      if (node->state() == Node::PENDING &&
+          node->is_zapped_during_weak_callback()) {
+        *(node->location()) = Smi::FromInt(kPhantomReferenceZap);
+      } else {
+        v->VisitPointer(node->location());
+      }
+    }
   }
 }
 
 
 void GlobalHandles::IdentifyWeakHandles(WeakSlotCallback f) {
   for (NodeIterator it(this); !it.done(); it.Advance()) {
     if (it.node()->IsWeak() && f(it.node()->location())) {
       it.node()->MarkPending();
     }
   }
@@ skipping 24 matching lines @@
   }
 }
 
 
 void GlobalHandles::IterateNewSpaceWeakIndependentRoots(ObjectVisitor* v) {
   for (int i = 0; i < new_space_nodes_.length(); ++i) {
     Node* node = new_space_nodes_[i];
     DCHECK(node->is_in_new_space_list());
     if ((node->is_independent() || node->is_partially_dependent()) &&
         node->IsWeakRetainer()) {
-      v->VisitPointer(node->location());
+      if (node->is_zapped_during_weak_callback()) {
+        *(node->location()) = Smi::FromInt(kPhantomReferenceZap);
+      } else {
+        v->VisitPointer(node->location());
+      }
     }
   }
 }
 
 
 bool GlobalHandles::IterateObjectGroups(ObjectVisitor* v,
                                         WeakSlotCallbackWithHeap can_skip) {
   ComputeObjectGroupsAndImplicitReferences();
   int last = 0;
   bool any_group_was_visited = false;
@@ skipping 474 matching lines @@
   DCHECK_EQ(isolate->heap()->the_hole_value(), blocks_[block][offset]);
   blocks_[block][offset] = object;
   if (isolate->heap()->InNewSpace(object)) {
     new_space_indices_.Add(size_);
   }
   *index = size_++;
 }
 
 
 } }  // namespace v8::internal