two pass phantom collection

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 1b1ab19c74d203e45474f0eb3a37242246722994..9827909552b63cdf231f9cd337c588f468f3e0be 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -479,11 +479,13 @@ class WeakCallbackInfo {
  public:
   typedef void (*Callback)(const WeakCallbackInfo<T>& data);
 
-  WeakCallbackInfo(Isolate* isolate, T* parameter, void* internal_field1,
-                   void* internal_field2)
-      : isolate_(isolate), parameter_(parameter) {
-    internal_fields_[0] = internal_field1;
-    internal_fields_[1] = internal_field2;
+  WeakCallbackInfo(Isolate* isolate, T* parameter,
+                   void* internal_fields[kInternalFieldsInWeakCallback],
+                   Callback* callback)
+      : isolate_(isolate), parameter_(parameter), callback_(callback) {
+    for (int i = 0; i < kInternalFieldsInWeakCallback; ++i) {
+      internal_fields_[i] = internal_fields[i];
+    }
   }
 
   V8_INLINE Isolate* GetIsolate() const { return isolate_; }
@@ -499,10 +501,17 @@ class WeakCallbackInfo {
     return internal_fields_[1];
   }
 
+  bool IsFirstPass() const { return callback_ != nullptr; }
+  // If needed, set the callback for the second pass, even if it's the same one.
+  // Setting this to nullptr is unecessary and means there will be no second

[Erik Corry, 2015/03/26 10:07:10]

Comment is a bit confusing.  How about:

The default is nullptr, which means there will be no second pass.

Also, since this is part of V8's official API I think a bit more explanation is
in order.  As I understand it, this structure is passed to the first callback,
and the embedder can arrange for a second callback.  When is the second callback
called?

[dcarney, 2015/03/26 10:26:56]

Done.

+  // pass.  Setting this on the second pass will crash.
+  void SetSecondPassCallback(Callback callback) const { *callback_ = callback; }
+
  private:
   Isolate* isolate_;
   T* parameter_;
-  void* internal_fields_[2];
+  Callback* callback_;
+  void* internal_fields_[kInternalFieldsInWeakCallback];
 };