Introduce EmbedderHeapTracer

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index db2178b73becb2c7ad76a5f28e8b81e4c14d1c33..053010e7bde738dc87302704d54055c86d281a21 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5361,6 +5361,53 @@ class V8_EXPORT PersistentHandleVisitor {  // NOLINT
 enum class MemoryPressureLevel { kNone, kModerate, kCritical };
 
 /**
+ * Structure representing hidden fields of the v8 object. Embedder sets these
+ * fields on wrapper creation to be able to access the object wrapped by the
+ * wrapper.
+ */
+struct WrapperHiddenFields {

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

why not just std::pair<void*,void*>?

[Marcel Hlopko, 2016/03/30 18:51:12]

Because I didn't know it exists :) Done.

+  WrapperHiddenFields(void* wrapper_info_ptr, void* wrapper_source_ptr)
+      : wrapper_info(wrapper_info_ptr), wrapper_source(wrapper_source_ptr) {}
+  void* wrapper_info;
+  void* wrapper_source;
+};
+
+/**
+ * Interface for tracing through the embedder heap. During the v8 garbage
+ * collection, v8 collects hidden fields of all potential wrappers, and at the
+ * end of its marking phase iterates the collection and asks the embedder to
+ * trace through its heap and call Isolate::AddObjectToMarkingDeque for each js
+ * object reachable from the given one.
+ *
+ * Before the first call to the TraceWrappableFrom function v8 will call
+ * TraceRoots. When the v8 garbage collection is finished, v8 will call
+ * ClearTracingMarks.
+ */
+class EmbedderHeapTracer {
+ public:
+  /**
+   * V8 will call this method at the beginning of the gc cycle.
+   */
+  virtual void TraceRoots(Isolate* isolate) {}

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

= 0;

also add an empty line after this one

[Marcel Hlopko, 2016/03/30 18:51:12]

done

+  /**
+   * V8 will call this method with internal fields of a potential wrapper.
+   * Embedder is expected to trace its heap (synchronously) and call
+   * Isolate::AddObjectToMarkingDeque() with all wrappers reachable from the
+   * given one.
+   */
+  virtual void TraceWrappableFrom(Isolate* isolate,
+                                  WrapperHiddenFields internal_fields) {}

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

same here.

why not pass a const std::vector<HiddenFields>& ?

[Marcel Hlopko, 2016/03/30 18:51:12]

Done

+  /**
+   * V8 will call this method at the end of the gc cycle. Allocation is *not*
+   * allowed in the ClearTracingMarks.
+   */
+  virtual void ClearTracingMarks(Isolate* isolate) {}

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

= 0;

[Marcel Hlopko, 2016/03/30 18:51:11]

Done

+
+ protected:
+  virtual ~EmbedderHeapTracer() {}

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

= default;

[Marcel Hlopko, 2016/03/30 18:51:12]

Done

+};
+
+/**
  * Isolate represents an isolated instance of the V8 engine.  V8 isolates have
  * completely separate states.  Objects from one isolate must not be used in
  * other isolates.  The embedder can create multiple isolates and use them in
@@ -5587,6 +5634,13 @@ class V8_EXPORT Isolate {
   static Isolate* GetCurrent();
 
   /**
+   * Allows the embedder to add objects to the marking deque of the v8 garbage
+   * collector. Only allowed when the embedder is asked to trace its heap by
+   * EmbedderHeapTracer.
+   */
+  void AddObjectToMarkingDeque(PersistentBase<Object>* handle);

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

why not make this a member of PersistentBase?

[Marcel Hlopko, 2016/03/30 18:51:11]

Done

+
+  /**
    * Custom callback used by embedders to help V8 determine if it should abort
    * when it throws and no internal handler is predicted to catch the
    * exception. If --abort-on-uncaught-exception is used on the command line,
@@ -5829,6 +5883,16 @@ class V8_EXPORT Isolate {
   void RemoveGCPrologueCallback(GCCallback callback);
 
   /**
+   * Sets the embedder heap tracer for the isolate.
+   */
+  void SetEmbedderHeapTracer(EmbedderHeapTracer* tracer);
+
+  /**
+   * Gets embedder heap tracer associated with the isolate, or nullptr.
+   */
+  EmbedderHeapTracer* embedder_heap_tracer();

[jochen (gone - plz use gerrit), 2016/03/30 16:44:45]

shouldn't be needed

[Marcel Hlopko, 2016/03/30 18:51:11]

Sure it's not :) Removed.

+
+  /**
    * Enables the host application to receive a notification after a
    * garbage collection. Allocations are allowed in the callback function,
    * but the callback is not re-entrant: if the allocation inside it will