nan_object_wrap.h

/*********************************************************************
 * NAN - Native Abstractions for Node.js
 *
 * Copyright (c) 2018 NAN contributors
 *
 * MIT License <https://github.com/nodejs/nan/blob/master/LICENSE.md>
 ********************************************************************/

#ifndef NAN_OBJECT_WRAP_H_
#define NAN_OBJECT_WRAP_H_

#include <cstdint>

class ObjectWrap {
 public:
  ObjectWrap() {
    refs_ = 0;
  }


  virtual ~ObjectWrap() {
    if (persistent().IsEmpty()) {
      return;
    }

    persistent().ClearWeak();
    persistent().Reset();
  }


  template <class T>
  static inline T* Unwrap(v8::Local<v8::Object> object) {
    assert(!object.IsEmpty());
    assert(object->InternalFieldCount() > InternalFields::kSlot);
    // Cast to ObjectWrap before casting to T.  A direct cast from void
    // to T won't work right when T has more than one base class.
    void* ptr = GetInternalFieldPointer(object, InternalFields::kSlot);
    ObjectWrap* wrap = static_cast<ObjectWrap*>(ptr);
    return static_cast<T*>(wrap);
  }


  inline v8::Local<v8::Object> handle() const {
    return New(handle_);
  }


  inline Persistent<v8::Object>& persistent() {
    return handle_;
  }


 protected:
  // These fields must match internal v8 EmbedderDataSlot Indicies.
  // unfortunately they are not centralized anywhere so we have to make our own
  // chromium examples:
  // https://source.chromium.org/chromium/chromium/src/+/936f9f68:v8/src/heap/embedder-tracing.h;l=176
  // https://source.chromium.org/chromium/chromium/src/+/936f9f68:gin/public/wrapper_info.h;l=19
  enum InternalFields {
    kWrapperType,
    kSlot,
    kInternalFieldCount
  };

  inline void Wrap(v8::Local<v8::Object> object) {
    assert(persistent().IsEmpty());
    assert(object->InternalFieldCount() > InternalFields::kSlot);
    /**
     * In Isolates(contexts?) with cppgc enabled, v8 assumes the first type slot
     * is a pointer to a "type info" struct which has an "Embedder ID" at the
     * first 2 bytes. If the first 2 bytes at `this` contain an ID that matches
     * an internal one then multiple issues can happen(mostly it will segfault).
     * Simple fix is to set the slot to an ID that doesn't match any internal
     * embedders(notably blink) so v8 doesn't try to mess with it.
     **/
    SetInternalFieldPointer(object, InternalFields::kWrapperType, &kEmbedderId);
    SetInternalFieldPointer(object, InternalFields::kSlot, this);
    persistent().Reset(object);
    MakeWeak();
  }

#if defined(V8_MAJOR_VERSION) && (V8_MAJOR_VERSION > 4 ||                      \
  (V8_MAJOR_VERSION == 4 && defined(V8_MINOR_VERSION) && V8_MINOR_VERSION >= 3))

  inline void MakeWeak() {
    persistent().v8::PersistentBase<v8::Object>::SetWeak(
        this, WeakCallback, v8::WeakCallbackType::kParameter);
#if NODE_MAJOR_VERSION < 10
    // FIXME(bnoordhuis) Probably superfluous in older Node.js versions too.
    persistent().MarkIndependent();
#endif
  }

#elif NODE_MODULE_VERSION > NODE_0_10_MODULE_VERSION

  inline void MakeWeak() {
    persistent().v8::PersistentBase<v8::Object>::SetWeak(this, WeakCallback);
    persistent().MarkIndependent();
  }

#else

  inline void MakeWeak() {
    persistent().persistent.MakeWeak(this, WeakCallback);
    persistent().MarkIndependent();
  }

#endif

  /* Ref() marks the object as being attached to an event loop.
   * Refed objects will not be garbage collected, even if
   * all references are lost.
   */
  virtual void Ref() {
    assert(!persistent().IsEmpty());
    persistent().ClearWeak();
    refs_++;
  }

  /* Unref() marks an object as detached from the event loop.  This is its
   * default state.  When an object with a "weak" reference changes from
   * attached to detached state it will be freed. Be careful not to access
   * the object after making this call as it might be gone!
   * (A "weak reference" means an object that only has a
   * persistent handle.)
   *
   * DO NOT CALL THIS FROM DESTRUCTOR
   */
  virtual void Unref() {
    assert(!persistent().IsEmpty());
    assert(!persistent().IsWeak());
    assert(refs_ > 0);
    if (--refs_ == 0)
      MakeWeak();
  }

  int refs_;  // ro

 private:
  NAN_DISALLOW_ASSIGN_COPY_MOVE(ObjectWrap)
#if defined(V8_MAJOR_VERSION) && (V8_MAJOR_VERSION > 4 ||                      \
  (V8_MAJOR_VERSION == 4 && defined(V8_MINOR_VERSION) && V8_MINOR_VERSION >= 3))

  static void
  WeakCallback(v8::WeakCallbackInfo<ObjectWrap> const& info) {
    ObjectWrap* wrap = info.GetParameter();
    assert(wrap->refs_ == 0);
    wrap->handle_.Reset();
    delete wrap;
  }

#elif NODE_MODULE_VERSION > NODE_0_10_MODULE_VERSION

  static void
  WeakCallback(v8::WeakCallbackData<v8::Object, ObjectWrap> const& data) {
    ObjectWrap* wrap = data.GetParameter();
    assert(wrap->refs_ == 0);
    assert(wrap->handle_.IsNearDeath());
    wrap->handle_.Reset();
    delete wrap;
  }

#else

  static void WeakCallback(v8::Persistent<v8::Value> value, void *data) {
    ObjectWrap *wrap = static_cast<ObjectWrap*>(data);
    assert(wrap->refs_ == 0);
    assert(wrap->handle_.IsNearDeath());
    wrap->handle_.Reset();
    delete wrap;
  }

#endif
  Persistent<v8::Object> handle_;

  // this can be any value so long as it's 16bits and doesn't conflict
  // with any chromium IDs:
  // https://source.chromium.org/chromium/chromium/src/+/936f9f68:gin/public/gin_embedders.h;l=18
  uint16_t kEmbedderId = 0xBEEF;
};


#endif  // NAN_OBJECT_WRAP_H_