gin/array_buffer.cc

// Copyright 2013 The Chromium 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 "gin/array_buffer.h"

#include <stdlib.h>

namespace gin {

COMPILE_ASSERT(V8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT == 2,
               array_buffers_must_have_two_internal_fields);

static const int kBufferViewPrivateIndex = 0;

// ArrayBufferAllocator -------------------------------------------------------

void* ArrayBufferAllocator::Allocate(size_t length) {
  return calloc(1, length);
}

void* ArrayBufferAllocator::AllocateUninitialized(size_t length) {
  return malloc(length);
}

void ArrayBufferAllocator::Free(void* data, size_t length) {
  free(data);
}

ArrayBufferAllocator* ArrayBufferAllocator::SharedInstance() {
  static ArrayBufferAllocator* instance = new ArrayBufferAllocator();
  return instance;
}

// BufferView::Private --------------------------------------------------------

// This class exists to solve a tricky lifetime problem. The V8 API doesn't
// want to expose a direct view into the memory behind an array buffer because
// V8 might deallocate that memory during garbage collection. Instead, the V8
// API forces us to externalize the buffer and take ownership of the memory.
// In order to know when to free the memory, we need to figure out both when
// we're done with it and when V8 is done with it.
//
// To determine whether we're done with the memory, every view we have into
// the array buffer takes a reference to the BufferView::Private object that
// actually owns the memory. To determine when V8 is done with the memory, we
// open a weak handle to the ArrayBuffer object. When we receive the weak
// callback, we know the object is about to be garbage collected and we can
// drop V8's implied reference to the memory.
//
// The final subtlety is that we need every BufferView into the same array
// buffer to AddRef the same BufferView::Private. To make that work, we store a
// pointer to the BufferView::Private object in an internal field of the
// ArrayBuffer object.
//
class BufferView::Private {
 public:
  static scoped_refptr<Private> From(v8::Isolate* isolate,
                                     v8::Handle<v8::ArrayBuffer> array);

  void AddRef();
  void Release();

  void* buffer() const { return buffer_; }
  size_t length() const { return length_; }

 private:
  Private(v8::Isolate* isolate, v8::Handle<v8::ArrayBuffer> array);
  ~Private();

  static void WeakCallback(
      const v8::WeakCallbackData<v8::ArrayBuffer, Private>& data);

  size_t ref_count_;
  v8::Persistent<v8::ArrayBuffer> array_buffer_;
  void* buffer_;
  size_t length_;
};

scoped_refptr<BufferView::Private> BufferView::Private::From(
    v8::Isolate* isolate, v8::Handle<v8::ArrayBuffer> array) {
  if (array->IsExternal()) {
    return make_scoped_refptr(static_cast<Private*>(
        array->GetAlignedPointerFromInternalField(kBufferViewPrivateIndex)));
  }
  return make_scoped_refptr(new Private(isolate, array));
}

void BufferView::Private::AddRef() {
  ++ref_count_;
}

void BufferView::Private::Release() {
  if (--ref_count_)
    return;
  delete this;
}

BufferView::Private::Private(v8::Isolate* isolate,
                             v8::Handle<v8::ArrayBuffer> array)
    : ref_count_(0),
      array_buffer_(isolate, array) {
  // Take ownership of the array buffer.
  v8::ArrayBuffer::Contents contents = array->Externalize();
  buffer_ = contents.Data();
  length_ = contents.ByteLength();

  array->SetAlignedPointerInInternalField(kBufferViewPrivateIndex, this);

  AddRef();  // Balanced in WeakCallback.
  array_buffer_.SetWeak(this, WeakCallback);
}

BufferView::Private::~Private() {
  ArrayBufferAllocator::SharedInstance()->Free(buffer_, length_);
}

void BufferView::Private::WeakCallback(
    const v8::WeakCallbackData<v8::ArrayBuffer, Private>& data) {
  Private* parameter = data.GetParameter();
  parameter->array_buffer_.Reset();
  parameter->Release();  // Balanced in BufferView::Private::Private.
}

// BufferView -----------------------------------------------------------------

BufferView::BufferView(v8::Isolate* isolate,
                       v8::Handle<v8::ArrayBufferView> view) {
  Initialize(isolate, view->Buffer());
  uint8_t* ptr = static_cast<uint8_t*>(bytes_);
  bytes_ = static_cast<void*>(ptr + view->ByteOffset());
  num_bytes_ = view->ByteLength();
}

BufferView::BufferView(v8::Isolate* isolate,
                       v8::Handle<v8::ArrayBuffer> array) {
  Initialize(isolate, array);
}

BufferView::~BufferView() {
}

void BufferView::Initialize(v8::Isolate* isolate,
                            v8::Handle<v8::ArrayBuffer> array) {
  private_ = BufferView::Private::From(isolate, array);
  bytes_ = private_->buffer();
  num_bytes_ = private_->length();
}

}  // namespace gin