Begin implementing V8 bindings for Mojo

mojo/public/bindings/js/v8_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 "mojo/public/bindings/js/v8_array_buffer.h"
+
+#include <stdlib.h>
+
+namespace mojo {
+namespace js {
+
+MOJO_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 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_;
+};
+
+BufferView::Private* BufferView::Private::From(
+    v8::Isolate* isolate, v8::Handle<v8::ArrayBuffer> array) {
+  if (array->IsExternal()) {
+    return static_cast<BufferView::Private*>(
+        array->GetAlignedPointerFromInternalField(kBufferViewPrivateIndex));
+  }
+  return new Private(isolate, array);
+}
+
+void BufferView::Private::AddRef() {
+  ++ref_count_;
+}
+
+void BufferView::Private::Release() {
+  --ref_count_;
+  if (ref_count_)
+    return;
+  ArrayBufferAllocator::SharedInstance()->Free(buffer_, length_);
+  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() {
+}
+
+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() {
+  private_->Release();  // Balanced in BufferView::Initialize.
+}
+
+void BufferView::Initialize(v8::Isolate* isolate,
+                            v8::Handle<v8::ArrayBuffer> array) {
+  private_ = BufferView::Private::From(isolate, array);
+  private_->AddRef();  // Balanced in BufferView::~BufferView.
+  bytes_ = private_->buffer();
+  num_bytes_ = private_->length();
+}
+
+}  // namespace js
+}  // mojo