Mojo: Use new message APIs to reduce copying

mojo/public/cpp/bindings/lib/message_buffer.h

+// Copyright 2016 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.
+
+#ifndef MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_LIB_MESSAGE_BUFFER_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_LIB_MESSAGE_BUFFER_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <utility>
+
+#include "base/macros.h"
+#include "mojo/public/cpp/bindings/lib/buffer.h"
+#include "mojo/public/cpp/system/message.h"
+
+namespace mojo {
+namespace internal {
+
+// A fixed-size Buffer implementation using a Mojo message object for storage.
+class MessageBuffer : public Buffer {
+ public:
+  // Initializes this buffer to carry a fixed byte capacity and no handles.
+  MessageBuffer(size_t capacity, bool zero_initialized);
+
+  // Initializes this buffer from an existing Mojo MessageHandle.
+  MessageBuffer(ScopedMessageHandle message, uint32_t num_bytes);
+
+  ~MessageBuffer() override;
+
+  void* data() const { return buffer_; }
+  uint32_t data_num_bytes() const { return data_num_bytes_; }
+
+  // Buffer:
+  void* Allocate(size_t delta) override;
+
+  ScopedMessageHandle TakeMessage() { return std::move(message_); }
+
+ private:
+  uint32_t data_num_bytes_ = 0;
+  ScopedMessageHandle message_;
+  void* buffer_;
+
+  uint32_t bytes_claimed_ = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(MessageBuffer);
+};
+
+}  // namespace internal
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_LIB_MESSAGE_BUFFER_H_