[mojo-edk] Add explicit message object APIs

mojo/edk/system/message_for_transit.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_EDK_SYSTEM_MESSAGE_FOR_TRANSIT_H_
+#define MOJO_EDK_SYSTEM_MESSAGE_FOR_TRANSIT_H_
+
+#include <stdint.h>

[Anand Mistry (off Chromium), 2016/04/19 12:57:35]

#include <memory> for unique_ptr

[Ken Rockot(use gerrit already), 2016/04/19 18:11:25]

Done

+
+#include "base/macros.h"
+#include "mojo/edk/system/dispatcher.h"
+#include "mojo/edk/system/ports_message.h"
+#include "mojo/edk/system/system_impl_export.h"
+
+namespace mojo {
+namespace edk {
+
+// MessageForTransit holds onto a PortsMessage which may be sent via
+// |MojoWriteMessage()| or which may have been received on a pipe endpoint.
+// Instances of this class are exposed to Mojo system API consumers via the
+// opaque pointers used with |MojoCreateMessage()|, |MojoDestroyMessage()|,
+// |MojoWriteMessageNew()|, and |MojoReadMessageNew()|.
+class MOJO_SYSTEM_IMPL_EXPORT MessageForTransit {
+ public:
+#pragma pack(push, 1)
+  // Header attached to every message.
+  struct MessageHeader {
+    // The number of serialized dispatchers included in this header.
+    uint32_t num_dispatchers;
+
+    // Total size of the header, including serialized dispatcher data.
+    uint32_t header_size;
+  };
+
+  // Header for each dispatcher in a message, immediately following the message
+  // header.
+  struct DispatcherHeader {
+    // The type of the dispatcher, correpsonding to the Dispatcher::Type enum.
+    int32_t type;
+
+    // The size of the serialized dispatcher, not including this header.
+    uint32_t num_bytes;
+
+    // The number of ports needed to deserialize this dispatcher.
+    uint32_t num_ports;
+
+    // The number of platform handles needed to deserialize this dispatcher.
+    uint32_t num_platform_handles;
+  };
+#pragma pack(pop)
+
+  ~MessageForTransit();
+
+  // A static constructor for building outbound messages.
+  static MojoResult Create(
+      MessageForTransit** message,
+      uint32_t num_bytes,
+      const Dispatcher::DispatcherInTransit* dispatchers,
+      uint32_t num_dispatchers);
+
+  // A static constructor for wrapping inbound messages.
+  static MessageForTransit* WrapPortsMessage(
+      std::unique_ptr<PortsMessage> message);
+
+  const void* bytes() const;
+  void* mutable_bytes();
+  size_t num_bytes() const;
+
+  size_t num_handles() const;
+
+  std::unique_ptr<PortsMessage>* message();

[Anand Mistry (off Chromium), 2016/04/19 12:57:35]

Returning a pointer to a unique_ptr seems weird. Why not just return
PortsMessage*, and not const? If it's to release the PortsMessage, add an
explicit ReleaseMessage which returns the unique_ptr.

[Ken Rockot(use gerrit already), 2016/04/19 18:11:25]

The rationale was that a caller may or may not want to take ownership. In
practice though it doesn't matter and it's fine to always take ownership, so
changed it to TakePortsMessage which returns a unique_ptr.

+
+ private:
+  explicit MessageForTransit(std::unique_ptr<PortsMessage> message);
+
+  std::unique_ptr<PortsMessage> message_;
+
+  DISALLOW_COPY_AND_ASSIGN(MessageForTransit);
+};
+
+}  // namespace edk
+}  // namespace mojo
+
+#endif  // MOJO_EDK_SYSTEM_MESSAGE_FOR_TRANSIT_H_