[mojo-edk] Add explicit message object APIs

mojo/edk/system/message_for_transit.h

Index: mojo/edk/system/message_for_transit.h
diff --git a/mojo/edk/system/message_for_transit.h b/mojo/edk/system/message_for_transit.h
new file mode 100644
index 0000000000000000000000000000000000000000..be035b04794489384453c351c0810016ec01014a
--- /dev/null
+++ b/mojo/edk/system/message_for_transit.h
@@ -0,0 +1,113 @@
+// 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>
+
+#include <memory>
+
+#include "base/macros.h"
+#include "base/memory/ptr_util.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(
+      std::unique_ptr<MessageForTransit>* message,
+      uint32_t num_bytes,
+      const Dispatcher::DispatcherInTransit* dispatchers,
+      uint32_t num_dispatchers);
+
+  // A static constructor for wrapping inbound messages.
+  static std::unique_ptr<MessageForTransit> WrapPortsMessage(
+      std::unique_ptr<PortsMessage> message) {
+    return base::WrapUnique(new MessageForTransit(std::move(message)));
+  }
+
+  const void* bytes() const {
+    DCHECK(message_);
+    return static_cast<const void*>(
+        static_cast<const char*>(message_->payload_bytes()) +
+            header()->header_size);
+  }
+
+  void* mutable_bytes() {
+    DCHECK(message_);
+    return static_cast<void*>(
+        static_cast<char*>(message_->mutable_payload_bytes()) +
+            header()->header_size);
+  }
+
+  size_t num_bytes() const {
+    size_t header_size = header()->header_size;
+    DCHECK_GE(message_->num_payload_bytes(), header_size);
+    return message_->num_payload_bytes() - header_size;
+  }
+
+  size_t num_handles() const { return header()->num_dispatchers; }
+
+  std::unique_ptr<PortsMessage> TakePortsMessage() {
+    return std::move(message_);
+  }
+
+ private:
+  explicit MessageForTransit(std::unique_ptr<PortsMessage> message);
+
+  const MessageForTransit::MessageHeader* header() const {
+    DCHECK(message_);
+    return static_cast<const MessageForTransit::MessageHeader*>(
+        message_->payload_bytes());
+  }
+
+  std::unique_ptr<PortsMessage> message_;
+
+  DISALLOW_COPY_AND_ASSIGN(MessageForTransit);
+};
+
+}  // namespace edk
+}  // namespace mojo
+
+#endif  // MOJO_EDK_SYSTEM_MESSAGE_FOR_TRANSIT_H_