Create a broker interface for the new Mojo EDK so that the browser can create and duplicate messa...

mojo/edk/system/token_serializer_messages_win.h

+// Copyright 2015 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_TOKEN_SERIALIZER_MESSAGES_WIN_H_
+#define MOJO_EDK_SYSTEM_TOKEN_SERIALIZER_MESSAGES_WIN_H_
+
+#include <stdint.h>
+
+#include "base/compiler_specific.h"
+
+namespace mojo {
+namespace edk {
+
+// This header defines the message format between ChildTokenSerializer and
+// ParentTokenSerializer.
+
+enum MessageId {
+  // The reply is two HANDLEs.
+  CREATE_PLATFORM_CHANNEL_PAIR = 0,
+  // The reply is tokens of the same count of passed in handles.
+  HANDLE_TO_TOKEN,
+  // The reply is handles of the same count of passed in tokens.
+  TOKEN_TO_HANDLE,
+};
+
+// Definitions of the raw bytes sent between ChildTokenSerializer and
+// ParentTokenSerializer.
+
+struct ALIGNAS(4) TokenSerializerMessage {

[brucedawson, 2015/11/25 20:44:11]

What is the purpose of the ALIGNAS(4) directive here? VC++ 2015 doesn't like it:

warning C4359: 'mojo::edk::TokenSerializerMessage': Alignment specifier is less
than actual alignment (8), and will be ignored.

The problem is the uint64_t member which (even in 32-bit x86 builds with VC++)
has 8-byte alignment. And, it appears that __declspec(align()) can only be used
to increase the alignment, not decrease it.

Was the goal to change the *packing* of the struct, or the alignment? If the
packing of the struct was set to 4 then that would ensure that it was optimally
packed without hitting this problem, and would also reduce the alignment
requirements.

As it stands the struct is already optimally packed, although it's 8-byte
alignment requirement may affect structures that it is placed in.

I think that the fix is to remove the ALIGNAS(4) directive. VC++ 2013 is already
ignoring it - VC++ 2015 is just more honest about it.

+  uint32_t size;
+  MessageId id;
+  // Data, if any, follows.
+  union {
+    HANDLE handles[1];  // If HANDLE_TO_TOKEN.
+    uint64_t tokens[1];  // If TOKEN_TO_HANDLE.
+  };
+};
+
+const int kTokenSerializerMessageHeaderSize =
+    sizeof(uint32_t) + sizeof(MessageId);
+
+}  // namespace edk
+}  // namespace mojo
+
+#endif  // MOJO_EDK_SYSTEM_TOKEN_SERIALIZER_MESSAGES_WIN_H_