Explicitly serialise platform file attachments instead of doing them implicitly as a mojo handle at…

ipc/mojo/ipc_mojo_handle_attachment.h

 // Copyright (c) 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 IPC_MOJO_IPC_MOJO_HANDLE_ATTACHMENT_H_
 #define IPC_MOJO_IPC_MOJO_HANDLE_ATTACHMENT_H_
 
 #include "base/files/file.h"
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "ipc/ipc_export.h"
 #include "ipc/ipc_message_attachment.h"
 #include "mojo/public/cpp/system/handle.h"
 
 namespace IPC {
 
 namespace internal {
 
 // A MessageAttachment that holds a MojoHandle.
-// * On the sending side, every Mojo handle is a MessagePipe. This is because
-//   any platform files are wrapped by PlatformFileAttachment.
-// * On the receiving side, the handle can be either MessagePipe or wrapped
-//   platform file: All files, not only MessagePipes are wrapped as a
-//   MojoHandle. The message deserializer should know which type of the object
-//   the handle wraps.
+// This can hold any type of transferrable Mojo handle (i.e. message pipe, data
+// pipe, etc), but the receiver is expected to know what type of handle to
+// expect.
 class IPC_MOJO_EXPORT MojoHandleAttachment : public MessageAttachment {
  public:
   explicit MojoHandleAttachment(mojo::ScopedHandle handle);
 
   Type GetType() const override;
 
 #if defined(OS_POSIX)
-  // Returns wrapped file if it wraps a file, or
-  // an invalid fd otherwise. The ownership of handle
-  // is passed to the caller.
+  // Should not be called.
   base::PlatformFile TakePlatformFile() override;
 #endif  // OS_POSIX
 
   // Returns the owning handle transferring the ownership.
   mojo::ScopedHandle TakeHandle();
 
  private:
   ~MojoHandleAttachment() override;
   mojo::ScopedHandle handle_;
 
   DISALLOW_COPY_AND_ASSIGN(MojoHandleAttachment);
 };
 
 }  // namespace internal
 }  // namespace IPC
 
 #endif  // IPC_MOJO_IPC_MOJO_HANDLE_ATTACHMENT_H_