Implement a trivial Mojo EchoService that can be connected to over DBus

mojo/shell/external_service.mojom

Index: mojo/shell/external_service.mojom
diff --git a/mojo/shell/external_service.mojom b/mojo/shell/external_service.mojom
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+// Copyright 2014 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.
+
+module mojo {
+
+[Peer=ExternalServiceHost]

[DaveMoore, 2014/04/22 20:39:51]

Why can't you just use the Shell / ShellClient interface here? Even if you need
to interpose yourself between the real shell and this client you could simply
proxy both sides. If you don't I think you can just hand the shell_handle off to
the dbus service, which would implement the ShellClient interface.

[Chris Masone, 2014/04/22 21:16:30]

This interface exists to allow me to hand the ShellHandle off to the dbus
service, because I couldn't figure out another way to do it :)

DBus provides a primitive that allows me to pass file descriptors across, and by
the time I'm in the DBusServiceLoader code, I have a ShellHandle and (as far as
I could tell, please correct if I'm wrong) no way to recover the FD of the
MessagePipe that underlies it.  So, instead I create a new FD pair and pass one
over DBus, which I use to get (what I believe is) a MessagePipe set up between
the two processes.

It's probably true that I could manually craft and send a message containing the
ShellHandle over the pipe I just created, if that'd be preferred. It seemed like
defining this little API just automatically handled marshalling and such for me,
so I did it this way.

I would prefer to have some way to get the ShellHandle directly to the
externally running process, without have to create a MessagePipe for this sole
purpose, but I couldn't figure out how to do so with the primitives I had.

+interface ExternalService {
+  Activate(handle<message_pipe> shell_handle);
+};
+
+[Peer=ExternalService]
+interface ExternalServiceHost {
+};
+
+}