Specification for org.freedesktop.DisplayManager DBus API?

[Mathieu Parent]

(Note 1: please CC me)
(Note 2: I have CC-ed two contacts from gnome-shell, one from gdm, one
from lightdm, and the system-logind ML)

Hello,

There are currently several ways to ask the DM to switch to the login screen:
- gdm3 uses org.gnome.DisplayManager.LocalDisplayFactory CreateTransientDisplay
- lightdm uses org.freedesktop.DisplayManager SwitchToGreeter
- old gdm, and kdm seems to use a socket without DBus
- ...

I am not involved in those projects, but I recently needed to switch
to LightDM to have guest sessions, while keeping gnome-shell
([debian-guest-sessions]). gnome-shell send a CreateTransientDisplay
when "Switch user", without any effect.

I proposed a patch to gnome-shell [gnome-shell-SwitchToGreeter], which
currently waiting for an implementation in gdm [gdm-SwitchToGreeter]
and a written specification.

This mail is to start a discussion about this DisplayManager specification.

The current DBus interfaces used by lightDM seems to be a good
starting point [lightDM-API], but some of its API may overlap with
logind.

Also, how can this work in the Wayland world?

What is the way forward to have the same API in all DM?

Refs:
[debian-guest-sessions]:
http://anonscm.debian.org/cgit/users/sathieu/guest-sessions.git/tree/
[gnome-shell-SwitchToGreeter]: https://bugzilla.gnome.org/show_bug.cgi?id=745940
[gdm-SwitchToGreeter]: https://bugzilla.gnome.org/show_bug.cgi?id=745938
[lightDM-API]: http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/vivid/lightdm/vivid/view/head:/src/lightdm.c#L784

Regards

Mathieu Parent