Identifying the real display server

[Simon McVittie]

On 13/04/15 16:36, Mattias Andrée wrote:
> Have you considered that you may want to have two
> displays (for example two instances of X or two
> instances of Mir) with the same $XDG_RUNTIME_DIR?

Then you can use DISPLAY and/or the X-replacement's equivalent, just
like you can still use DBUS_SESSION_BUS_ADDRESS if you want to have more
than one instance of the D-Bus session bus with the same
XDG_RUNTIME_DIR. But I don't think this should be required in the
simpler situations that ought to "just work", like running an X app from
the context of a per-user service where the user in question has exactly
one graphical desktop.

Note that this is per-(user,machine), not per-machine: the unusual case
here is having more than one Xserver *per user* per machine. That's
quite an unusual thing to do even with multi-seat, because in my
experience, people are usually in one physical place at any given time :-)

If you do want to be on more than one display device at a time, it seems
like it would be better to be able to attach/detach output devices to a
single DISPLAY-equivalent and move windows between them (as opposed to
"I can't usefully edit that document, because I left it open with
unsaved changes on seat0", or worse, "I can't read my email without
kill(1), because I left Thunderbird running on seat0"). That can't work
if each application is bound to a display by the initial values of its
environment variables.

>> dbus 1.9 defaults to using $XDG_RUNTIME_DIR/bus if it
> 
> I hope you mean $XDG_RUNTIME_DIR/dbus, otherwise someone
> need to make an amendment to dbus.

I do mean XDG_RUNTIME_DIR/bus. The initial proposal was
XDG_RUNTIME_DIR/dbus/user_bus_socket (as seen in user-session-units and
other early work on having one bus per user), but the sd-bus/kdbus
developers wanted X_R_D/bus, which had in fact been sd-bus' default for
quite a while before this was merged into dbus:

"""
In the systemd context we tried to avoid using the name "dbus" too much
for the generic bits. We just refer to the "bus", without any further
naming, since we want to ensure that there's no doubt that this is the
one and only bus that matters on the OS, and not just one project among
many.
""" - https://bugs.freedesktop.org/show_bug.cgi?id=61303#c27

I wanted to get this merged rather than painting naming-related bike
sheds: partly because many "session" services, e.g. dconf, really want
to be user services; partly because the kdbus developers have indicated
that they will never support multiple session buses per user, and I
wanted to start getting implementation experience for that mode before
kdbus lands; and partly because the other option for a default way to
connect to D-Bus is dbus-launch, which is tied to X11 anyway, so
Wayland/Mir is a natural opportunity to get rid of it.

-- 
Simon McVittie
Collabora Ltd. <http://www.collabora.com/>