[systemd-devel] question regarding DBUS_SESSION_BUS_ADDRESS in multiseat environment

[Mantas Mikulėnas]

On Fri, Dec 2, 2016 at 10:28 AM, Mohan R <mohan43u at gmail.com> wrote:

> Hi,
>
> I'm working on a display-manager for a multiseat environment. Here is how
> we use logind
>
> 1. create new seat through udev
> 2. we set the XDG_SEAT in pamenv before calling pam_open_session()
> 3. we take XDG_SESSION_ID, XDG_RUNTIME_DIR and DBUS_SESSION_BUS_ADDRESS
> from pam_systemd
>
> Let say if a user already have a session(session0) in a seat (customseat0)
> and he want to start another session in another seat (customseat1). Our
> display manager would get the same DBUS_SESSION_BUS_ADDRESS from
> pam_systemd for session1.
>
> As we cannot use the same address for two sessions, we have to start
> dbus-session manually. Using the address provided by dbus-session as
> DBUS_SESSION_BUS_ADDRESS, we then proceed with starting gnome-keyring and
> gnome-session.
>
> Problem is, processes forked by 'systemd --user' will have
> DBUS_SESSION_BUS_ADDRESS="unix:path=$XDG_RUNTIME_DIR/bus" but processes
> forked by gnome-session will have DBUS_SESSION_BUS_ADDRESS=$
> DBUS_SESSION_PROVIDED_ADDRESS.
>
> Is there any way to make pam_systemd provides uniq
> DBUS_SESSION_BUS_ADDRESS for every session (may be
> unix:path=$XDG_RUNTIME_DIR/$XDG_SESSION_ID/bus)? or is there any way to
> ask 'systemd --user' to provide different DBUS_SESSION_BUS_ADDRESS to the
> childs?
>

No, only one graphical session at a time is supported. While you can start
additional sessions using `dbus-run-session` or the old `dbus-launch` (and
have them run independently, "traditional way"), systemd --user will only
ever use the main user bus.

Also, if you made systemd specify different bus envvars for different
services, you wouldn't be able to start the same service for *both*
sessions at once...

(*Technically,* I suppose it would be possible to make logind start
per-session instances of --user and so on and so on, but as it is nothing
like that is supported.)

-- 
Mantas Mikulėnas <grawity at gmail.com>
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