Bug#835887: gnome-session: don't run dbus-launch if XDG_RUNTIME_DIR/bus is available

[Simon McVittie]

Source: gnome-session
Version: 3.20.2-1
Severity: normal
Tags: upstream
User: dbus at packages.debian.org
Usertags: dbus-launch dbus-launch-unless-dsba

As described in <https://lists.debian.org/debian-devel/2016/08/msg00554.html>
I'm trying to reduce how much dbus-launch is used in Debian.
gnome-session has code that explicitly runs dbus-launch if
DBUS_SESSION_BUS_ADDRESS isn't set.

The major D-Bus implementations (libdbus, GDBus, sd-bus) now have a
fallback code path, before trying X11 autolaunch, when
DBUS_SESSION_BUS_ADDRESS is unset: if XDG_RUNTIME_DIR is set, and
XDG_RUNTIME_DIR/bus exists, is a socket and is owned by the
process's uid, then they will use it. In particular, dbus-user-session
sets up that situation.

For the moment, dbus-user-session does make sure DBUS_SESSION_BUS_ADDRESS
is set, to be nice to packages that don't have this fallback path.
However, I'd like to avoid requiring that in future, by adapting
the dbus-launch code in gnome-session and its forks to look for
XDG_RUNTIME_DIR/bus before trying dbus-launch.

I'll open an upstream bug after doing this MBF.

Thanks,
    S