Bug#836118: obus: don't run dbus-launch if XDG_RUNTIME_DIR/bus is available

[Simon McVittie <smcv@debian.org>]

Source: obus
Version: 1.1.5-5
Severity: wishlist
Tags: upstream
User: dbus@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.
While searching packages for (mis)uses of dbus-launch, I noticed that
obus implements the fallback from DBUS_SESSION_BUS_ADDRESS to the
autolaunch: transport.

This is a correct thing to do. However, 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 code path.
dbus-launch --autolaunch also returns an address pointing to
$XDG_RUNTIME_DIR/bus, without requiring X11, if that socket is available.
However, I'd like to avoid requiring those in future, by adapting
the fallback code path in other implementations like oBus to
look for $XDG_RUNTIME_DIR/bus.

Thanks,
    S