Use the existing user D-Bus session ($XDG_RUNTIME_DIR/bus) instead of spawning dbus-launch

[Fantu]

Issue

require_dbus_session() in cinnamon-session/main.c only checks the
DBUS_SESSION_BUS_ADDRESS environment variable: if it is unset, it re-execs
itself under dbus-launch --exit-with-session, even when a user D-Bus session
bus is already running at $XDG_RUNTIME_DIR/bus (the default on any systemd
system, and also with elogind + dbus configured for the user bus).

This spawns a second session bus, separate from the user bus: D-Bus
activated services and portals end up split across the two buses. On systems
without the dbus-launch binary (Debian with only dbus-user-session
installed) the session fails to start entirely.

X11 logins are usually unaffected because display managers wrap the session
with a script that exports DBUS_SESSION_BUS_ADDRESS (e.g.
/etc/X11/Xsession.d/20dbus_xdg-runtime on Debian-based distros). But the
Wayland session (cinnamon-session-cinnamon --wayland) does not go through
those wrappers, so whether the fallback triggers depends on the display
manager. It is also easy to demonstrate directly:

$ env -u DBUS_SESSION_BUS_ADDRESS cinnamon-session --version

re-execs under dbus-launch and spawns a new dbus-daemon --session just to
print the version.

Suggested fix

Do what current gnome-session does (gnome-session/leader-main.c): ask GDBus
for the session bus address before falling back to dbus-launch. GDBus already
falls back to $XDG_RUNTIME_DIR/bus when the variable is unset (GLib >= 2.50):

address = g_dbus_address_get_for_bus_sync (G_BUS_TYPE_SESSION, NULL, NULL);
if (address != NULL) {
        g_setenv ("DBUS_SESSION_BUS_ADDRESS", address, TRUE);
        return TRUE;
}
/* existing dbus-launch fallback for non-systemd/non-elogind setups */

Exporting the variable keeps every child process of the session on the same
bus. The dbus-launch path would remain as a last resort, so nothing changes
for setups without a user bus.

Originally reported in Debian in 2016 as part of a cross-desktop effort to
reduce reliance on dbus-launch: https://bugs.debian.org/835846 (gnome-session
dropped dbus-launch entirely; the check described there — a socket at
$XDG_RUNTIME_DIR/bus owned by the current uid — is what the GDBus call
implements).

I can send a PR if this approach is acceptable.