Bug#1004448: gnome-session: should manage session with systemd --user, if available

[Simon McVittie]

Control: found -1 40.1.1-2
Control: severity -1 normal

On Thu, 27 Jan 2022 at 20:23:36 +0000, Simon McVittie wrote:
> Recent-ish upstream versions of gnome-session have been able to manage the
> whole login session as a group of `systemd --user` services:
> https://wiki.gnome.org/Initiatives/SystemdUser

In fact gnome-session in stable already does this, and it appears
to have been disabled by mistake in testing/unstable. The patch
debian/patches/debian/Make-sure-to-pass-systemd-when-we-re-managing-the-user-se.patch
was removed about a year ago, but we're still configuring with
-Dsystemd_session=enable rather than -Dsystemd_session=default, which
means systemd --user session management is still opt-in but we're no longer
opting in.

With -Dsystemd_session=default, I believe we would now have the intended
semantics: users of dbus-user-session (the majority) get a systemd-based
session, and dbus-x11/sysvinit systems get the old code path.

    smcv