This patch was initially prompted by a report on a Fedora update [1], that the
upgrade causes systemd-resolved.service and systemd-networkd.service to be
re-enabled. We generally want to preserve the enablement of all services during
upgrades, so a reset like this is not expected.
Both services declare two symlinks in their [Install] sections, for their dbus
names and for multi-user.target.wants/. It turns out that both services were
only partially enabled, because their dbus unit symlinks
/etc/systemd/system/dbus-org.freedesktop.{resolve1,network1}.service were
created, by the symlinks in /etc/systemd/system/multi-user.target.wants/ were
not. This means that the units could be activated by dbus, but not in usual
fashion using systemctl start. Our tools make it rather hard to figure out when
something like this happens, and it is definitely an area for improvement on its
own. The symlink in .wants/ was filtered out by during packaging, but the dbus
symlink was left in (I assume by mistake).
Let's simplify things by not creating the symlinks statically during 'ninja
install'. This means that the units shipped by systemd have to be enabled in
the usual fashion, which in turns means that [Install] section and presets
become the "single source of truth" and we don't have two sets of conflicting
configuration.
Let's consider a few cases:
- developer: a developer installs systemd from git on a running system, and they
don't want the installation to reset enablement of anything. So this change is
either positive for them, or has no effect (if they have everything at
defaults).
- package creation: we want to create symlinks using 'preset-all' and 'preset'
on upgraded packages, we don't want to have any static symlinks. This change
will remove the need to filter out symlinks in packaging and of course fix
the original report.
- installation of systemd from scratch: this change means that without
'preset-all' the system will not be functional. This case could be affected
negatively by this change, but I think it's enough of a corner case to accept
this. In practice I expect people to build a package, not installl directly
into the file system, so this might not even matter in practice.
Creating those symlinks was probably the right thing in the beginning, but
nowadays the preset system is very well established and people expect it to
be honoured. Ignoring the presets and doing static configuration is not welcome
anymore.
Note: during package installation, either 'preset-all' or 'preset getty@.service
machines.target remote-cryptsetup.target remote-fs.target
systemd-networkd.service systemd-resolved.service
systemd-networkd-wait-online.service systemd-timesyncd.service' should be called.
[1] https://bodhi.fedoraproject.org/updates/FEDORA-2019-616045ca76
This also changes that .netdev files are loaded in ascending order.
Otherwise, when NetDev.ifname= setting conflicts with other .netdev file,
then .netdev file with large prefix number wins.
