Previously we synced just before calling switch-to-configuration.
That prevents corruption of the Nix store, but it can leave the boot
loader configuration and kernel files in /boot corrupted. So do the
sync after installing the boot loader.
Usually timers.target is pulled in by basic.target, but we don't
restart basic.target. So timers.target wouldn't be started when
coming from an older systemd.
During a configuration switch, changed units are stopped in the old
configuration, then started in the new configuration (i.e. after
running the activation script and running "systemctl daemon-reload").
This ensures that services are stopped using the ExecStop/ExecStopPost
commands from the old configuration.
However, for some services it's undesirable to stop them; in
particular dhcpcd, which deconfigures its network interfaces when it
stops. This is dangerous when doing remote upgrades - usually things
go right (especially because the switch script ignores SIGHUP), but
not always (see 9aa69885f0). Likewise,
sshd should be kept running for as long as possible to prevent a
lock-out if the switch fails.
So the new option ‘stopIfChanged = false’ causes "systemctl restart"
to be used instead of "systemctl stop" followed by "systemctl start".
This is only proper for services that don't have stop commands. (And
it might not handle dependencies properly in some cases, but I'm not
sure.)
If a service has a corresponding socket unit, then stop the socket
before stopping the service. This prevents it from being restarted
behind our backs. Also, don't restart the service; it will be
restarted on demand via the socket.
