As with many things, we have scenarios where we don't want to boot on a
disk / bootloader and also we don't want to boot directly.
Sometimes, we want to boot through an OptionROM of our NIC, e.g. netboot
scenarios or let the firmware decide something, e.g. UEFI PXE (or even
UEFI OptionROM!).
This is composed of:
- `directBoot.enable`: whether to direct boot or not
- `directBoot.initrd`: enable overriding the
`config.system.build.initialRamdisk` defaults, useful for
netbootRamdisk for example.
This makes it possible.
This essentially backports
https://github.com/systemd/systemd/pull/27791. `systemd-networkd.service`
is sent the `SIGTERM` signal, but it is not required to be stopped
before `initrd-switch-root.target` is reached, despite the use of
`systemctl isolate initrd-switch-root.target`. This is because when
there is no ordering at all between two units, and a transaction stops
one and starts the other, the two operations can happen
simultaneously. This means the service could still be running when
`switch-root` actually occurs. Then, stage 2 systemd will see the
service still running and decide it doesn't need to add a start
operation for it to its initial transaction. Finally, the service
exits, but only after it's already too late. If, however, there is any
ordering at all between a stopping unit and a starting unit, then the
stop operation will be done first. This way, we ensure that the
service is properly exited before doing `switch-root`.
This is something to keep in mind going forward. There may be other
services that need this treatment. These `before` and `conflicts`
definitions are the correct way to ensure a unit is actually stopped
before you reach initrd-switch-root
not using this any more, and really don't have the energy to deal with
neither the fallout of ubnt not officially supporting mongodb newer than
3.6, nor the hacks nixpkgs contains to work around that.
Since 816614bd62, the service is set to use the exim user so that
systemd takes care of the credentials ownership. The executable is
still required to run as root, to then drop privileges. The prefix '+'
that was used however interfers with the use of privilege restrictions
and other sandboxing options. Since we only want to escape the "User"
setting, we can use the '!' prefix instead.
