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switch-to-configuration: add sysinit-reactivation manual section

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nikstur 2023-12-25 23:45:52 +01:00
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nixos/doc/manual/development/unit-handling.section.md
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@ -63,3 +63,42 @@ checks:
is **restart**ed with the others. If it is set, both the service and the
socket are **stop**ped and the socket is **start**ed, leaving socket
activation to start the service when it's needed.
## Sysinit reactivation {#sec-sysinit-reactivation}
[`sysinit.target`](https://www.freedesktop.org/software/systemd/man/latest/systemd.special.html#sysinit.target)
is a systemd target that encodes system initialization (i.e. early startup). A
few units that need to run very early in the bootup process are ordered to
finish before this target is reached. Probably the most notable one of these is
`systemd-tmpfiles-setup.service`. We will refer to these units as "sysinit
units".
"Normal" systemd units, by default, are ordered AFTER `sysinit.target`. In
other words, these "normal" units expect all services ordered before
`sysinit.target` to have finished without explicity declaring this dependency
relationship for each dependency. See the [systemd
bootup](https://www.freedesktop.org/software/systemd/man/latest/bootup.html)
for more details on the bootup process.
When restarting both a unit ordered before `sysinit.target` as well as one
after, this presents a problem because they would be started at the same time
as they do not explicitly declare their dependency relations.
To solve this, NixOS has an artificial `sysinit-reactivation.target` which
allows you to ensure that services ordered before `sysinit.target` are
restarted correctly. This applies both to the ordering between these sysinit
services as well as ensuring that sysinit units are restarted before "normal"
units.
To make an existing sysinit service restart correctly during system switch, you
have to declare:
```nix
systemd.services.my-sysinit = {
requiredBy = [ "sysinit-reactivation.target" ];
before = [ "sysinit-reactivation.target" ];
restartTriggers = [ config.environment.etc."my-sysinit.d".source ];
};
```
You need to configure appropriate `restartTriggers` specific to your service.