Starting up services configured with batou.lib.service.Service via systemctl start may not be necessary on NixOS

[Ma27]

To be precise, I'm talking about the following code-snippet:

https://github.com/flyingcircusio/batou_ext/blob/2.4.2/src/batou_ext/nix.py#L298-L305

I'm not sure if this is actually needed:

• each service generated like this has a wantedBy = [ "multi-user.target" ];:

https://github.com/flyingcircusio/fc-nixos/blob/11b6b49aaaa04145496a208cf23d1d7515dab0b2/nixos/platform/systemd.nix#L74-L78

• inactive services that are wanted by the multi-user.target are started on switch-to-configuration.pl: See https://nixos.org/manual/nixos/stable/#sec-unit-handling (multi-user.target gets started on a switch which starts up inactive units).

• a rebuild causing a switch is done on the nixos platform implementation: https://github.com/flyingcircusio/batou_ext/blob/2.4.2/src/batou_ext/nix.py#L290

The reason why I'm bringing this up is a practical one: in some cases you want to deploy services without starting them (which is effectively forced by the current implementation).

An example is a bunch of queue consumers that must not be started before a certain component is finished. What I tried was to temporarily mask the services during a deployment, however then systemctl is-active returns false causing the component to attempt starting up the unit which fails because the unit got masked before.

For context, see also FC-31563.

cc @zagy