improve all-formats module: support customizing and creating formats + add tests + add readme

[DavHau]

While the all-formats.nix module (introduced via #241) allows to import all formats at once, it lacks an interface to customize formats or add formats.

This is fixed by adding the top-level option formatConfigs (attrsOf deferredModule).

All format modules created under config.formatConfigs are mapped so their outputs are available under config.formats which has also been moved to the top-level (previously config.system.formats).

Done:

• expose the all-formats module via nixosModules
• add option formatConfigs (pre-populated with all existing formats)
• move option system.formats -> formats
• add test for customizing a format
• add example to readme
• add test similar to the new readme example ensuring that formats can be customized and created

Open questions:

• should the readme section Using in a Flake be removed now, as the new section Using as a nixos-module supports all its features and should be simpler to use (no need to call a function for each format etc.)?

[mayl]

Probably a dumb question: if I nixos-rebuild --flake .#my-machine what does this build? Just some kind of store path with the nixos-system in it?

[DavHau]

Yes, and this isn't changed by the all-formats module.

[mayl]

This is clear if there is only one machine defined per flake. Is there a convenient way you know of to "namespace" these formats so I could do something like nix build .#machine1.vm and nix build .#machine2.virtualbox?

[mayl]

Ok, not sure if it's right or not, but looks like the way I've kind of done this before without violating the flakes package schema is to make an unstructured output:

outputs { ... }: {
  ...
  my-machine = nixpkgs.lib.getAttrs ["vm" "virtualbox"] self.nixosConfigurations.my-machine.config.formats;
};

Which then can then be used:

nix build .#my-machine.vm # build the qemu vm
nix build .#my-machine.virtualbox # build the virtualbox vm

[DavHau]

This is clear if there is only one machine defined per flake. Is there a convenient way you know of to "namespace" these formats so I could do something like nix build .#machine1.vm and nix build .#machine2.virtualbox?

Defining packages outputs is not necessary. You can just simply:

nix build .#nixosConfigurations.machine1.config.formats.vm

...or

nix build .#nixosConfigurations.machine2.config.formats.virtualbox

Creating an unstructured output is another way how to achieve this.

[DavHau]

Via 34bbb3e I now removed the misleading packages output from the example flake and for the nix build commands use the more explicit referencing.

[mayl]

I used comments to ask a couple questions, but I've tried out your branch and it seems to work well for me. This seems really cool, and would be very helpful for how I use nixos-generators.

Work for a future PR, but I'm thinking it would still be useful to have a helper function to wrap up some of this boilerplate. I'm thinking something that took a configuration module and then automatically injected the all-formats import, called lib.nixosSystem and generated the various flake output attrs in one go. Maybe best done with a flake-parts flake module?

[DavHau]

I'm thinking something that took a configuration module and then automatically injected the all-formats import, called lib.nixosSystem and generated the various flake output attrs in one go. Maybe best done with a flake-parts flake module?

Making a flake-parts module sounds like a good way to achieve what you describe.

[Lassulus]

alright, finally had some time to test this, looks pretty nice!

[MICHAELABICK]

This seems to work great! I'm not really super experienced with Nix but is there any way to get the image filename path like the nixos-generate script does? It looks like passing the --print-out-paths option to nix build gets part of the way there but only provides the output directory.

[Lassulus]

This seems to work great! I'm not really super experienced with Nix but is there any way to get the image filename path like the nixos-generate script does? It looks like passing the --print-out-paths option to nix build gets part of the way there but only provides the output directory.

this is now taken care of in #263