nixos/klipper: preserve SAVE_CONFIG for nixos-managed config

[kira-bruneau]

Klipper macros that use SAVE_CONFIG (eg. bed mesh calibration, PID tuning, ...) don't currently work with a nixos-managed config.

This can be worked around by using services.klipper.mutableConfig = true, but then you lose out on being able to configure klipper from NixOS.

This changes the default behaviour so that:

1. The config is stored in services.klipper.configDir instead of /etc
2. The config is copied instead of symlinked (keeping a timestamped backup of the existing config)
3. The SAVE_CONFIG section from the backup is copied over into the new config
4. The backup is deleted if the final config is identical

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[cab404]

I do support the «deprecate immutable config» part. However, I do want to retain the ability to change the rest of the config without changing nixos configuration.

[kira-bruneau]

Oh yep, you should still be able to do that. If you want to quickly test a change without committing to it in your NixOS config, you can just add something to the SAVE_CONFIG section (although the comment there does point out you probably shouldn't modify that section, because klipper could stamp over anything you change there).

Also, if you wanted to permanently persist something outside of your Nix config, you can add an include section in settings that points to a path that isn't manged by NixOS.

[cab404]

Also, if you wanted to permanently persist something outside of your Nix config, you can add an include section in settings that points to a path that isn't manged by NixOS.

But I do want to have a default config!

Maybe this should just replace immutable config instead, as it basically retains the same semantics, yet fixing the SAVE_CONFIG problem?

[kira-bruneau]

But I do want to have a default config!

I'm not really sure what you mean by that, I think there might be a misunderstanding? With this PR, what I mentioned would still allow for a default config.

Maybe this should just replace immutable config instead, as it basically retains the same semantics, yet fixing the SAVE_CONFIG problem?

I think this PR already does this? (but I feel unclear on what you meant earlier, so I might be missing something)

You got me thinking though, maybe it would be simpler to patch klipper to automatically include /etc/printer.cfg as a default config? That way we wouldn't have to do this weird workaround to merge an immutable config into a mutable one.

[cab404]

But I do want to have a default config!

I'm not really sure what you mean by that, I think there might be a misunderstanding? With this PR, what I mentioned would still allow for a default config.

What I mean is that I want default config while retaining the option to edit it in arbitrary locations. E.g if I am building an image for a generic printer platform, and I want to provide a default for it, but not to restrict end user in any way.

[kira-bruneau]

Ohhh ok thanks! That makes a lot more sense!

Basically what I wanted was to be able to manage all the settings from NixOS, with the exception of SAVE_CONFIG. I'd only want to modify printer.cfg manually to quickly test something, and then I would move it over to my NixOS config when I was ok with the change.

It's sounds like you designed mutableConfig to basically keep NixOS from touching the config, except for initialization. I think it was wrong for me to put the behaviour I wanted into the mutableConfig option then.

How would you feel about a new managedConfig option that replaces "immutable" configs? Then mutableConfig could be deprecated in favour of setting managedConfig = false.

[cab404]

I really do feel that your changes should be enabled by default, as they are strictly better than the previous default — so maybe just use them if mutableConfig = false?

[kira-bruneau]

Oh yeah that would be ideal, but the reason I thought of the new option was because the change would be breaking. So we'd need some kind of transitory option between old state versions and new ones.

[cab404]

Oh yeah that would be ideal, but the reason I thought of the new option was because the change would be breaking. So we'd need some kind of transitory option between old state versions and new ones.

Hmm, I don't really see how it would be breaking

[kira-bruneau]

Oh wait sorry, your right. I'm not sure why I thought it would be earlier 😅

[kira-bruneau]

I really do feel that your changes should be enabled by default, as they are strictly better than the previous default — so maybe just use them if mutableConfig = false?

Ok, it should be good now!

[cab404]

LGTM!

Got stuck for a second thinking of removed chmod, but I guess that should work with piping instead of cp <3

[cab404]

@kira-bruneau Thank you so much for your PR <3