[OS-Release] Set image_id to NixOS-based system build toplevel on NixOS systems

[mgrzybek]

Context

• NixOS is based on derivations. The system's state produces a unique hash.
• image_id is empty on NixOS systems.

Why is it useful?

While managing upgrades, checking the drift is very important. Comparing the live image_id with the desired state from some nix-build evaluation is very practical.

This is why this patch checks if we are running on a NixOS system (NAME env variable) and gets the hash from the symlink at /run/current-system.
The test grabs the hash from the local NixOS system or creates a fake

[mgrzybek]

Hi @SuperQ and @discordianfish,

This PR is ready for human validation. The changes are quite small, I hope the code is fine for you.

In the end I removed some content from os_release_test.go because reading/creating a fake /run/current-system is complicated.

Regards,
Mathieu

[mweinelt]

NixOS Infra uses a small python script, that exposes various things as a textfile. Ideally we could extract, package and expose that with node-exporter downstream in nixpkgs.

https://github.com/NixOS/infra/tree/main/modules/prometheus/nixos-exporter

[mgrzybek]

I managed to adapt my code in order to get the data directly from the nix store sqlite database:

• creating some fixtures: fake symlinks and a nix store DB
• making some paths configurable when testing

However the make common-unused target prevents me from adding a dependancy into go.mod. Basically I need github.com/mattn/go-sqlite3.

Is it acceptable to add this dependancy? What's the procedure?

[SuperQ]

This seems like an issue for NixOS, it needs to include an IMAGE_ID in the os-release file.

[mweinelt]

https://github.com/NixOS/nixpkgs/blob/67d39b38ad43310b5f87b17cd8c1dde60b845fa5/nixos/modules/misc/version.nix#L33-L61

[mgrzybek]

I'm not a NixOS expert but you seem to tell me that I can set IMAGE_ID and IMAGE_VERSION using my nix configuration.

Such as:

system.image = {
  version = system.build.toplevel; # Using lib.strings.splitString to extract the hash
  id = "";
}

Is it right?

[mweinelt]

Yes, that looks correct. I suppose we really only set it for actual images.