nixos/loki: Implement configuration verification

[jpds]

Description of changes

Fixes: #293088

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/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 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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[emilylange]

This should use cfg.package not pkgs.grafana-loki :(

[drupol]

I'll provide the PR to fix this, I should have seen this.

[drupol]

#312294

[mjm]

Can we make this optional? If you use -config.expand-env=true to expand environment variables in your config, the validation may end up failing even though it will be fine at runtime:

failed parsing config: /nix/store/85dzsqn3qjhzqfjmaiz95xgr4x9jag4c-loki-config.json: parse "http://${AWS_ACCESS_KEY_ID}:${AWS_SECRET_ACCESS_KEY}@garage.service.consul:3902": net/url: invalid userinfo. Use `-config.expand-env=true` flag if you want to expand environment variables in your config file

[emilylange]

Thanks for reporting!

Taking a second look it's not just services.loki.extraFlags that breaks it.
The use of services.loki.configFile will error out as well.

services.loki.configFile is a string of a path that is not copied to the build sandbox.
It may only exist on the destination host, or only after the action script, etc.

I am tempted to add a conditional to only ever run that config validation/verification step if both cfg.extraFlags == [] and cfg.configFile == null are true.
I can't think of other cases were the validation step would fail.

Not sure if you are asking for a boolean option in services.loki.
But I think I would prefer it to have it somewhat "automagical".

Will open a PR with my proposed fix shortly.

[mjm]

I was thinking of a boolean option, similar to services.prometheus.checkConfig. But your automatic suggestion would work for me just as well.

[emilylange]

• nixos/loki: skip config validation when it's impossible to validate #312785