CS SYD - Announcing autodocodec-nix and Nix integration for opt-env-conf

NorfairKing · July 29, 2024, 4:02pm

https://cs-syd.eu/posts/2024-07-29-announcing-autodocodec-nix

enobayram · July 31, 2024, 1:17pm

I’ve also often found myself wondering about the best way to integrate the Haskell-side option parsing with the Nix module options. This is a clean approach, but it forces you to commit generated code and add CI steps to make sure it stays in sync.

Even if you’re OK with committing generated code, I think it would still be very frustrating to have the module options go out of sync with the Haskell parser and to get option parsing failures while the deployed systemd service is starting. Could be great to add a --config-parse-only way to run your service binary so that you could implement a Nix derivation that will check the given Nix configuration against the service binary getting run, so that you’d get configuration errors at nix build time of your NixOS image, instead of at service start time (which could be after you’ve already run your database migrations, for instance).

NorfairKing · July 31, 2024, 6:17pm

Even if you’re OK with committing generated code, I think it would still be very frustrating to have the module options go out of sync with the Haskell parser and to get option parsing failures while the deployed systemd service is starting.

There’s a function for that:

github.com/NorfairKing/opt-env-conf

opt-env-conf-test/src/OptEnvConf/Test.hs

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            specify "produces the same reference documentation as before" $
              pureGoldenReferenceDocumentation path progname parser
          
          pureGoldenReferenceDocumentation :: FilePath -> String -> Parser a -> GoldenTest Text
          pureGoldenReferenceDocumentation path progname parser =
            pureGoldenTextFile path $
              renderChunksText With24BitColours $
                renderReferenceDocumentation progname $
                  parserDocs parser
          
          goldenSettingsNixOptionsSpec :: forall a. (HasCallStack) => (HasParser a) => FilePath -> Spec
          goldenSettingsNixOptionsSpec path = withFrozenCallStack $ goldenParserNixOptionsSpec (settingsParser @a) path
          
          goldenParserNixOptionsSpec :: (HasCallStack) => Parser a -> FilePath -> Spec
          goldenParserNixOptionsSpec parser path = withFrozenCallStack $ do
            specify "produces the nix options as before" $
              pureGoldenNixOptions path parser
          
          pureGoldenNixOptions :: FilePath -> Parser a -> GoldenTest Text
          pureGoldenNixOptions path parser =
            pureGoldenTextFile path $

Could be great to add a --config-parse-only way to run your service binary so that you could implement a Nix derivation that will check the given Nix configuration against the service binary getting run, so that you’d get configuration errors at nix build time of your NixOS image, instead of at service start time (which could be after you’ve already run your database migrations, for instance).

That exists as --run-settings-check:

github.com/NorfairKing/opt-env-conf

nix/overrides.nix

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                  echo 'Could not find ${exeName} in completion script.'
                  exit 1
                }
              '';
            });
          installCompletions = exeNames: drv:
            foldr installCompletion drv exeNames;
          installManpagesAndCompletions = exeNames: drv:
            installManpages exeNames (installCompletions exeNames drv);
          
          makeSettingsCheckScript = name: exe: args: env: writeShellApplication {
            inherit name;
            runtimeEnv = env;
            text = ''
              ${exe} --run-settings-check ${concatStringsSep " " args}
            '';
          };
          
          # A nix build that does a settings check.
          # Note that this will only work if the option parsing does not rely on any
          # files not available in the nix build sandbox.

enobayram · August 1, 2024, 7:12am

Thanks for your response!

I like your framing of it as a golden test case, it’s so obvious in hindsight but hadn’t thought of it that way.

The --run-settings-check would also be great to run as a derivation under system.checks:

{
  config.system.checks = [ ( pkgs.runCommand "My Service config check" {} ''
    ${cfg.package} --run-settings-check ${concatStringsSep " " cfg.args}
  '')];
}

I’ll make sure to check out autodocodec-nix the next time I’m writing a NixOS module for a Haskell service, thanks for sharing it!