refactor: major refactor into proper reusable modules. No logical changes.

modules/repo/secrets.nix

@@ -0,0 +1,94 @@
+{
+  config,
+  inputs,
+  lib,
+  pkgs,
+  ...
+}: let
+  inherit
+    (lib)
+    assertMsg
+    attrNames
+    literalExpression
+    mapAttrs
+    mdDoc
+    mkIf
+    mkOption
+    types
+    ;
+
+  # If the given expression is a bare set, it will be wrapped in a function,
+  # so that the imported file can always be applied to the inputs, similar to
+  # how modules can be functions or sets.
+  constSet = x:
+    if builtins.isAttrs x
+    then (_: x)
+    else x;
+
+  # Try to access the extra builtin we loaded via nix-plugins.
+  # Throw an error if that doesn't exist.
+  rageImportEncrypted = assert assertMsg (builtins ? extraBuiltins.rageImportEncrypted) "The extra builtin 'rageImportEncrypted' is not available, so repo.secrets cannot be decrypted. Did you forget to use `defineNixExtraBuiltins` or use the appropriate ad-hoc command line arguments?";
+    builtins.extraBuiltins.rageImportEncrypted;
+
+  # This "imports" an encrypted .nix.age file by evaluating the decrypted content.
+  importEncrypted = path:
+    constSet (
+      if builtins.pathExists path
+      then rageImportEncrypted inputs.self.secretsConfig.masterIdentities path
+      else {}
+    );
+
+  cfg = config.repo;
+in {
+  options.repo = {
+    defineNixExtraBuiltins = mkOption {
+      default = false;
+      type = types.bool;
+      description = mdDoc ''
+        Add nix-plugins and the correct extra-builtin-files definition to this host's
+        nix configuration, so that it can be used to decrypt the secrets in this repository.
+      '';
+    };
+
+    secretFiles = mkOption {
+      default = {};
+      type = types.attrsOf types.path;
+      example = literalExpression "{ local = ./secrets.nix.age; }";
+      description = mdDoc ''
+        This file manages the origin for this machine's repository-secrets. Anything that is
+        technically not a secret in the classical sense (i.e. that it has to be protected
+        after it has been deployed), but something you want to keep secret from the public;
+        Anything that you wouldn't want people to see on GitHub, but that can live unencrypted
+        on your own devices. Consider it a more ergonomic nix alternative to using git-crypt.
+
+        All of these secrets may (and probably will be) put into the world-readable nix-store
+        on the build and target hosts. You'll most likely want to store personally identifiable
+        information here, such as:
+          - MAC Addreses
+          - Static IP addresses
+          - Your full name (when configuring your users)
+          - Your postal address (when configuring e.g. home-assistant)
+          - ...
+
+        Each path given here must be an age-encrypted .nix file. For each attribute `<name>`,
+        the corresponding file will be decrypted, imported and exposed as {option}`repo.secrets.<name>`.
+      '';
+    };
+
+    secrets = mkOption {
+      readOnly = true;
+      default = mapAttrs (_: x: importEncrypted x inputs) cfg.secretFiles;
+      type = types.unspecified;
+      description = "Exposes the loaded repo secrets. This option is read-only.";
+    };
+  };
+
+  config = {
+    # Make sure not to reference the extra-builtins file directly but
+    # at least via its parent folder so it can access relative files.
+    nix.extraOptions = mkIf cfg.defineNixExtraBuiltins ''
+      plugin-files = ${pkgs.nix-plugins}/lib/nix/plugins
+      extra-builtins-file = ${inputs.self.outPath}/nix/extra-builtins.nix
+    '';
+  };
+}