nix: add structural settings

.github/CODEOWNERS

@@ -21,6 +21,9 @@
 
 /modules/misc/news.nix                                @rycee
 
+/modules/misc/nix.nix                                 @polykernel
+/tests/modules/misc/nix                               @polykernel
+
 /modules/misc/nixpkgs-disabled.nix                    @thiagokokada
 
 /modules/misc/numlock.nix                             @evanjs

modules/misc/nix.nix

@@ -6,8 +6,98 @@ let
 
   cfg = config.nix;
 
+  nixPackage = cfg.package;
+
+  isNixAtLeast = versionAtLeast (getVersion nixPackage);
+
+  nixConf = assert isNixAtLeast "2.2";
+    let
+
+      mkValueString = v:
+        if v == null then
+          ""
+        else if isInt v then
+          toString v
+        else if isBool v then
+          boolToString v
+        else if isFloat v then
+          floatToString v
+        else if isList v then
+          toString v
+        else if isDerivation v then
+          toString v
+        else if builtins.isPath v then
+          toString v
+        else if isString v then
+          v
+        else if isCoercibleToString v then
+          toString v
+        else
+          abort "The nix conf value: ${toPretty { } v} can not be encoded";
+
+      mkKeyValue = k: v: "${escape [ "=" ] k} = ${mkValueString v}";
+
+      mkKeyValuePairs = attrs:
+        concatStringsSep "\n" (mapAttrsToList mkKeyValue attrs);
+
+    in pkgs.writeTextFile {
+      name = "nix.conf";
+      text = ''
+        # WARNING: this file is generated from the nix.settings option in
+        # your Home Manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.
+        # Do not edit it!
+        ${mkKeyValuePairs cfg.settings}
+        ${cfg.extraOptions}
+      '';
+      checkPhase =
+        if pkgs.stdenv.hostPlatform != pkgs.stdenv.buildPlatform then ''
+          echo "Ignoring validation for cross-compilation"
+        '' else ''
+          echo "Validating generated nix.conf"
+          ln -s $out ./nix.conf
+          set -e
+          set +o pipefail
+          NIX_CONF_DIR=$PWD \
+            ${cfg.package}/bin/nix show-config ${
+              optionalString (isNixAtLeast "2.3pre")
+              "--no-net --option experimental-features nix-command"
+            } \
+            |& sed -e 's/^warning:/error:/' \
+            | (! grep '${
+              if cfg.checkConfig then "^error:" else "^error: unknown setting"
+            }')
+          set -o pipefail
+        '';
+    };
+
+  semanticConfType = with types;
+    let
+      confAtom = nullOr (oneOf [ bool int float str path package ]) // {
+        description =
+          "Nix config atom (null, bool, int, float, str, path or package)";
+      };
+    in attrsOf (either confAtom (listOf confAtom));
+
+  jsonFormat = pkgs.formats.json { };
+
 in {
   options.nix = {
+    enable = mkEnableOption ''
+      the Nix configuration module
+    '' // {
+      default = true;
+      visible = false;
+    };
+
+    package = mkOption {
+      type = types.nullOr types.package;
+      default = null;
+      example = literalExpression "pkgs.nix";
+      description = ''
+        The Nix package that the configuration should be generated for.
+      '';
+    };
+
     registry = mkOption {
       type = types.attrsOf (types.submodule (let
         inputAttrs = types.attrsOf
@@ -68,19 +158,81 @@ in {
         User level flake registry.
       '';
     };
+
     registryVersion = mkOption {
       type = types.int;
       default = 2;
       internal = true;
       description = "The flake registry format version.";
     };
+
+    checkConfig = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        If enabled (the default), checks for data type mismatches and that Nix
+        can parse the generated nix.conf.
+      '';
+    };
+
+    extraOptions = mkOption {
+      type = types.lines;
+      default = "";
+      example = ''
+        keep-outputs = true
+        keep-derivations = true
+      '';
+      description =
+        "Additional text appended to <filename>nix.conf</filename>.";
+    };
+
+    settings = mkOption {
+      type = types.submodule { freeformType = semanticConfType; };
+      default = { };
+      example = literalExpression ''
+        {
+          use-sandbox = true;
+          show-trace = true;
+          system-features = [ "big-parallel" "kvm" "recursive-nix" ];
+        }
+      '';
+      description = ''
+        Configuration for Nix, see
+        <link xlink:href="https://nixos.org/manual/nix/stable/#sec-conf-file"/> or
+        <citerefentry>
+          <refentrytitle>nix.conf</refentrytitle>
+          <manvolnum>5</manvolnum>
+        </citerefentry> for avalaible options.
+        The value declared here will be translated directly to the key-value pairs Nix expects.
+        </para>
+        <para>
+        Configuration specified in <option>nix.extraOptions</option> which will be appended
+        verbatim to the resulting config file.
+      '';
+    };
   };
 
-  config = mkIf (cfg.registry != { }) {
-    xdg.configFile."nix/registry.json".text = builtins.toJSON {
-      version = cfg.registryVersion;
-      flakes =
-        mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
+  config = mkIf cfg.enable {
+    assertions = [{
+      assertion = cfg.settings == { } || cfg.package != null;
+      message = ''
+        A corresponding Nix package must be specified via `nix.package` for generating
+        nix.conf.
+      '';
+    }];
+
+    xdg.configFile = {
+      "nix/registry.json" = mkIf (cfg.registry != { }) {
+        source = jsonFormat.generate "registry.json" {
+          version = cfg.registryVersion;
+          flakes =
+            mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
+        };
+      };
+
+      "nix/nix.conf" = mkIf (cfg.settings != { }) { source = nixConf; };
     };
   };
+
+  meta.maintainers = [ maintainers.polykernel ];
 }

tests/default.nix

@@ -45,6 +45,7 @@ import nmt {
     ./modules/files
     ./modules/home-environment
     ./modules/misc/fontconfig
+    ./modules/misc/nix
     ./modules/programs/alacritty
     ./modules/programs/alot
     ./modules/programs/aria2

tests/modules/misc/nix/default.nix

@@ -0,0 +1,5 @@
+{
+  nix-empty-settings = ./empty-settings.nix;
+  nix-example-settings = ./example-settings.nix;
+  nix-example-registry = ./example-registry.nix;
+}

tests/modules/misc/nix/empty-settings.nix

@@ -0,0 +1,13 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    nix = { package = config.lib.test.mkStubPackage { }; };
+
+    nmt.script = ''
+      assertPathNotExists home-files/.config/nix
+    '';
+  };
+}

tests/modules/misc/nix/example-registry-expected.json

@@ -0,0 +1,17 @@
+{
+  "flakes": [
+    {
+      "exact": true,
+      "from": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      },
+      "to": {
+        "owner": "my-org",
+        "repo": "my-nixpkgs",
+        "type": "github"
+      }
+    }
+  ],
+  "version": 2
+}

tests/modules/misc/nix/example-registry.nix

@@ -0,0 +1,25 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    nix = {
+      registry = {
+        nixpkgs = {
+          to = {
+            type = "github";
+            owner = "my-org";
+            repo = "my-nixpkgs";
+          };
+        };
+      };
+    };
+
+    nmt.script = ''
+      assertFileContent \
+        home-files/.config/nix/registry.json \
+        ${./example-registry-expected.json}
+    '';
+  };
+}

tests/modules/misc/nix/example-settings-expected.conf

@@ -0,0 +1,7 @@
+# WARNING: this file is generated from the nix.settings option in
+# your Home Manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.
+# Do not edit it!
+show-trace = true
+system-features = big-parallel kvm recursive-nix
+use-sandbox = true
+

tests/modules/misc/nix/example-settings.nix

@@ -0,0 +1,33 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    nix = {
+      package = config.lib.test.mkStubPackage {
+        version = lib.getVersion pkgs.nixStable;
+        buildScript = ''
+          target=$out/bin/nix
+          mkdir -p "$(dirname "$target")"
+
+          echo -n "true" > "$target"
+
+          chmod +x "$target"
+        '';
+      };
+
+      settings = {
+        use-sandbox = true;
+        show-trace = true;
+        system-features = [ "big-parallel" "kvm" "recursive-nix" ];
+      };
+    };
+
+    nmt.script = ''
+      assertFileContent \
+        home-files/.config/nix/nix.conf \
+        ${./example-settings-expected.conf}
+    '';
+  };
+}