xdg: add module

When enabled this module will cause Home Manager to manage the user
environment XDG variables. When disabled, then Home Manager will use
the XDG variables taken from the user environment.

modules/default.nix

@@ -19,6 +19,7 @@ let
     ./misc/news.nix
     ./misc/nixpkgs.nix
     ./misc/pam.nix
+    ./misc/xdg.nix
     ./programs/bash.nix
     ./programs/beets.nix
     ./programs/browserpass.nix

modules/misc/xdg.nix

@@ -0,0 +1,138 @@
+{ options, config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.xdg;
+
+  fileType = basePath: (types.loaOf (types.submodule (
+    { name, config, ... }: {
+      options = {
+        target = mkOption {
+          type = types.str;
+          apply = p: "${cfg.configHome}/${p}";
+          description = ''
+            Path to target file relative to <varname>${basePath}</varname>.
+          '';
+        };
+
+        text = mkOption {
+          default = null;
+          type = types.nullOr types.lines;
+          description = "Text of the file.";
+        };
+
+        source = mkOption {
+          type = types.path;
+          description = ''
+            Path of the source file. The file name must not start
+            with a period since Nix will not allow such names in
+            the Nix store.
+            </para><para>
+            This may refer to a directory.
+          '';
+        };
+
+        executable = mkOption {
+          type = types.bool;
+          default = false;
+          description = "Whether the file should be executable.";
+        };
+      };
+
+      config = {
+        target = mkDefault name;
+        source = mkIf (config.text != null) (
+          let
+            file = pkgs.writeTextFile {
+              inherit (config) text executable;
+              name = "user-etc-" + baseNameOf name;
+            };
+          in
+            mkDefault file
+        );
+      };
+    }
+  )));
+
+  defaultCacheHome = "${config.home.homeDirectory}/.cache";
+  defaultConfigHome = "${config.home.homeDirectory}/.config";
+  defaultDataHome = "${config.home.homeDirectory}/.local/share";
+
+  getXdgDir = name: fallback:
+    let
+      value = builtins.getEnv name;
+    in
+      if value != "" then value else fallback;
+
+in
+
+{
+  options.xdg = {
+    enable = mkEnableOption "management of XDG base directories";
+
+    cacheHome = mkOption {
+      type = types.path;
+      defaultText = "~/.cache";
+      description = ''
+        Absolute path to directory holding application caches.
+      '';
+    };
+
+    configFile = mkOption {
+      type = fileType "xdg.configHome";
+      default = {};
+      description = ''
+        Attribute set of files to link into the user's XDG
+        configuration home.
+      '';
+    };
+
+    configHome = mkOption {
+      type = types.path;
+      defaultText = "~/.config";
+      description = ''
+        Absolute path to directory holding application configurations.
+      '';
+    };
+
+    dataHome = mkOption {
+      type = types.path;
+      defaultText = "~/.local/share";
+      description = ''
+        Absolute path to directory holding application data.
+      '';
+    };
+  };
+
+  config = mkMerge [
+    (mkIf cfg.enable {
+      xdg.cacheHome = mkDefault defaultCacheHome;
+      xdg.configHome = mkDefault defaultConfigHome;
+      xdg.dataHome = mkDefault defaultDataHome;
+
+      home.sessionVariables = {
+        XDG_CACHE_HOME = cfg.cacheHome;
+        XDG_CONFIG_HOME = cfg.configHome;
+        XDG_DATA_HOME = cfg.dataHome;
+      };
+    })
+
+    (mkIf (!cfg.enable) {
+      xdg.cacheHome = getXdgDir "XDG_CACHE_HOME" defaultCacheHome;
+      xdg.configHome = getXdgDir "XDG_CONFIG_HOME" defaultConfigHome;
+      xdg.dataHome = getXdgDir "XDG_DATA_HOME" defaultDataHome;
+    })
+
+    {
+      home.file =
+        let
+          f = n: v: {
+            inherit (v) source target;
+            mode = if v.executable then "777" else "444";
+          };
+        in mapAttrsToList f cfg.configFile;
+    }
+  ];
+}