mopidy: add module

This is adapted from the `services.mopidy` NixOS module. The
difference is the setting can be configured with Nix language, taking
advantage of generators from nixpkgs. The module is also suited more
for user-specific configuration, removing the `extraConfigFiles` and
`dataDir` option.

modules/services/mopidy.nix

@@ -0,0 +1,136 @@
+{ config, options, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.services.mopidy;
+
+  # The configuration format of Mopidy. It seems to use configparser with
+  # some quirky handling of its types. You can see how they're handled in
+  # `mopidy/config/types.py` from the source code.
+  toMopidyConf = generators.toINI {
+    mkKeyValue = generators.mkKeyValueDefault {
+      mkValueString = v:
+        if isList v then
+          "\n  " + concatStringsSep "\n  " v
+        else
+          generators.mkValueStringDefault { } v;
+    } " = ";
+  };
+
+  mopidyEnv = pkgs.buildEnv {
+    name = "mopidy-with-extensions-${pkgs.mopidy.version}";
+    paths = closePropagation cfg.extensionPackages;
+    pathsToLink = [ "/${pkgs.mopidyPackages.python.sitePackages}" ];
+    buildInputs = [ pkgs.makeWrapper ];
+    postBuild = ''
+      makeWrapper ${pkgs.mopidy}/bin/mopidy $out/bin/mopidy \
+        --prefix PYTHONPATH : $out/${pkgs.mopidyPackages.python.sitePackages}
+    '';
+  };
+
+  # Nix-representable format for Mopidy config.
+  mopidyConfFormat = { }: {
+    type = with types;
+      let
+        valueType = nullOr (oneOf [ bool float int str (listOf valueType) ])
+          // {
+            description = "Mopidy config value";
+          };
+      in attrsOf (attrsOf valueType);
+
+    generate = name: value: pkgs.writeText name (toMopidyConf value);
+  };
+
+  settingsFormat = mopidyConfFormat { };
+
+in {
+  meta.maintainers = [ hm.maintainers.foo-dogsquared ];
+
+  options.services.mopidy = {
+    enable = mkEnableOption "Mopidy music player daemon";
+
+    extensionPackages = mkOption {
+      type = with types; listOf package;
+      default = [ ];
+      example = literalExpression
+        "with pkgs; [ mopidy-spotify mopidy-mpd mopidy-mpris ]";
+      description = ''
+        Mopidy extensions that should be loaded by the service.
+      '';
+    };
+
+    settings = mkOption {
+      type = settingsFormat.type;
+      default = { };
+      example = literalExpression ''
+        {
+          file = {
+            media_dirs = [
+              "$XDG_MUSIC_DIR|Music"
+              "~/library|Library"
+            ];
+            follow_symlinks = true;
+            excluded_file_extensions = [
+              ".html"
+              ".zip"
+              ".jpg"
+              ".jpeg"
+              ".png"
+            ];
+          };
+
+          # Please don't put your mopidy-spotify configuration in the public. :)
+          # Think of your Spotify Premium subscription!
+          spotify = {
+            client_id = "CLIENT_ID";
+            client_secret = "CLIENT_SECRET";
+          };
+        }
+      '';
+      description = ''
+        Configuration written to
+        <filename>$XDG_CONFIG_HOME/mopidy/mopidy.conf</filename>.
+        </para><para>
+        See <link xlink:href="https://docs.mopidy.com/en/latest/config/"/> for
+        more details.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    assertions =
+      [ (hm.assertions.assertPlatform "services.mopidy" pkgs platforms.linux) ];
+
+    xdg.configFile."mopidy/mopidy.conf".source =
+      settingsFormat.generate "mopidy-${config.home.username}" cfg.settings;
+
+    systemd.user.services.mopidy = {
+      Unit = {
+        Description = "mopidy music player daemon";
+        Documentation = [ "https://mopidy.com/" ];
+        After = [ "network.target" "sound.target" ];
+      };
+
+      Service = { ExecStart = "${mopidyEnv}/bin/mopidy"; };
+
+      Install.WantedBy = [ "default.target" ];
+    };
+
+    systemd.user.services.mopidy-scan = {
+      Unit = {
+        Description = "mopidy local files scanner";
+        Documentation = [ "https://mopidy.com/" ];
+        After = [ "network.target" "sound.target" ];
+      };
+
+      Service = {
+        ExecStart = "${mopidyEnv}/bin/mopidy local scan";
+        Type = "oneshot";
+      };
+
+      Install.WantedBy = [ "default.target" ];
+    };
+  };
+}