Update the locate database using a systemd service

This makes it easier to update the database manually ("systemctl start
update-locatedb").

Also, use modern module syntax.

modules/misc/locate.nix

@@ -1,60 +1,62 @@
-{pkgs, config, ...}:
+{ config, pkgs, ... }:
+
+with pkgs.lib;
 
-###### interface
 let
-  inherit (pkgs.lib) mkOption;
 
-  options = {
-    services = {
-      locate = {
-
-        enable = mkOption {
-          default = false;
-          example = true;
-          description = ''
-            If enabled, NixOS will periodically update the database of
-            files used by the <command>locate</command> command.
-          '';
-        };
-
-        period = mkOption {
-          default = "15 02 * * *";
-          description = ''
-            This option defines (in the format used by cron) when the
-            locate database is updated.
-            The default is to update at 02:15 (at night) every day.
-          '';
-        };
-
-      };
-
-    };
-  };
-in
-
-###### implementation
-let
   locatedb = "/var/cache/locatedb";
 
-  updatedbCmd =
-    "${config.services.locate.period}  root  " +
-    "mkdir -m 0755 -p $(dirname ${locatedb}) && " +
-    "nice -n 19 ${pkgs.utillinux}/bin/ionice -c 3 " +
-    "updatedb --localuser=nobody --output=${locatedb} > /var/log/updatedb 2>&1";
 in
 
 {
-  require = [
-    # ../../upstart-jobs/cron.nix # config.services.cron
-    options
-  ];
 
-  services = {
+  ###### interface
-    cron = {
+
-      systemCronJobs =
+  options = {
-        pkgs.lib.optional
+
-          config.services.locate.enable
+    services.locate = {
-          updatedbCmd;
+
+      enable = mkOption {
+        default = false;
+        example = true;
+        description = ''
+          If enabled, NixOS will periodically update the database of
+          files used by the <command>locate</command> command.
+        '';
+      };
+
+      period = mkOption {
+        default = "15 02 * * *";
+        description = ''
+          This option defines (in the format used by cron) when the
+          locate database is updated.
+          The default is to update at 02:15 (at night) every day.
+        '';
+      };
+
     };
+
   };
+
+  ###### implementation
+
+  config = {
+
+    systemd.services.update-locatedb =
+      { description = "Update Locate Database";
+        path  = [ pkgs.su ];
+        script =
+          ''
+            mkdir -m 0755 -p $(dirname ${locatedb})
+            exec updatedb --localuser=nobody --output=${locatedb}
+          '';
+        serviceConfig.Nice = 19;
+        serviceConfig.IOSchedulingClass = "idle";
+      };
+
+    services.cron.systemCronJobs = optional config.services.locate.enable
+      "${config.services.locate.period} root ${config.systemd.package}/bin/systemctl start update-locatedb.service";
+
+  };
+
 }