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

@@ -1,12 +1,20 @@
-{pkgs, config, ...}:
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+
+  locatedb = "/var/cache/locatedb";
+
+in
+
+{
 
   ###### interface
-let
-  inherit (pkgs.lib) mkOption;
 
   options = {
-    services = {
-      locate = {
+
+    services.locate = {
 
       enable = mkOption {
         default = false;
@@ -29,32 +37,26 @@ let
     };
 
   };
-  };
-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
+  config = {
 
-{
-  require = [
-    # ../../upstart-jobs/cron.nix # config.services.cron
-    options
-  ];
-
-  services = {
-    cron = {
-      systemCronJobs =
-        pkgs.lib.optional
-          config.services.locate.enable
-          updatedbCmd;
+    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";
+
   };
+
 }