Add support for systemd timers

modules/services/misc/nix-gc.nix

@@ -11,36 +11,38 @@ in
   ###### interface
 
   options = {
+
     nix.gc = {
 
       automatic = mkOption {
         default = false;
         type = types.bool;
-        description = "
-          Automatically run the garbage collector at specified dates.
-        ";
+        description = "Automatically run the garbage collector at a specific time.";
       };
 
       dates = mkOption {
-        default = "15 03 * * *";
-        type = types.string;
-        description = "
-          Run the garbage collector at specified dates to avoid full
-          hard-drives.
-        ";
+        default = "00:43";
+        type = types.uniq types.string;
+        description = ''
+          Specification (in the format described by
+          <citerefentry><refentrytitle>systemd.time</refentrytitle>
+          <manvolnum>5</manvolnum></citerefentry>) of the time at
+          which the garbage collector will run.
+        '';
       };
 
       options = mkOption {
         default = "";
         example = "--max-freed $((64 * 1024**3))";
-        type = types.string;
-        description = "
+        type = types.uniq types.string;
+        description = ''
           Options given to <filename>nix-collect-garbage</filename> when the
           garbage collector is run automatically.
-        ";
+        '';
       };
 
     };
+
   };
 
 
@@ -48,10 +50,11 @@ in
 
   config = {
 
-    services.cron.systemCronJobs = mkIf cfg.automatic (singleton
-      "${cfg.dates} root ${config.systemd.package}/bin/systemctl start nix-gc.service");
+    #systemd.timers.nix-gc.enable = cfg.automatic;
+    systemd.timers.nix-gc.enable = true;
+    systemd.timers.nix-gc.timerConfig.OnCalendar = cfg.dates;
 
-    systemd.services."nix-gc" =
+    systemd.services.nix-gc =
       { description = "Nix Garbage Collector";
         path  = [ config.environment.nix ];
         script = "exec nix-collect-garbage ${cfg.options}";

modules/system/boot/systemd-unit-options.nix

@@ -223,6 +223,26 @@ rec {
 
   };
 
+
+  timerOptions = unitOptions // {
+
+    timerConfig = mkOption {
+      default = {};
+      example = { OnCalendar = "Sun 14:00:00"; Unit = "foo.service"; };
+      type = types.attrs;
+      description = ''
+        Each attribute in this set specifies an option in the
+        <literal>[Timer]</literal> section of the unit.  See
+        <citerefentry><refentrytitle>systemd.timer</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> and
+        <citerefentry><refentrytitle>systemd.time</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> for details.
+      '';
+    };
+
+  };
+
+
   mountOptions = unitOptions // {
 
     what = mkOption {

modules/system/boot/systemd.nix

@@ -278,6 +278,18 @@ let
         '';
     };
 
+  timerToUnit = name: def:
+    { inherit (def) wantedBy enable;
+      text =
+        ''
+          [Unit]
+          ${attrsToSection def.unitConfig}
+
+          [Timer]
+          ${attrsToSection def.timerConfig}
+        '';
+    };
+
   mountToUnit = name: def:
     { inherit (def) wantedBy enable;
       text =
@@ -410,6 +422,13 @@ in
       description = "Definition of systemd socket units.";
     };
 
+    systemd.timers = mkOption {
+      default = {};
+      type = types.attrsOf types.optionSet;
+      options = [ timerOptions unitConfig ];
+      description = "Definition of systemd timer units.";
+    };
+
     systemd.mounts = mkOption {
       default = [];
       type = types.listOf types.optionSet;
@@ -552,6 +571,7 @@ in
       mapAttrs' (n: v: nameValuePair "${n}.target" (targetToUnit n v)) cfg.targets
       // mapAttrs' (n: v: nameValuePair "${n}.service" (serviceToUnit n v)) cfg.services
       // mapAttrs' (n: v: nameValuePair "${n}.socket" (socketToUnit n v)) cfg.sockets
+      // mapAttrs' (n: v: nameValuePair "${n}.timer" (timerToUnit n v)) cfg.timers
       // listToAttrs (map
                    (v: let n = escapeSystemdPath v.where;
                        in nameValuePair "${n}.mount" (mountToUnit n v)) cfg.mounts);