Make it easier to define timer units for services

Systemd services now have a startAt attribute.  If set, NixOS will
automatically emit a timer unit that causes the service to start at
the specified time.  For example:

  systemd.services.foo =
    { script = "... bla bla ...";
      startAt = "02:15";
    };

causes the given script to be started at 02:15 every day.

modules/programs/venus.nix

@@ -167,11 +167,7 @@ in
         serviceConfig.User = "${cfg.user}";
         serviceConfig.Group = "${cfg.group}";
         environment.OPENSSL_X509_CERT_FILE = "/etc/ssl/certs/ca-bundle.crt";
-      };
-
-    systemd.timers.venus =
-      { wantedBy = [ "timers.target" ];
-        timerConfig.OnCalendar = cfg.dates;
+        startOn = cfg.dates;
       };
 
   };

modules/services/misc/nix-gc.nix

@@ -48,22 +48,14 @@ in
 
   ###### implementation
 
-  config = mkMerge [
+  config = {
 
-    { systemd.services.nix-gc =
-        { description = "Nix Garbage Collector";
-          path  = [ config.environment.nix ];
-          script = "exec nix-collect-garbage ${cfg.options}";
-        };
-    }
+    systemd.services.nix-gc =
+      { description = "Nix Garbage Collector";
+        serviceConfig.ExecStart = "${config.environment.nix}/bin/nix-collect-garbage ${cfg.options}";
+        startAt = optionalString cfg.automatic cfg.dates;
+      };
 
-    (mkIf cfg.automatic {
-      systemd.timers.nix-gc =
-        { wantedBy = [ "timers.target" ];
-          timerConfig.OnCalendar = cfg.dates;
-        };
-    })
-
-  ];
+  };
 
 }

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

@@ -229,6 +229,20 @@ rec {
       '';
     };
 
+    startAt = mkOption {
+      type = types.uniq types.string;
+      default = "";
+      example = "Sun 14:00:00";
+      description = ''
+        Automatically start this unit at the given date/time, which
+        must be in the format described in
+        <citerefentry><refentrytitle>systemd.time</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry>.  This is equivalent
+        to adding a corresponding timer unit with
+        <option>OnCalendar</option> set to the value given here.
+      '';
+    };
+
   };
 
 

modules/system/boot/systemd.nix

@@ -628,6 +628,14 @@ in
 
     users.extraGroups.systemd-journal.gid = config.ids.gids.systemd-journal;
 
+    # Generate timer units for all services that have a ‘startAt’ value.
+    systemd.timers =
+      mapAttrs (name: service:
+        { wantedBy = [ "timers.target" ];
+          timerConfig.OnCalendar = service.startAt;
+        })
+        (filterAttrs (name: service: service.startAt != "") cfg.services);
+
     # FIXME: These are borrowed from upstream systemd.
     systemd.services."systemd-update-utmp" =
       { description = "Update UTMP about System Reboot/Shutdown";