add simple rsnapshot module

modules/module-list.nix

@@ -75,6 +75,7 @@
   ./services/backup/mysql-backup.nix
   ./services/backup/postgresql-backup.nix
   ./services/backup/sitecopy-backup.nix
+  ./services/backup/rsnapshot.nix
   ./services/databases/4store-endpoint.nix
   ./services/databases/4store.nix
   ./services/databases/firebird.nix

modules/services/backup/rsnapshot.nix

@@ -0,0 +1,56 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let cfg = config.services.rsnapshot;
+in
+{
+  options = {
+    services.rsnapshot = {
+      enable = mkEnableOption "rsnapshot backups";
+
+      configuration = mkOption {
+        default = "";
+        example = ''
+          retain hourly 24
+          retain daily 365
+          backup /home/ localhost/
+        '';
+        type = types.lines;
+        description = ''
+        rsnapshot configuration option in addition to the defaults from rsnapshot and this module.
+        '';
+      };
+
+      cronIntervals = mkOption {
+        default = {};
+        example = { "hourly" = "0 * * * *"; "daily" = "50 21 * * *"; };
+        type = types.attrsOf types.string;
+        description = ''
+        Periodicity at which intervals should be run by cron.
+        Note that the intervals also have to exist in configuration
+        as retain options.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [ pkgs.rsnapshot ];
+
+    services.cron.systemCronJobs =
+      mapAttrsToList (interval: time: "${time} root ${pkgs.rsnapshot}/bin/rsnapshot ${interval}") cfg.cronIntervals;
+
+    environment.etc."rsnapshot.conf".source = with pkgs; writeText "gen-rsnapshot.conf" (''
+        config_version	1.2
+        cmd_cp	${coreutils}/bin/cp
+        cmd_rsync	${rsync}/bin/rsync
+        cmd_ssh	${openssh}/bin/ssh
+        cmd_logger	${inetutils}/bin/logger
+        cmd_du	${coreutils}/bin/du
+        cmd_rsnapshot_diff	${rsnapshot}/bin/rsnapshot-diff
+        lockfile	/run/rsnapshot.pid
+
+      '' + cfg.configuration);
+  };
+}