nixos/restic: make it possible to use the existing backup cache for prune/check

Configures the `--cache-dir` parameter for the prune and check commands run after backing up. For `check`, also adds a `checkOpts` flag to enable using the cache, since that is disabled by default.

nixos/modules/services/backup/restic.nix

@@ -196,6 +196,18 @@ in
           ];
         };
 
+        checkOpts = mkOption {
+          type = types.listOf types.str;
+          default = [ ];
+          description = lib.mdDoc ''
+            A list of options for 'restic check', which is run after
+            pruning.
+          '';
+          example = [
+            "--with-cache"
+          ];
+        };
+
         dynamicFilesFrom = mkOption {
           type = with types; nullOr str;
           default = null;
@@ -270,8 +282,8 @@ in
               then if (backup.paths != null) then concatStringsSep " " backup.paths else ""
               else "--files-from ${filesFromTmpFile}";
             pruneCmd = optionals (builtins.length backup.pruneOpts > 0) [
-              (resticCmd + " forget --prune " + (concatStringsSep " " backup.pruneOpts))
-              (resticCmd + " check")
+              (resticCmd + " forget --prune --cache-dir=%C/restic-backups-${name} " + (concatStringsSep " " backup.pruneOpts))
+              (resticCmd + " check --cache-dir=%C/restic-backups-${name} " + (concatStringsSep " " backup.checkOpts))
             ];
             # Helper functions for rclone remotes
             rcloneRemoteName = builtins.elemAt (splitString ":" backup.repository) 1;

nixos/tests/restic.nix

@@ -68,6 +68,9 @@ import ./make-test-python.nix (
               package = pkgs.writeShellScriptBin "restic" ''
                 echo "$@" >> /tmp/fake-restic.log;
               '';
+
+              pruneOpts = [ "--keep-last 1" ];
+              checkOpts = [ "--some-check-option" ];
             };
           };
 
@@ -98,6 +101,7 @@ import ./make-test-python.nix (
           '${pkgs.restic}/bin/restic -r ${rcloneRepository} -p ${passwordFile} snapshots -c | grep -e "^1 snapshot"',
           "systemctl start restic-backups-custompackage.service",
           "grep 'backup .* /opt' /tmp/fake-restic.log",
+          "grep 'check .* --some-check-option' /tmp/fake-restic.log",
           "timedatectl set-time '2017-12-13 13:45'",
           "systemctl start restic-backups-remotebackup.service",
           "rm /opt/backupCleanupCommand",