Add ZFS/btrfs snapshot support to backups

Creates filesystem snapshots before backup for point-in-time consistency.
Uses mount namespaces to bind mount snapshots over original paths, so
restic records correct paths while reading from frozen snapshot data.

- Auto-detects filesystem type via findmnt
- Deterministic snapshot names using path hash
- Graceful fallback for unsupported filesystems

common/backups.nix

@@ -2,13 +2,101 @@
 
 let
   cfg = config.backup;
-  hostname = config.networking.hostName;
 
   mkRespository = group: "s3:s3.us-west-004.backblazeb2.com/D22TgIt0-main-backup/${group}";
 
+  findmnt = "${pkgs.util-linux}/bin/findmnt";
+  mount = "${pkgs.util-linux}/bin/mount";
+  umount = "${pkgs.util-linux}/bin/umount";
+  btrfs = "${pkgs.btrfs-progs}/bin/btrfs";
+  zfs = "/run/current-system/sw/bin/zfs";
+
+  # Creates snapshots and bind mounts them over original paths within the
+  # service's mount namespace, so restic sees correct paths but reads frozen data
+  snapshotHelperFn = ''
+    snapshot_for_path() {
+      local group="$1" path="$2" action="$3"
+      local pathhash fstype
+
+      pathhash=$(echo -n "$path" | sha256sum | cut -c1-8)
+      fstype=$(${findmnt} -n -o FSTYPE -T "$path" 2>/dev/null || echo "unknown")
+
+      case "$fstype" in
+        zfs)
+          local dataset mount subpath snapname snappath
+          dataset=$(${findmnt} -n -o SOURCE -T "$path")
+          mount=$(${findmnt} -n -o TARGET -T "$path")
+          subpath=''${path#"$mount"}
+          [[ "$subpath" != /* ]] && subpath="/$subpath"
+          snapname="''${dataset}@restic-''${group}-''${pathhash}"
+          snappath="''${mount}/.zfs/snapshot/restic-''${group}-''${pathhash}''${subpath}"
+          case "$action" in
+            create)
+              ${zfs} destroy "$snapname" 2>/dev/null || true
+              ${zfs} snapshot "$snapname"
+              ${mount} --bind "$snappath" "$path"
+              echo "$path"
+              ;;
+            destroy)
+              ${umount} "$path" 2>/dev/null || true
+              ${zfs} destroy "$snapname" 2>/dev/null || true
+              ;;
+          esac
+          ;;
+        btrfs)
+          local mount subpath snapdir snappath
+          mount=$(${findmnt} -n -o TARGET -T "$path")
+          subpath=''${path#"$mount"}
+          [[ "$subpath" != /* ]] && subpath="/$subpath"
+          snapdir="/.restic-snapshots/''${group}-''${pathhash}"
+          snappath="''${snapdir}''${subpath}"
+          case "$action" in
+            create)
+              ${btrfs} subvolume delete "$snapdir" 2>/dev/null || true
+              mkdir -p /.restic-snapshots
+              ${btrfs} subvolume snapshot -r "$mount" "$snapdir" >&2
+              ${mount} --bind "$snappath" "$path"
+              echo "$path"
+              ;;
+            destroy)
+              ${umount} "$path" 2>/dev/null || true
+              ${btrfs} subvolume delete "$snapdir" 2>/dev/null || true
+              ;;
+          esac
+          ;;
+        *)
+          echo "No snapshot support for $fstype ($path), using original" >&2
+          [ "$action" = "create" ] && echo "$path"
+          ;;
+      esac
+    }
+  '';
+
   mkBackup = group: paths: {
     repository = mkRespository group;
-    inherit paths;
+
+    dynamicFilesFrom = "cat /run/restic-backup-${group}/paths";
+
+    backupPrepareCommand = ''
+      mkdir -p /run/restic-backup-${group}
+      : > /run/restic-backup-${group}/paths
+
+      ${snapshotHelperFn}
+
+      for path in ${lib.escapeShellArgs paths}; do
+        snapshot_for_path ${lib.escapeShellArg group} "$path" create >> /run/restic-backup-${group}/paths
+      done
+    '';
+
+    backupCleanupCommand = ''
+      ${snapshotHelperFn}
+
+      for path in ${lib.escapeShellArgs paths}; do
+        snapshot_for_path ${lib.escapeShellArg group} "$path" destroy
+      done
+
+      rm -rf /run/restic-backup-${group}
+    '';
 
     initialize = true;
 
@@ -21,11 +109,11 @@ let
       ''--exclude-if-present ".nobackup"''
     ];
 
+    # Keeps backups from up to 6 months ago
     pruneOpts = [
       "--keep-daily 7" # one backup for each of the last n days
       "--keep-weekly 5" # one backup for each of the last n weeks
-      "--keep-monthly 12" # one backup for each of the last n months
-      "--keep-yearly 75" # one backup for each of the last n years
+      "--keep-monthly 6" # one backup for each of the last n months
     ];
 
     environmentFile = "/run/agenix/backblaze-s3-backups";
@@ -64,12 +152,25 @@ in
   };
 
   config = lib.mkIf (cfg.group != null) {
+    assertions = lib.mapAttrsToList (group: _: {
+      assertion = config.systemd.services."restic-backups-${group}".enable;
+      message = "Expected systemd service 'restic-backups-${group}' not found. The nixpkgs restic module may have changed its naming convention.";
+    }) cfg.group;
+
     services.restic.backups = lib.concatMapAttrs
       (group: groupCfg: {
         ${group} = mkBackup group groupCfg.paths;
       })
       cfg.group;
 
+    # Mount namespace lets us bind mount snapshots over original paths,
+    # so restic backs up from frozen snapshots while recording correct paths
+    systemd.services = lib.concatMapAttrs
+      (group: _: {
+        "restic-backups-${group}".serviceConfig.PrivateMounts = true;
+      })
+      cfg.group;
+
     age.secrets.backblaze-s3-backups.file = ../secrets/backblaze-s3-backups.age;
     age.secrets.restic-password.file = ../secrets/restic-password.age;