disallow unknown special tags

only known special tags is @prot for now.

src/borg/archiver/tag_cmd.py

@@ -3,7 +3,7 @@
 from ._common import with_repository, define_archive_filters_group
 from ..archive import Archive
 from ..constants import *  # NOQA
-from ..helpers import bin_to_hex, archivename_validator, tag_validator
+from ..helpers import bin_to_hex, archivename_validator, tag_validator, Error
 from ..manifest import Manifest
 
 from ..logger import create_logger
@@ -25,6 +25,16 @@ def tags_set(tags):
         else:
             archive_infos = manifest.archives.list_considering(args)
 
+        def check_special(tags):
+            if tags:
+                special = {tag for tag in tags_set(tags) if tag.startswith("@")}
+                if not special.issubset(SPECIAL_TAGS):
+                    raise Error("unknown special tags given.")
+
+        check_special(args.set_tags)
+        check_special(args.add_tags)
+        check_special(args.remove_tags)
+
         for archive_info in archive_infos:
             archive = Archive(manifest, archive_info.id, cache=cache)
             if args.set_tags:

src/borg/constants.py

@@ -124,6 +124,10 @@
 # tar related
 SCHILY_XATTR = "SCHILY.xattr."  # xattr key prefix in tar PAX headers
 
+# special tags
+# @PROT protects archives against accidential deletion or modification by delete, prune or recreate.
+SPECIAL_TAGS = frozenset(["@PROT"])
+
 # return codes returned by borg command
 EXIT_SUCCESS = 0  # everything done, no problems
 EXIT_WARNING = 1  # reached normal end of operation, but there were issues (generic warning)

src/borg/testsuite/archiver/tag_cmd_test.py

@@ -1,5 +1,8 @@
+import pytest
+
 from ...constants import *  # NOQA
 from . import cmd, generate_archiver_tests, RK_ENCRYPTION
+from ...helpers import Error
 
 pytest_generate_tests = lambda metafunc: generate_archiver_tests(metafunc, kinds="local")  # NOQA
 
@@ -45,3 +48,16 @@ def test_tag_set_noclobber_special(archivers, request):
     # it is possible though to use --set if the existing special tags are also given:
     output = cmd(archiver, "tag", "-a", "archive", "--set", "noclobber", "--set", "@PROT")
     assert "tags: @PROT,noclobber." in output
+
+
+def test_tag_set_only_known_special(archivers, request):
+    archiver = request.getfixturevalue(archivers)
+    cmd(archiver, "repo-create", RK_ENCRYPTION)
+    cmd(archiver, "create", "archive", archiver.input_path)
+    # user can't set / add / remove unknown special tags
+    with pytest.raises(Error):
+        cmd(archiver, "tag", "-a", "archive", "--set", "@UNKNOWN")
+    with pytest.raises(Error):
+        cmd(archiver, "tag", "-a", "archive", "--add", "@UNKNOWN")
+    with pytest.raises(Error):
+        cmd(archiver, "tag", "-a", "archive", "--remove", "@UNKNOWN")