fix --last, --first accepting negative values

also don't accept zero, because every use of these doesn't cover that case,
and it arguably doesn't make a lot of sense.

src/borg/archiver.py

@@ -44,7 +44,7 @@ from .crypto.key import key_creator, tam_required_file, tam_required, RepoKey, P
 from .crypto.keymanager import KeyManager
 from .helpers import EXIT_SUCCESS, EXIT_WARNING, EXIT_ERROR
 from .helpers import Error, NoManifestError, set_ec
-from .helpers import location_validator, archivename_validator, ChunkerParams
+from .helpers import positive_int_validator, location_validator, archivename_validator, ChunkerParams
 from .helpers import PrefixSpec, SortBySpec, HUMAN_SORT_KEYS
 from .helpers import BaseFormatter, ItemFormatter, ArchiveFormatter
 from .helpers import format_timedelta, format_file_size, parse_file_size, format_archive
@@ -2394,9 +2394,9 @@ class Archiver:
 
             if first_last:
                 group = filters_group.add_mutually_exclusive_group()
-                group.add_argument('--first', metavar='N', dest='first', default=0, type=int,
+                group.add_argument('--first', metavar='N', dest='first', default=0, type=positive_int_validator,
                                    help='consider first N archives after other filters were applied')
-                group.add_argument('--last', metavar='N', dest='last', default=0, type=int,
+                group.add_argument('--last', metavar='N', dest='last', default=0, type=positive_int_validator,
                                    help='consider last N archives after other filters were applied')
 
         parser = argparse.ArgumentParser(prog=self.prog, description='Borg - Deduplicated Backups',

src/borg/helpers.py

@@ -236,7 +236,7 @@ class Archives(abc.MutableMapping):
         if first:
             archives = archives[:first]
         elif last:
-            archives = archives[len(archives) - last:]
+            archives = archives[max(len(archives) - last, 0):]
         return archives
 
     def list_considering(self, args):
@@ -400,6 +400,14 @@ class Manifest:
         self.repository.put(self.MANIFEST_ID, self.key.encrypt(data))
 
 
+def positive_int_validator(value):
+    """argparse type for positive integers"""
+    int_value = int(value)
+    if int_value <= 0:
+        raise argparse.ArgumentTypeError('A positive integer is required: %s' % value)
+    return int_value
+
+
 def interval(s):
     """Convert a string representing a valid interval to a number of hours."""
     multiplier = {'H': 1, 'd': 24, 'w': 24 * 7, 'm': 24 * 31, 'y': 24 * 365}