give clean error msg for invalid nonce file, see #7967

this is a fwd port from 1.4-maint. as we don't have nonce files
any more in master, only the generally useful stuff has been ported.

- add Error / ErrorWithTraceback exception classes to RPC layer.
- add hex_to_bin helper

src/borg/helpers/__init__.py

@@ -25,7 +25,7 @@ from .fs import O_, flags_dir, flags_special_follow, flags_special, flags_base,
 from .fs import HardLinkManager
 from .misc import sysinfo, log_multi, consume
 from .misc import ChunkIteratorFileWrapper, open_item, chunkit, iter_separated, ErrorIgnoringTextIOWrapper
-from .parseformat import bin_to_hex, safe_encode, safe_decode
+from .parseformat import bin_to_hex, hex_to_bin, safe_encode, safe_decode
 from .parseformat import text_to_json, binary_to_json, remove_surrogates, join_cmd
 from .parseformat import eval_escapes, decode_dict, positive_int_validator, interval
 from .parseformat import PathSpec, SortBySpec, ChunkerParams, FilesCacheMode, partial_format, DatetimeWrapper

src/borg/helpers/parseformat.py

@@ -1,6 +1,7 @@
 import abc
 import argparse
 import base64
+import binascii
 import hashlib
 import json
 import os
@@ -10,7 +11,6 @@ import shlex
 import stat
 import uuid
 from typing import Dict, Set, Tuple, ClassVar, Any, TYPE_CHECKING, Literal
-from binascii import hexlify
 from collections import Counter, OrderedDict
 from datetime import datetime, timezone
 from functools import partial
@@ -33,7 +33,18 @@ if TYPE_CHECKING:
 
 
 def bin_to_hex(binary):
-    return hexlify(binary).decode("ascii")
+    return binascii.hexlify(binary).decode("ascii")
+
+
+def hex_to_bin(hex, length=None):
+    try:
+        binary = binascii.unhexlify(hex)
+        binary_len = len(binary)
+        if length is not None and binary_len != length:
+            raise ValueError(f"Expected {length} bytes ({2 * length} hex digits), got {binary_len} bytes.")
+    except binascii.Error as e:
+        raise ValueError(str(e)) from None
+    return binary
 
 
 def safe_decode(s, coding="utf-8", errors="surrogateescape"):

src/borg/remote.py

@@ -21,7 +21,7 @@ import borg.logger
 from . import __version__
 from .compress import Compressor
 from .constants import *  # NOQA
-from .helpers import Error, IntegrityError
+from .helpers import Error, ErrorWithTraceback, IntegrityError
 from .helpers import bin_to_hex
 from .helpers import get_limited_unpacker
 from .helpers import replace_placeholders
@@ -766,7 +766,11 @@ class RemoteRepository:
             error = unpacked["exception_class"]
             args = unpacked["exception_args"]
 
-            if error == "DoesNotExist":
+            if error == "Error":
+                raise Error(args[0])
+            elif error == "ErrorWithTraceback":
+                raise ErrorWithTraceback(args[0])
+            elif error == "DoesNotExist":
                 raise Repository.DoesNotExist(self.location.processed)
             elif error == "AlreadyExists":
                 raise Repository.AlreadyExists(self.location.processed)