BORG_MSGPACK_VERSION_CHECK=no to disable the version check, fixes #9109

(cherry picked from commit 2d63dc9a4f90283f92bf1269f17fb55a06ee25db)

docs/changes.rst

@@ -163,6 +163,8 @@ New features:
 - export-tar/import-tar: support for POSIX ACLs (PAX format)
 - list --format: add "inode" placeholder
 - improved tty-less progress reporting (--progress), #9055
+- BORG_MSGPACK_VERSION_CHECK=no to optionally disable the msgpack version
+  check; default is "yes", use at your own risk, #9109.
 
 Fixes:
 
@@ -888,6 +890,7 @@ New features:
 
       borg serve --socket  # server side (not started automatically!)
       borg -r socket:///path/to/repo ...  # client side
+
 - add get_runtime_dir / BORG_RUNTIME_DIR (contains e.g. .sock and .pid file)
 - support shell-style alternatives, like: sh:image.{png,jpg}, #7602
 

docs/usage/general/environment.rst.inc

@@ -78,6 +78,11 @@ General:
         When set to no (default: yes), system information (like OS, Python version, ...) in
         exceptions is not shown.
         Please only use for good reasons as it makes issues harder to analyze.
+    BORG_MSGPACK_VERSION_CHECK
+        Controls whether Borg checks the ``msgpack`` version.
+        The default is ``yes`` (strict check). Set to ``no`` to disable the version check and
+        allow any installed ``msgpack`` version. Use this at your own risk; malfunctioning or
+        incompatible ``msgpack`` versions may cause subtle bugs or repository data corruption.
     BORG_FUSE_IMPL
         Choose the low-level FUSE implementation borg shall use for ``borg mount``.
         This is a comma-separated list of implementation names, they are tried in the

src/borg/helpers/msgpack.py

@@ -46,6 +46,8 @@ Current behavior in msgpack terms
 - unpacks bin to bytes and raw to str (thus we need to convert to desired type if we want bytes from "raw")
 """
 
+import os
+
 from .datastruct import StableDict
 from ..constants import *  # NOQA
 
@@ -206,8 +208,14 @@ def is_slow_msgpack():
 
 def is_supported_msgpack():
     # DO NOT CHANGE OR REMOVE! See also requirements and comments in pyproject.toml.
+    # This function now also respects the env var BORG_MSGPACK_VERSION_CHECK.
+    # Set BORG_MSGPACK_VERSION_CHECK=no to disable the version check at your own risk.
     import msgpack
 
+    version_check = os.environ.get("BORG_MSGPACK_VERSION_CHECK", "yes").strip().lower()
+    if version_check == "no":
+        return True
+
     if msgpack.version in []:  # < add bad releases here to deny list
         return False
     return (1, 0, 3) <= msgpack.version[:3] <= (1, 1, 2)