check msgpack version, terminate for unsupported versions

setup.py

@@ -43,8 +43,10 @@ install_requires = [
     # if you can't satisfy the above requirement, these are versions that might
     # also work ok, IF you make sure to use the COMPILED version of msgpack-python,
     # NOT the PURE PYTHON fallback implementation: ==0.5.1, ==0.5.4
-    # using any other version is not supported by borg development, feel free to
-    # do it on your own risk (and after own testing).
+    #
+    # Please note:
+    # using any other version is not supported by borg development and
+    # any feedback related to issues caused by this will be ignored.
 ]
 
 # note for package maintainers: if you package borgbackup for distribution,

src/borg/archiver.py

@@ -57,7 +57,7 @@ from .helpers import Manifest
 from .helpers import hardlinkable
 from .helpers import StableDict
 from .helpers import check_python, check_extension_modules
-from .helpers import dir_is_tagged, is_slow_msgpack, yes, sysinfo
+from .helpers import dir_is_tagged, is_slow_msgpack, is_supported_msgpack, yes, sysinfo
 from .helpers import log_multi
 from .helpers import signal_handler, raising_signal_handler, SigHup, SigTerm
 from .helpers import ErrorIgnoringTextIOWrapper
@@ -4340,6 +4340,11 @@ class Archiver:
         if args.show_version:
             logging.getLogger('borg.output.show-version').info('borgbackup version %s' % __version__)
         self.prerun_checks(logger)
+        if not is_supported_msgpack():
+            logger.error("You do not have a supported msgpack[-python] version installed. Terminating.")
+            logger.error("This should never happen as specific, supported versions are required by our setup.py.")
+            logger.error("Do not contact borgbackup support about this.")
+            return set_ec(EXIT_ERROR)
         if is_slow_msgpack():
             logger.warning("Using a pure-python msgpack! This will result in lower performance.")
         if args.debug_profile:

src/borg/helpers.py

@@ -1290,6 +1290,12 @@ def is_slow_msgpack():
     return msgpack.Packer is msgpack.fallback.Packer
 
 
+def is_supported_msgpack():
+    # DO NOT CHANGE OR REMOVE! See also requirements and comments in setup.py.
+    return (0, 4, 6) <= msgpack.version <= (0, 5, 6) and \
+           msgpack.version not in [(0, 5, 0), (0, 5, 2), (0, 5, 3), (0, 5, 5)]
+
+
 FALSISH = ('No', 'NO', 'no', 'N', 'n', '0', )
 TRUISH = ('Yes', 'YES', 'yes', 'Y', 'y', '1', )
 DEFAULTISH = ('Default', 'DEFAULT', 'default', 'D', 'd', '', )