acls (linux): use surrogatescape error handling, fix test

surrogatescape will decode/encode invalid utf-8 sequences (if we do not get utf-8) in a round-tripping way.

borg/platform_linux.pyx

@@ -31,7 +31,7 @@ def acl_use_local_uid_gid(acl):
     """Replace the user/group field with the local uid/gid if possible
     """
     entries = []
-    for entry in acl.decode('ascii').split('\n'):
+    for entry in acl.decode('utf-8', 'surrogateescape').split('\n'):
         if entry:
             fields = entry.split(':')
             if fields[0] == 'user' and fields[1]:
@@ -39,7 +39,7 @@ def acl_use_local_uid_gid(acl):
             elif fields[0] == 'group' and fields[1]:
                 fields[1] = str(group2gid(fields[1], int(fields[3])))
             entries.append(':'.join(fields[:3]))
-    return ('\n'.join(entries)).encode('ascii')
+    return '\n'.join(entries).encode('utf-8', 'surrogatescape')
 
 
 cdef acl_append_numeric_ids(acl):

borg/testsuite/platform.py

@@ -86,8 +86,9 @@ class PlatformLinuxTestCase(BaseTestCase):
         acl = b'\n'.join([nothing_special, user_entry, group_entry])
         self.set_acl(file.name, access=acl, numeric_owner=False)
         acl_access = self.get_acl(file.name)[b'acl_access']
-        self.assert_in(user_entry, acl_access)
-        self.assert_in(group_entry, acl_access)
+        # set_acl did not find the local user/group here, so it fell back to the uid/gid:
+        self.assert_in(user_entry_numeric, acl_access)
+        self.assert_in(group_entry_numeric, acl_access)
         acl_access_numeric = self.get_acl(file.name, numeric_owner=True)[b'acl_access']
         self.assert_in(user_entry_numeric, acl_access_numeric)
         self.assert_in(group_entry_numeric, acl_access_numeric)