Backport: remove support for shake_* hashes

This is a backport of #6117
Fixes #6082

src/borg/helpers.py

@@ -1898,6 +1898,9 @@ class ArchiveFormatter(BaseFormatter):
 
 
 class ItemFormatter(BaseFormatter):
+    # we provide the hash algos from python stdlib (except shake_*).
+    # shake_* is not provided because it uses an incompatible .digest() method to support variable length.
+    hash_algorithms = hashlib.algorithms_guaranteed.difference({'shake_128', 'shake_256'})
     KEY_DESCRIPTIONS = {
         'bpath': 'verbatim POSIX path, can contain any character except NUL',
         'path': 'path interpreted as text (might be missing non-text characters, see bpath)',
@@ -1914,7 +1917,7 @@ class ItemFormatter(BaseFormatter):
         ('type', 'mode', 'uid', 'gid', 'user', 'group', 'path', 'bpath', 'source', 'linktarget', 'flags'),
         ('size', 'csize', 'dsize', 'dcsize', 'num_chunks', 'unique_chunks'),
         ('mtime', 'ctime', 'atime', 'isomtime', 'isoctime', 'isoatime'),
-        tuple(sorted(hashlib.algorithms_guaranteed)),
+        tuple(sorted(hash_algorithms)),
         ('archiveid', 'archivename', 'extra'),
         ('health', )
     )
@@ -1988,7 +1991,7 @@ class ItemFormatter(BaseFormatter):
             'ctime': partial(self.format_time, 'ctime'),
             'atime': partial(self.format_time, 'atime'),
         }
-        for hash_function in hashlib.algorithms_guaranteed:
+        for hash_function in self.hash_algorithms:
             self.add_key(hash_function, partial(self.hash_item, hash_function))
         self.used_call_keys = set(self.call_keys) & self.format_keys
 
@@ -2060,6 +2063,7 @@ class ItemFormatter(BaseFormatter):
         return item.get_size(compressed=True)
 
     def hash_item(self, hash_function, item):
+        assert hash_function in self.hash_algorithms
         if 'chunks' not in item:
             return ""
         hash = hashlib.new(hash_function)