ChunkerParams: reject even window size for buzhash, fixes #8868

docs/internals/data-structures.rst

@@ -657,7 +657,7 @@ can be used to tune the chunker parameters, the default is:
 - CHUNK_MIN_EXP = 19 (minimum chunk size = 2^19 B = 512 kiB)
 - CHUNK_MAX_EXP = 23 (maximum chunk size = 2^23 B = 8 MiB)
 - HASH_MASK_BITS = 21 (target chunk size ~= 2^21 B = 2 MiB)
-- HASH_WINDOW_SIZE = 4095 [B] (`0xFFF`)
+- HASH_WINDOW_SIZE = 4095 [B] (`0xFFF`) (must be an odd number)
 
 The buzhash table is altered by XORing it with a seed randomly generated once
 for the repository, and stored encrypted in the keyfile. This is to prevent

docs/misc/create_chunker-params.txt

@@ -18,7 +18,7 @@ determined by the windows contents rather than the min/max. chunk size).
 Default: 21 (statistically, chunks will be about 2^21 == 2MiB in size)
 
 HASH_WINDOW_SIZE: the size of the window used for the rolling hash computation.
-Default: 4095B
+Must be an odd number. Default: 4095B
 
 
 Trying it out
@@ -114,4 +114,3 @@ $ ls -l /extra/repo-xl/index*
 
 $ du -sk /extra/repo-xl/
 14253464    /extra/repo-xl/
-

src/borg/helpers/parseformat.py

@@ -131,6 +131,8 @@ def ChunkerParams(s):
             reject_or_warn('min. chunk size exponent must not be less than 6 (2^6 = 64B min. chunk size)', False)
         if chunk_max > 23:
             reject_or_warn('max. chunk size exponent must not be more than 23 (2^23 = 8MiB max. chunk size)', True)
+        if window_size % 2 == 0:
+            reject_or_warn("window_size must be an uneven (odd) number", False)
         return CH_BUZHASH, chunk_min, chunk_max, chunk_mask, window_size
     raise argparse.ArgumentTypeError('invalid chunker params')