ChunkerParams: reject buzhash params that do not fit the chunker buffer (1.4-maint)

src/borg/helpers/parseformat.py

@@ -137,6 +137,10 @@ def ChunkerParams(s):
             raise argparse.ArgumentTypeError('min. chunk size exponent must not be less than 6 (2^6 = 64B min. chunk size)')
         if chunk_max > 23:
             raise argparse.ArgumentTypeError('max. chunk size exponent must not be more than 23 (2^23 = 8MiB max. chunk size)')
+        # the chunker needs the hash window plus one byte beyond the min. chunk size to fit into
+        # a max. chunk size sized buffer, see the assertion in Chunker.__cinit__.
+        if window_size + 2 ** chunk_min + 1 > 2 ** chunk_max:
+            raise argparse.ArgumentTypeError('required: window_size + 2^chunk_min + 1 <= 2^chunk_max')
         if window_size % 2 == 0:
             raise argparse.ArgumentTypeError("window_size must be an uneven (odd) number")
         return CH_BUZHASH, chunk_min, chunk_max, chunk_mask, window_size

src/borg/testsuite/helpers.py

@@ -391,6 +391,11 @@ def test_chunkerparams():
             ChunkerParams('buzhash,5,7,6,4095')  # too small min. size
     with pytest.raises(ArgumentTypeError):
         ChunkerParams('buzhash,19,24,21,4095')  # too big max. size
+    with pytest.raises(ArgumentTypeError):
+        ChunkerParams('buzhash,10,12,11,4095')  # window_size + 2^chunk_min + 1 > 2^chunk_max
+    with pytest.raises(ArgumentTypeError):
+        ChunkerParams('19,19,19,4095')  # dito, chunk_min == chunk_max
+    assert ChunkerParams('10,13,11,4095') == ('buzhash', 10, 13, 11, 4095)  # just fits: 4095 + 1024 + 1 <= 8192
     if borg2:
         with pytest.raises(ArgumentTypeError):
             ChunkerParams('buzhash,23,19,21,4095')  # violates min <= mask <= max