tests: only warn about "invalid" chunker params, fixes borgbackup#7588

we previously allowed some weird chunker params, so we just warn
about them instead of rejecting them.

e.g. one could use super small chunk sizes. borg can do that,
but you'll end up with a huge amount of chunks and very large
hash tables (RAM usage) to manage them.

also, one can violate the min <= mask <= max chunker param condition
as in borgbackup#7586 and it will somehow work, but will likely not dedup as good
as when not violating that.

borg2 **will** reject such strange chunker params, see borgbackup#7586 for
some ideas how to "fix" your repos.

src/borg/helpers/parseformat.py

@@ -95,10 +95,16 @@ def interval(s):
 
 
 def ChunkerParams(s):
+    def reject_or_warn(msg, reject):
+        if reject:
+            raise argparse.ArgumentTypeError(msg)
+        else:
+            logger.warning(msg)
+
     params = s.strip().split(',')
     count = len(params)
     if count == 0:
-        raise argparse.ArgumentTypeError('no chunker params given')
+        reject_or_warn('no chunker params given', True)
     algo = params[0].lower()
     if algo == CH_FIXED and 2 <= count <= 3:  # fixed, block_size[, header_size]
         block_size = int(params[1])
@@ -109,28 +115,22 @@ def ChunkerParams(s):
             # or in-memory chunk management.
             # choose the block (chunk) size wisely: if you have a lot of data and you cut
             # it into very small chunks, you are asking for trouble!
-            raise argparse.ArgumentTypeError('block_size must not be less than 64 Bytes')
+            reject_or_warn('block_size must not be less than 64 Bytes', False)
         if block_size > MAX_DATA_SIZE or header_size > MAX_DATA_SIZE:
-            raise argparse.ArgumentTypeError(
-                'block_size and header_size must not exceed MAX_DATA_SIZE [%d]' % MAX_DATA_SIZE
-            )
+            reject_or_warn('block_size and header_size must not exceed MAX_DATA_SIZE [%d]' % MAX_DATA_SIZE, True)
         return algo, block_size, header_size
     if algo == 'default' and count == 1:  # default
         return CHUNKER_PARAMS
     # this must stay last as it deals with old-style compat mode (no algorithm, 4 params, buzhash):
     if algo == CH_BUZHASH and count == 5 or count == 4:  # [buzhash, ]chunk_min, chunk_max, chunk_mask, window_size
         chunk_min, chunk_max, chunk_mask, window_size = (int(p) for p in params[count - 4:])
         if not (chunk_min <= chunk_mask <= chunk_max):
-            raise argparse.ArgumentTypeError('required: chunk_min <= chunk_mask <= chunk_max')
+            reject_or_warn('required: chunk_min <= chunk_mask <= chunk_max', False)
         if chunk_min < 6:
             # see comment in 'fixed' algo check
-            raise argparse.ArgumentTypeError(
-                'min. chunk size exponent must not be less than 6 (2^6 = 64B min. chunk size)'
-            )
+            reject_or_warn('min. chunk size exponent must not be less than 6 (2^6 = 64B min. chunk size)', False)
         if chunk_max > 23:
-            raise argparse.ArgumentTypeError(
-                'max. chunk size exponent must not be more than 23 (2^23 = 8MiB max. chunk size)'
-            )
+            reject_or_warn('max. chunk size exponent must not be more than 23 (2^23 = 8MiB max. chunk size)', True)
         return CH_BUZHASH, chunk_min, chunk_max, chunk_mask, window_size
     raise argparse.ArgumentTypeError('invalid chunker params')
 

src/borg/testsuite/helpers.py

@@ -349,16 +349,20 @@ def test_chunkerparams():
     assert ChunkerParams('fixed,4096') == ('fixed', 4096, 0)
     assert ChunkerParams('fixed,4096,200') == ('fixed', 4096, 200)
     # invalid values checking
+    borg2 = False  # for borg < 2, we only emit a warning, but do not raise ArgumentTypeError for some cases
     with pytest.raises(ArgumentTypeError):
         ChunkerParams('crap,1,2,3,4')  # invalid algo
-    with pytest.raises(ArgumentTypeError):
-        ChunkerParams('buzhash,5,7,6,4095')  # too small min. size
+    if borg2:
+        with pytest.raises(ArgumentTypeError):
+            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,23,19,21,4095')  # violates min <= mask <= max
-    with pytest.raises(ArgumentTypeError):
-        ChunkerParams('fixed,63')  # too small block size
+    if borg2:
+        with pytest.raises(ArgumentTypeError):
+            ChunkerParams('buzhash,23,19,21,4095')  # violates min <= mask <= max
+    if borg2:
+        with pytest.raises(ArgumentTypeError):
+            ChunkerParams('fixed,63')  # too small block size
     with pytest.raises(ArgumentTypeError):
         ChunkerParams('fixed,%d,%d' % (MAX_DATA_SIZE + 1, 4096))  # too big block size
     with pytest.raises(ArgumentTypeError):