[GitHub] [arrow] jorisvandenbossche commented on issue #34634: [C++] "replace_with_mask" kernel produces invalid result (wrong validity bitmap) when array is a boolean ChunkedArray

["jorisvandenbossche (via GitHub)" <gi...@apache.org>]

jorisvandenbossche commented on issue #34634:
URL: https://github.com/apache/arrow/issues/34634#issuecomment-1477566537

   The same issue happens for the fill_forward/backward kernels:
   
   ```
   In [37]: pc.fill_null_forward(pa.array([True, False, None]))
   Out[37]: 
   <pyarrow.lib.BooleanArray object at 0x7f0276f33820>
   [
     true,
     false,
     false
   ]
   
   In [38]: pc.fill_null_forward(pa.chunked_array([[True, False, None]]))
   Out[38]: 
   <pyarrow.lib.ChunkedArray object at 0x7f0276ff56c0>
   [
   <Invalid array: Buffer #1 too small in array of type bool and length 3: expected at least 1 byte(s), got 0
   /home/joris/scipy/repos/arrow/cpp/src/arrow/array/validate.cc:116  ValidateLayout(*data.type)>
   ]
   ```
   
   Looking at the implementation, I think the issue is with pre-allocating the result arrays for fixed width types. This uses `->byte_width()`, but that only works for fixed width types with at least 1 byte per element, while boolean uses bits, and so this `->byte_width()` returns 0 (basically that should never be used if the type can be boolean):
   
   https://github.com/apache/arrow/blob/69118b2d26f2fbbbe65ad30dbe167d74b70fe791/cpp/src/arrow/compute/kernels/vector_replace.cc#L424-L426
   
   So in case of boolean arrays, we are allocating here a data buffer of length 0.
   
   ---
   
   @lukemanley or @pstorozenko if someone of you would like to try fixing this, I would be happy to provide some guidance 


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