[Python] compute.replace_substring_regex sometimes returns incorrect offsets, causing crashes/ub

[asfimport]

I've come across examples where calling pyarrow.compute.replace_substring_regex caused a segfault once using the result. After some experimentation, I found that the problem lies in the offsets buffer in the result of the computation.

Here is a docker file that reproduces the problem in a few lines (though without an immediate crash):

FROM python:3.8
RUN pip install pyarrow
RUN echo "import pyarrow; \
    import pyarrow.compute; \
    options = pyarrow.compute.ReplaceSubstringOptions('a', ''); \
    values = [''] * 16; \
    arr = pyarrow.array(values, pyarrow.string()); \
    res = pyarrow.compute.replace_substring_regex(arr, options=options); \
    offsets = res.buffers()[1]; \
    assert any(offset != 0 for offset in offsets[-4:]);" > /test.py
RUN python /test.py

The docker image installs pyarrow (4.0.0 at the time of submitting this issue), and then runs python code which creates an array of 16 empty strings, and calls replace_substring_regex on the array.
The offsets buffer's last 4 bytes (representing the last offset) are checked to be non-zero, which fails.

Everything but the last offset looks fine: the valid buffer, the rest of the offsets, and the data buffer.

I have more elaborate examples of arrays which return a random value for the last offset, causing crashes sooner than simply 0 at the end.
Another hint which might help, the problem occurs at multiples of 16, i.e. changing 16 to 32, 48, etc. still shows the problem, but other values don't have a problem.
 
When I cloned the latest master, built arrow, and run the example - there was no problem. But since I didn't see the issue here on JIRA, I thought I should probably post it. I have no idea if I'm building correctly, and maybe I'm adding a bug to a bug :)

Environment: ubuntu 20.04 or macos catalina running docker engine 20.10.2 and python 3.8.6
Reporter: Dror Speiser

Related issues:

• [C++][Compute] replace_substring_regex() creates invalid arrays => crash (is superceded by)

_{Note: This issue was originally created as ARROW-12889. Please see the migration documentation for further details.}

[asfimport]

Dror Speiser:
This is same as #ARROW-12774 !

Closing...