[Python] `KeyValueMetadata.key()` segfaults on an out-of-range index

[marinelay]

Describe the bug, including details regarding any error messages, version, and platform.

Summary

KeyValueMetadata.key(0) and KeyValueMetadata.value(0) cause a segmentation fault when called on an empty KeyValueMetadata object.

Versions

pyarrow 25.0.0, CPython 3.12.3, Ubuntu 24.04 x86_64, glibc 2.39.

Reproducer

Each call below reproduces independently.

import pyarrow as pa
pa.KeyValueMetadata().key(0)

import pyarrow as pa
pa.KeyValueMetadata().value(0)

Running either call produces:

Segmentation fault (core dumped)

An UBSan source build reports an out-of-range std::vector<std::string>::operator[] access from src/arrow/util/key_value_metadata.cc:169 or :175.

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This was found while fuzzing Python C extension modules for a small research project.
It looks like a bug to me, but I would appreciate a check.
I would expect IndexError rather than an unchecked native vector access.

Component(s)

Python

[waterWang]

take

[waterWang]

I've filed a fix: PR #51020 adds bounds checks in the Python bindings so out-of-range/negative indexes raise IndexError instead of segfaulting (reproduced on pyarrow 25.0.1).