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GH-51044: [Python] Reject read-only readinto destinations (#51126)
### Rationale for this change `BufferReader.readinto()` currently segfaults when passed a read-only destination such as `bytes` or a read-only `memoryview`. `py_buffer()` creates an immutable Arrow buffer, `mutable_data()` returns a null pointer, and the C++ read path attempts to copy into that pointer. A Python API misuse should raise a Python exception rather than terminate the interpreter. ### What changes are included in this PR? - Validate that a `NativeFile.readinto()` destination is mutable before obtaining its writable pointer. - Raise `TypeError` for immutable destinations. - Add regression coverage for `bytes` and read-only `memoryview` destinations. ### Are these changes tested? Yes. The focused `readinto` tests pass against a locally compiled patched `pyarrow.lib`. The stock 25.0.1 wheel exits with status 139 for the issue reproducer, while the patched build raises the expected `TypeError`; the writable `bytearray` control continues to read successfully. ### Are there any user-facing changes? Yes. Passing a read-only destination to `NativeFile.readinto()` now raises `TypeError` instead of terminating the interpreter. Writable-buffer behavior is unchanged. Fixes #51044. This contribution was developed with assistance from OpenAI Codex. I reviewed, tested, and take responsibility for the patch and this description. * GitHub Issue: #51044 Authored-by: Darren Carreras <283775510+carrerasdarren-cell@users.noreply.github.com> Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>
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‎python/pyarrow/io.pxi‎

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@@ -548,6 +548,8 @@ cdef class NativeFile(_Weakrefable):
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handle = self.get_input_stream()
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py_buf = py_buffer(b)
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if not py_buf.buffer.get().is_mutable():
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raise TypeError("readinto() argument must be a writable buffer")
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buf_len = py_buf.size
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buf = py_buf.buffer.get().mutable_data()
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‎python/pyarrow/tests/test_io.py‎

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@@ -1024,6 +1024,13 @@ def test_nativefile_write_memoryview():
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assert buf.to_pybytes() == data * 3
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@pytest.mark.parametrize("dst_buf", [b"a", memoryview(b"a")])
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def test_native_file_readinto_rejects_readonly_buffer(dst_buf):
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with pa.BufferReader(b"x") as f:
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with pytest.raises(TypeError, match="writable buffer"):
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f.readinto(dst_buf)
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# ----------------------------------------------------------------------
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# Mock output stream
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