GH-51229: [Python] Raise instead of crashing on unopened resize

[1fanwang]

Rationale for this change

Calling resize on a memory-mapped file that has never been opened takes down the Python process with a SIGSEGV, so an application cannot catch or recover from it.

What changes are included in this PR?

The resize path now applies the same open-state check the other file operations already use, before it reaches the native resize call. A directly constructed object raises ValueError.

Are these changes tested?

The regression runs in-process and checks ValueError and its message with pytest.raises. It does not spawn a subprocess.

Separate, 20-second-bounded processes reproduced the original crash and the exception after the fix. Both used native Arrow 26.0.0-SNAPSHOT; the reused patched extension has byte-identical resize source to this PR.

Raw logs

$ python -c 'import pyarrow as pa; pa.MemoryMappedFile().resize(0)'
Before:
returncode=-11

After:
ValueError: I/O operation on closed file
returncode=1

$ cd python
$ python -m pytest pyarrow/tests/test_io.py -k memory_map_resize -q
..                                                                       [100%]
2 passed, 161 deselected, 1 warning in 1.02s

Are there any user-facing changes?

Yes. Misuse of a directly constructed memory-mapped file now raises a Python exception instead of terminating the process.

This PR contains a "Critical Fix". It fixes a process crash reachable from ordinary Python-level object state.

• GitHub Issue: [Python] MemoryMappedFile().resize() segfaults when no file has been opened #51229

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• GitHub Issue: [Python] MemoryMappedFile().resize() segfaults when no file has been opened #51229

[github-actions]

⚠️ GitHub issue #51229 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟢 Approval recommended

The change correctly prevents a reproducible segfault by aligning resize() with existing open-state checks, and the regression is covered by a subprocess-based test.

Pull request overview

This PR addresses a Python-level crash in pyarrow.MemoryMappedFile.resize() when MemoryMappedFile() is constructed without opening a file, by adding an open-state guard so the misuse raises a catchable exception instead of segfaulting.

Changes:

• Add _assert_open() to the MemoryMappedFile.resize() wrapper before calling the native resize implementation.
• Add a regression test that exercises the behavior in a subprocess so a segfault would be observable without killing the pytest runner.

File summaries

File	Description
python/pyarrow/io.pxi	Adds an open-state assertion to MemoryMappedFile.resize() to prevent dereferencing a null native handle.
python/pyarrow/tests/test_io.py	Adds a subprocess-based regression test ensuring uninitialized MemoryMappedFile().resize() raises ValueError instead of crashing.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

The new regression test currently runs the crashing call in-process, so a future regression could abort the entire pytest process rather than failing cleanly.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

[1fanwang]

Keeping it in-process per #51246 (comment); with the fix it raises rather than crashing.

[AlenkaF]

Only the PR description needs an update.

[1fanwang]

The testing description now matches the in-process regression.

[github-actions]

⚠️ GitHub issue #51229 has been automatically assigned in GitHub to PR creator.