Release BytesIO and StringIO buffers on close - #9230

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

⚙️ Run configuration

• Configuration used: Repository: RustPython/RustPython/.coderabbit.yml
• Review profile: CHILL
• Plan: Advanced
• Run ID: ddbed40f-63d6-4023-a73e-531d76344401

📥 Commits

Reviewing files that changed from the base of the PR and between c08375b and f26e909.

📒 Files selected for processing: 2

• crates/vm/src/stdlib/_io.rs
• extra_tests/snippets/stdlib_io.py

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 7 remain after this review.

StringIO and BytesIO initialization and closure now update their contents while holding buffer locks. BytesIO buffer operations also check the closed state after acquiring the buffer. Tests cover stream reinitialization, reentrant closure, and storage capacity after closure.

Changes: IO stream buffer lifecycle

Layer / File(s)	Summary
Stream reinitialization crates/vm/src/stdlib/_io.rs, extra_tests/snippets/stdlib_io.py	Initialization replaces stream contents under the buffer lock and resets the closed state. Tests cover reinitialization and reentrant closure during reinitialization.
Stream closure and storage release crates/vm/src/stdlib/_io.rs	Closing either stream clears its contents while holding the buffer lock. A test checks that each backing vector has zero capacity after closure.
Guarded buffer access and state updates crates/vm/src/stdlib/_io.rs	StringIO state restoration uses its closed-checking accessor. BytesIO truncation, state restoration, and buffer export check the closed state while holding the relevant buffer lock.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: youknowone

The reviewed buffer-lifecycle changes have no identified issue that needs correction before merge.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 53.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 15 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: releasing BytesIO and StringIO backing buffers when the streams close.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

🧪 Generate unit tests (beta)

• Create a new PR

• X
• Mastodon
• Reddit
• LinkedIn