Fix race condition in statesampler_fast.pyx

[derrickaw]

1. Race condition seen when reading nsecs.
2. Fixing it and adding test.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[derrickaw]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces thread-safe reading of the nsecs property in ScopedState by acquiring a lock during the read operation, and adds a concurrent test case to verify behavior under thread contention. The review feedback suggests releasing the Python GIL using with nogil: when acquiring the C-level lock to prevent blocking the entire Python interpreter under contention.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a race condition identified when reading the nsecs property in statesampler_fast.pyx. By introducing explicit locking mechanisms during the read process, the changes ensure data consistency when multiple threads interact with the state sampler. A regression test has been included to validate the fix and prevent future occurrences of this issue.

Highlights

• Thread Safety: Implemented thread-safe access to the nsecs property in ScopedState by acquiring the sampler lock during read operations.
• Testing: Added a new test case test_concurrent_nsecs_reads to verify that concurrent reads of nsecs function correctly under thread contention.

New Features

🧠 You can now enable Memory (public preview) to help Gemini Code Assist learn from your team's feedback. This makes future code reviews more consistent and personalized to your project's style. Click here to enable Memory in your admin console.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize the Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counterproductive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for GitHub and other Google products, sign up here.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request introduces thread-safe access to the nsecs property in ScopedState by acquiring a lock within a nogil block, and adds a concurrent test test_concurrent_nsecs_reads to verify this behavior under thread contention. The review feedback highlights two important improvements: first, in statesampler_fast.pyx, accessing self.sampler.lock inside the with nogil block is unsafe if self.sampler is None (which could cause a segmentation fault), so the lock should be extracted to a local variable beforehand; second, in statesampler_test.py, the try...finally block should be expanded to wrap all operations after sampler.start() to guarantee that the sampler is stopped and resources are cleaned up even if an exception occurs during setup.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[github-actions]

Assigning reviewers:

R: @tvalentyn for label python.

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[tvalentyn]

Thanks!