Fix Scanner thread-safety in active transaction managers

[brfrn169]

Description

This PR applies to versions 3.16 and earlier.

It fixes a thread-safety issue in the scanner in active transaction managers. This change is a backport of part of the change introduced in #3082.

Related issues and/or PRs

• Support batch operation in Transaction API #3082

Changes made

• Fixed a thread-safety issue in the scanner in active transaction managers.

Checklist

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes.
• I have considered whether similar issues could occur in other products, components, or modules if this PR is for bug fixes.
• Any remaining open issues linked to this PR are documented and up-to-date (Jira, GitHub, etc.).
• Tests (unit, integration, etc.) have been added for the changes.
• My changes generate no new warnings.
• Any dependent changes in other PRs have been merged and published.

Additional notes (optional)

N/A

Release notes

N/A

[gemini-code-assist]

Summary of Changes

Hello @brfrn169, 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 resolves a critical thread-safety vulnerability affecting scanner operations in active transaction managers. By carefully synchronizing access to scanner methods, the changes ensure data consistency and prevent potential race conditions that could arise in multi-threaded environments, particularly when transactions are rolled back concurrently.

Highlights

• Thread-Safety Fix: Addressed a thread-safety issue within the scanner operations of active transaction managers, specifically for versions 3.16 and earlier.
• Synchronization Implementation: Implemented synchronization around Scanner methods (one, all, close, iterator) within the ActiveTransaction class to prevent race conditions when rollback() is called from a different thread.
• Backport: This change is a backport of a fix originally introduced in pull request Support batch operation in Transaction API #3082.

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 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 counter productive. 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.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

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. ↩

[Copilot]

Pull Request Overview

This PR addresses a thread-safety issue in the scanner within active transaction managers for versions 3.16 and earlier. The fix ensures that Scanner operations are properly synchronized when the rollback() method may be called from an expiration handler in a different thread.

Key Changes:

• Wraps Scanner instances returned by getScanner() with synchronized delegates
• Adds documentation explaining the thread-safety requirements for ActiveTransaction classes
• Applies identical fixes to both TwoPhaseCommitTransactionManager and DistributedTransactionManager

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
ActiveTransactionManagedTwoPhaseCommitTransactionManager.java	Wraps Scanner with synchronized delegate to prevent concurrent access during rollback
ActiveTransactionManagedDistributedTransactionManager.java	Applies identical Scanner synchronization fix for distributed transactions

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[gemini-code-assist]

Code Review

This pull request aims to fix a thread-safety issue in the scanner within active transaction managers by synchronizing access to the scanner's methods. The approach of wrapping the Scanner and synchronizing its methods on the ActiveTransaction instance is correct. However, the implementation for the iterator() method is incomplete, as it does not synchronize the methods of the returned Iterator, leaving a potential race condition. I've provided suggestions to fully fix the thread-safety issue.