feat: add func walSetKeepVersion.

[xiao-77]

Description

Please briefly describe the code changes in this pull request.

Checklist

Please check the items in the checklist if applicable.

• Is the user manual updated?
• Are the test cases passed and automated?
• Is there no significant decrease in test coverage?

[gemini-code-assist]

Summary of Changes

Hello @xiao-77, 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 introduces a critical enhancement to the Write-Ahead Log (WAL) system by implementing a 'keep version' mechanism. This feature allows specifying a minimum version for WAL entries that must be preserved, preventing their automatic deletion during routine snapshot operations. This provides more robust control over data retention within the WAL, ensuring that essential log data is not prematurely removed.

Highlights

• New keepVersion Field: A new int64_t keepVersion field has been added to the SWal structure to store the minimum version number of WAL entries that should be retained.
• Implemented walSetKeepVersion Function: A new function walSetKeepVersion has been implemented to allow external components to set the keepVersion for a given WAL instance. This function includes parameter validation and proper locking.
• WAL Deletion Logic Update: The walEndSnapshot function has been modified to incorporate the keepVersion constraint, ensuring that no WAL entries with a version greater than or equal to keepVersion are deleted during snapshot cleanup.
• Comprehensive Unit Tests: New unit tests have been added to walMetaTest.cpp to cover the functionality of walSetKeepVersion, including basic setting, interaction with deletion logic, and concurrent updates.

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[gemini-code-assist]

Code Review

This pull request introduces a keepVersion feature to the WAL, preventing automatic deletion of WAL files from a certain version onwards. The implementation looks mostly correct, adding the necessary field, initialization, a setter function, and the core logic to respect the constraint during snapshot cleanup. However, there are a couple of significant issues. A critical issue exists in walSetKeepVersion where the return codes from lock and unlock operations are ignored, potentially leading to race conditions. Additionally, a test case intended to verify concurrency is implemented sequentially, failing to provide any thread-safety guarantees. I've provided detailed comments on these points.