enhance replication error handling and improve performance with CachePadded struct

[isdaniel]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @isdaniel, 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 refines the replication mechanism by focusing on both robustness and performance. It introduces more precise error handling for PostgreSQL's PQgetCopyData function, ensuring that specific protocol errors are immediately identified and reported. A significant performance optimization is implemented by using cache-line padding for shared LSN feedback counters, which helps prevent false sharing in multi-threaded environments. Additionally, the changes include an improvement in the replication stream's error recovery logic, allowing for quicker termination on permanent errors, and a minor adjustment to byte reading for efficiency.

Highlights

• Enhanced Replication Error Handling: Improved error handling for PQgetCopyData by explicitly distinguishing and reporting protocol errors (return code -2) instead of treating them as 'would block' conditions. This provides more specific diagnostics for replication issues.
• Performance Improvement with CachePadded Struct: Introduced and applied a CachePadded wrapper to the flushed_lsn and applied_lsn atomic counters within SharedLsnFeedback. This aligns these frequently accessed atomic variables to cache lines, mitigating false sharing and improving performance in concurrent scenarios.
• Optimized Replication Stream Error Recovery: Added a check in the replication stream processing to immediately return if an encountered error is permanent. This prevents unnecessary retry attempts for unrecoverable errors, improving efficiency and responsiveness to critical failures.
• Refactored Byte Reading in Stream: Changed the method for reading message data in the replication stream from reader.read_bytes to reader.read_bytes_buf, potentially offering performance or memory efficiency benefits by reading directly into a buffer.

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[gemini-code-assist]

Code Review

This pull request introduces several valuable improvements. The error handling is enhanced by correctly interpreting PQgetCopyData return codes and by adding logic to fail-fast on permanent errors within the retry loop. Performance is also addressed by introducing a CachePadded struct to mitigate false sharing for LSN tracking and by using a more efficient zero-copy buffer read for WAL messages. The changes are solid, and I have a couple of minor suggestions to make the new additions even more robust and complete.

[isdaniel]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces significant improvements to both error handling and performance. The error handling is made more robust by correctly processing error codes from libpq and by implementing a mechanism to avoid retrying on permanent errors. Performance is enhanced by introducing a CachePadded struct to prevent false sharing in concurrent LSN tracking, and by using zero-copy buffer reads to reduce allocations. The changes are well-implemented and include relevant tests. I have one suggestion regarding the Clone implementation on SharedLsnFeedback to improve clarity.