Buffer BufferedLogger by newline to avoid log splitting

[shunping]

• Update BufferedLogger.Write to search for newlines and accumulate partial log lines in the builder instead of immediately emitting them as separate entries.
• Update FlushAtDebug and FlushAtError to flush any remaining trailing text in the builder on exit or flush events.
• Fix bug in buffered_logging_test.go where log list assertions only verified the first element of logCatcher.msgs instead of checking all gathered log messages.
• Add TestBufferedLogger/partial_write_splitting to verify correct chunked write buffering and line assembly behavior.

Background

Previously, when the worker segvfault, the logs from fault handler will be split in different log entries in Cloud Logging UI, making them very inconvenient to read.

After this fix, the logs will be buffered and split by newline.

Internal bug id: 315054330.

[shunping]

r: @tvalentyn

[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 improves the log formatting for the BufferedLogger by introducing newline-aware buffering. Previously, logs were emitted immediately, leading to fragmented entries in the Cloud Logging UI. The changes ensure that log messages are properly assembled and split by newlines before being flushed, resulting in cleaner and more readable logs. Additionally, the test suite has been updated to be more robust in verifying log output.

Highlights

• Log Buffering Logic: Updated BufferedLogger.Write to intelligently split input by newlines, ensuring that log entries are not fragmented in the Cloud Logging UI.
• Flush Handling: Modified FlushAtDebug and FlushAtError to ensure any remaining partial log lines in the builder are flushed during exit or periodic flush events.
• Test Improvements: Enhanced buffered_logging_test.go to verify all log messages rather than just the first, and added a new test case for partial write splitting.

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[gemini-code-assist]

Code Review

This pull request updates the Go SDK's BufferedLogger to properly handle partial writes by splitting incoming log messages on newlines and buffering incomplete lines. It also ensures any remaining partial lines are flushed and adds tests to verify this behavior. The review feedback suggests optimizing the line-splitting logic in the Write method to avoid unnecessary string allocations and multiple scans by using strings.IndexByte in a loop.

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