dispatcher: storage / redo sink dispatcher blocked smartly by the inflight bytes

[3AceShowHand]

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Issue Number: close #xxx

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[gemini-code-assist]

Summary of Changes

Hello @3AceShowHand, 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 significantly enhances the BasicDispatcher's event handling for asynchronous sinks, particularly CloudStorageSinkType and RedoSinkType. The core improvement is the introduction of an "in-flight budget" that allows the dispatcher to continue processing DML events without blocking, even when a substantial amount of data is awaiting acknowledgment from the sink. To maintain data consistency and schema integrity, especially for cloud storage, DDL and SyncPoint events are now intelligently deferred until all prior DML operations have been flushed. This change aims to optimize throughput while ensuring transactional correctness, with new metrics providing visibility into the dispatcher's blocked state.

Highlights

• In-flight Budget for Asynchronous Sinks: Introduced a per-dispatcher in-flight budget for CloudStorageSinkType and RedoSinkType to manage the amount of unacknowledged DML data, allowing for non-blocking DML processing and improved throughput.
• Deferred Block Events: Implemented a mechanism to defer DDL and SyncPoint events for CloudStorageSinkType until all preceding DML events are flushed. This ensures correct schema-version visibility and prevents out-of-order writes, maintaining data consistency.
• Refactored Block Event Handling: The DealWithBlockEvent function was refactored into dealWithBlockEvent and executeNonBlockingBlockEvent to support asynchronous processing of non-blocking events, enhancing the dispatcher's responsiveness.
• New Metrics for Monitoring: Added Prometheus metrics (AsyncSinkInflightBudgetBlockedDispatcherCount, AsyncSinkInflightBudgetBlockedDuration) to provide visibility into the performance and blocking status of dispatchers utilizing the new in-flight budget mechanism.
• Improved Redo Log File Cache Management: Enhanced memory management in the redo log writer by modifying fileCache to use a pointer to a byte slice from a sync.Pool and adding explicit releaseFileCache calls for proper buffer recycling.

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[gemini-code-assist]

Code Review

This pull request introduces an intelligent in-flight byte budget for storage and redo sinks to improve flow control. A key part of this is deferring block events like DDLs until all in-flight DMLs are flushed, which is crucial for sinks like cloud storage to maintain data consistency. The implementation is well-structured, with the core logic cleanly encapsulated in the new inflight_budget.go file, and is well-supported by new metrics and tests. The code is generally of high quality, but I've identified a potential issue in the redo writer's buffer management that could impact performance over time.

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@3AceShowHand: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-error-log-review	5fec0c4	link	true	/test pull-error-log-review

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