eventservice: improve dml processing efficiency

[asddongmen]

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[gemini-code-assist]

Code Review

This pull request introduces parallel decoding of DML events in the eventScanner and dmlProcessor to improve performance, along with a fast-path cache (dmlTypeFilterCache) to skip decoding for ignored event types early. The review feedback highlights a critical bug where the dmlTypeFilterCache array size of 3 is too small to cache delete events (index 3), causing the cache to be bypassed. Additionally, the reviewer suggested optimizing sequential execution by conditionally bypassing the buffering and cloning of raw KV entries when parallel decoding is disabled.

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[gemini-code-assist]

Define a boolean flag enableParallel to check if parallel decoding is actually enabled and possible. This allows us to conditionally bypass buffering and cloning for sequential runs, avoiding unnecessary allocations and CPU overhead.

Suggested change

	txnRows := make([]*common.RawKVEntry, 0)
	enableParallel := s.parallelDecodeWorkers > 1 && s.newMounter != nil && !dispatcher.info.IsOutputRawChangeEvent()
	txnRows := make([]*common.RawKVEntry, 0)

[gemini-code-assist]

If parallel decoding is enabled, buffer and clone the raw KV entry. Otherwise, directly append the row sequentially to avoid the memory allocation and CPU overhead of cloning.

		if enableParallel {
			txnRows = append(txnRows, cloneRawKVEntry(rawEvent))
		} else {
			if err = processor.appendRow(rawEvent); err != nil {
				log.Error("append row failed", zap.Error(err),
					zap.Stringer("dispatcherID", session.dispatcherStat.id),
					zap.Int64("tableID", tableID),
					zap.Uint64("startTs", rawEvent.StartTs),
					zap.Uint64("commitTs", rawEvent.CRTs),
					zap.Int64("mode", s.mode))
				return false, err
			}
		}

[gemini-code-assist]

The dmlTypeFilterCache array size of 3 is too small to cache RowTypeDelete events. In TiCDC, common.RowType is defined as:

const (
	RowTypeUnknown RowType = iota
	RowTypeInsert
	RowTypeUpdate
	RowTypeDelete
)

This means RowTypeDelete has an integer value of 3. Since the array size is 3, any attempt to cache or retrieve a delete event will result in an out-of-bounds index (idx >= 3), causing the cache to be completely bypassed for all delete events. Increasing the array size to 4 fixes this issue and ensures delete events are cached correctly.

Suggested change

	dmlTypeFilterCache [3]struct {
	valid bool
	ignore bool
	}
	dmlTypeFilterCache [4]struct {
	valid bool
	ignore bool
	}