This is an automated cherry-pick of pingcap#2367

Signed-off-by: ti-chi-bot <ti-community-prow-bot@tidb.io>

cdc/processor/pipeline/sink.go

@@ -142,7 +142,38 @@ func (n *sinkNode) flushSink(ctx pipeline.NodeContext, resolvedTs model.Ts) (err
 }
 
 func (n *sinkNode) emitEvent(ctx pipeline.NodeContext, event *model.PolymorphicEvent) error {
+<<<<<<< HEAD
 	n.eventBuffer = append(n.eventBuffer, event)
+=======
+	if event == nil || event.Row == nil {
+		return nil
+	}
+
+	colLen := len(event.Row.Columns)
+	preColLen := len(event.Row.PreColumns)
+	config := ctx.ChangefeedVars().Info.Config
+
+	// This indicates that it is an update event,
+	// and after enable old value internally by default(but disable in the configuration).
+	// We need to handle the update event to be compatible with the old format.
+	if !config.EnableOldValue && colLen != 0 && preColLen != 0 && colLen == preColLen {
+		if shouldSplitUpdateEvent(event) {
+			deleteEvent, insertEvent, err := splitUpdateEvent(event)
+			if err != nil {
+				return errors.Trace(err)
+			}
+			// NOTICE: Please do not change the order, the delete event always comes before the insert event.
+			n.eventBuffer = append(n.eventBuffer, deleteEvent, insertEvent)
+		} else {
+			// If the handle key columns are not updated, PreColumns is directly ignored.
+			event.Row.PreColumns = nil
+			n.eventBuffer = append(n.eventBuffer, event)
+		}
+	} else {
+		n.eventBuffer = append(n.eventBuffer, event)
+	}
+
+>>>>>>> f2526c1e (sink(refactor): Refine functions and improve event splitting code (#2367))
 	if len(n.eventBuffer) >= defaultSyncResolvedBatch {
 		if err := n.flushRow2Sink(ctx); err != nil {
 			return errors.Trace(err)
@@ -151,6 +182,70 @@ func (n *sinkNode) emitEvent(ctx pipeline.NodeContext, event *model.PolymorphicE
 	return nil
 }
 
+// shouldSplitUpdateEvent determines if the split event is needed to align the old format based on
+// whether the handle key column has been modified.
+// If the handle key column is modified,
+// we need to use splitUpdateEvent to split the update event into a delete and an insert event.
+func shouldSplitUpdateEvent(updateEvent *model.PolymorphicEvent) bool {
+	// nil event will never be split.
+	if updateEvent == nil {
+		return false
+	}
+
+	handleKeyCount := 0
+	equivalentHandleKeyCount := 0
+	for i := range updateEvent.Row.Columns {
+		if updateEvent.Row.Columns[i].Flag.IsHandleKey() && updateEvent.Row.PreColumns[i].Flag.IsHandleKey() {
+			handleKeyCount++
+			colValueString := model.ColumnValueString(updateEvent.Row.Columns[i].Value)
+			preColValueString := model.ColumnValueString(updateEvent.Row.PreColumns[i].Value)
+			if colValueString == preColValueString {
+				equivalentHandleKeyCount++
+			}
+		}
+	}
+
+	// If the handle key columns are not updated, so we do **not** need to split the event row.
+	return !(handleKeyCount == equivalentHandleKeyCount)
+}
+
+// splitUpdateEvent splits an update event into a delete and an insert event.
+func splitUpdateEvent(updateEvent *model.PolymorphicEvent) (*model.PolymorphicEvent, *model.PolymorphicEvent, error) {
+	if updateEvent == nil {
+		return nil, nil, errors.New("nil event cannot be split")
+	}
+
+	// If there is an update to handle key columns,
+	// we need to split the event into two events to be compatible with the old format.
+	// NOTICE: Here we don't need a full deep copy because our two events need Columns and PreColumns respectively,
+	// so it won't have an impact and no more full deep copy wastes memory.
+	deleteEvent := *updateEvent
+	deleteEventRow := *updateEvent.Row
+	deleteEventRowKV := *updateEvent.RawKV
+	deleteEvent.Row = &deleteEventRow
+	deleteEvent.RawKV = &deleteEventRowKV
+
+	deleteEvent.Row.Columns = nil
+	for i := range deleteEvent.Row.PreColumns {
+		// NOTICE: Only the handle key pre column is retained in the delete event.
+		if !deleteEvent.Row.PreColumns[i].Flag.IsHandleKey() {
+			deleteEvent.Row.PreColumns[i] = nil
+		}
+	}
+	// Align with the old format if old value disabled.
+	deleteEvent.Row.TableInfoVersion = 0
+
+	insertEvent := *updateEvent
+	insertEventRow := *updateEvent.Row
+	insertEventRowKV := *updateEvent.RawKV
+	insertEvent.Row = &insertEventRow
+	insertEvent.RawKV = &insertEventRowKV
+	// NOTICE: clean up pre cols for insert event.
+	insertEvent.Row.PreColumns = nil
+
+	return &deleteEvent, &insertEvent, nil
+}
+
 func (n *sinkNode) flushRow2Sink(ctx pipeline.NodeContext) error {
 	for _, ev := range n.eventBuffer {
 		err := ev.WaitPrepare(ctx)