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

@@ -154,54 +154,17 @@ func (n *sinkNode) emitEvent(ctx pipeline.NodeContext, event *model.PolymorphicE
 	// 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 {
-		handleKeyCount := 0
-		equivalentHandleKeyCount := 0
-		for i := range event.Row.Columns {
-			if event.Row.Columns[i].Flag.IsHandleKey() && event.Row.PreColumns[i].Flag.IsHandleKey() {
-				handleKeyCount++
-				colValueString := model.ColumnValueString(event.Row.Columns[i].Value)
-				preColValueString := model.ColumnValueString(event.Row.PreColumns[i].Value)
-				if colValueString == preColValueString {
-					equivalentHandleKeyCount++
-				}
+		if shouldSplitUpdateEvent(event) {
+			deleteEvent, insertEvent, err := splitUpdateEvent(event)
+			if err != nil {
+				return errors.Trace(err)
 			}
-		}
-
-		// If the handle key columns are not updated, PreColumns is directly ignored.
-		if handleKeyCount == equivalentHandleKeyCount {
+			// 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 {
-			// 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 := *event
-			deleteEventRow := *event.Row
-			deleteEventRowKV := *event.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
-			n.eventBuffer = append(n.eventBuffer, &deleteEvent)
-
-			insertEvent := *event
-			insertEventRow := *event.Row
-			insertEventRowKV := *event.RawKV
-			insertEvent.Row = &insertEventRow
-			insertEvent.RawKV = &insertEventRowKV
-
-			// NOTICE: clean up pre cols for insert event.
-			insertEvent.Row.PreColumns = nil
-			n.eventBuffer = append(n.eventBuffer, &insertEvent)
 		}
 	} else {
 		n.eventBuffer = append(n.eventBuffer, event)
@@ -215,6 +178,70 @@ 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)