add one unit test.

cdc/sink/dmlsink/mq/manager/kafka_manager.go

@@ -170,7 +170,7 @@ func (m *kafkaTopicManager) fetchAllTopicsPartitionsNum(
 	}
 
 	// it may happen the following case:
-	// 1. use create the default topic with partition number set as 3 manually
+	// 1. user create the default topic with partition number set as 3 manually
 	// 2. set the partition-number as 2 in the sink-uri.
 	// in the such case, we should use 2 instead of 3 as the partition number.
 	_, ok := numPartitions[m.defaultTopic]

cdc/sink/util/helper_test.go

@@ -51,6 +51,17 @@ func TestPartition(t *testing.T) {
 	partitionsNum, err = manager.GetPartitionNum(ctx, "new-topic")
 	require.NoError(t, err)
 	require.Equal(t, int32(2), partitionsNum)
+
+	// assume a topic already exist, the not default topic won't be affected by the default topic's partition number.
+	err = adminClient.CreateTopic(ctx, &kafka.TopicDetail{
+		Name:          "new-topic-2",
+		NumPartitions: 3,
+	}, false)
+	require.NoError(t, err)
+
+	partitionsNum, err = manager.GetPartitionNum(ctx, "new-topic-2")
+	require.NoError(t, err)
+	require.Equal(t, int32(3), partitionsNum)
 }
 
 func TestGetTopic(t *testing.T) {