TiFlash can not refresh schema automatically after loss column modification

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

# table supplier should be large. (maybe more than 100 MiB)
# It is to make sure TiFlash can get the temporary schema in TiDB.
mysql> show create table supplier;
+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                         |
+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| supplier | CREATE TABLE `supplier` (
  `S_SUPPKEY` bigint NOT NULL,
  `S_NAME` char(25) NOT NULL,
  `S_ADDRESS` varchar(40) NOT NULL,
  `S_NATIONKEY` bigint NOT NULL,
  `S_PHONE` char(15) NOT NULL,
  `S_ACCTBAL` decimal(15,2) NOT NULL,
  `S_COMMENT` varchar(101) NOT NULL,
  `tcol` varchar(256) DEFAULT NULL,
  PRIMARY KEY (`S_SUPPKEY`) /*T![clustered_index] CLUSTERED */
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

mysql> alter table supplier set tiflash replica 1;
Query OK, 0 rows affected (0.04 sec)

mysql> alter table supplier modify column tcol varchar(255) default null;
Query OK, 0 rows affected (19.33 sec)

TiFlash schema:

$ cat t_115.sql
ATTACH TABLE t_115
(
    s_suppkey Int64, 
    s_name StringV2, 
    s_address StringV2, 
    s_nationkey Int64, 
    s_phone StringV2, 
    s_acctbal Decimal(15, 2), 
    s_comment StringV2, 
    tcol Nullable(StringV2), 
    `_col$_tcol_0` Nullable(StringV2)
)
ENGINE = DeltaMerge(s_suppkey, '{"cols":[{"id":1,"name":{"L":"s_suppkey","O":"s_suppkey"},"offset":0,"state":5,"type":{"Charset":"binary","Collate":"binary","Decimal":0,"Flag":4099,"Flen":20,"Tp":8}},{"id":2,"name":{"L":"s_name","O":"s_name"},"offset":1,"state":5,"type":{"Charset":"utf8mb4","Collate":"utf8mb4_bin","Decimal":0,"Flag":4097,"Flen":25,"Tp":254}},{"id":3,"name":{"L":"s_address","O":"s_address"},"offset":2,"state":5,"type":{"Charset":"utf8mb4","Collate":"utf8mb4_bin","Decimal":0,"Flag":4097,"Flen":40,"Tp":15}},{"id":4,"name":{"L":"s_nationkey","O":"s_nationkey"},"offset":3,"state":5,"type":{"Charset":"binary","Collate":"binary","Decimal":0,"Flag":4097,"Flen":20,"Tp":8}},{"id":5,"name":{"L":"s_phone","O":"s_phone"},"offset":4,"state":5,"type":{"Charset":"utf8mb4","Collate":"utf8mb4_bin","Decimal":0,"Flag":4097,"Flen":15,"Tp":254}},{"id":6,"name":{"L":"s_acctbal","O":"s_acctbal"},"offset":5,"state":5,"type":{"Charset":"binary","Collate":"binary","Decimal":2,"Flag":4097,"Flen":15,"Tp":246}},{"id":7,"name":{"L":"s_comment","O":"s_comment"},"offset":6,"state":5,"type":{"Charset":"utf8mb4","Collate":"utf8mb4_bin","Decimal":0,"Flag":4097,"Flen":101,"Tp":15}},{"id":8,"name":{"L":"tcol","O":"tcol"},"offset":7,"state":5,"type":{"Charset":"utf8mb4","Collate":"utf8mb4_bin","Decimal":0,"Flag":0,"Flen":256,"Tp":15}},{"id":9,"name":{"L":"_col$_tcol_0","O":"_col$_tcol_0"},"offset":8,"state":3,"type":{"Charset":"utf8mb4","Collate":"utf8mb4_bin","Decimal":0,"Flag":0,"Flen":255,"Tp":15}}],"id":115,"index_info":[],"is_common_handle":false,"keyspace_id":4294967295,"name":{"L":"supplier","O":"supplier"},"pk_is_handle":true,"state":5,"tiflash_replica":{"Available":true,"Count":1},"update_timestamp":458808485532663817}', 0)

If there is no DDL trigger TiFlash sync table schema, tcol will not be drop and _col$_tcol_0 will not be renamed.

4. What is your TiFlash version? (Required)

v7.5 ~ master

Start with the t_115.sql schema and reproduce the supplier table workflow, including the TiFlash replica and ALTER TABLE MODIFY COLUMN commands. Trace the DDL trigger and TiFlash table-schema synchronization path described in the report. Done means the old tcol column is dropped and _col$_tcol_0 is renamed after the column modification without manual intervention.