DDL allowed on a MERGE child table that is only READ-locked through its MERGE parent, once any other table is WRITE-locked

CREATE TABLE m1 (a INT) ENGINE=MyISAM;

CREATE TABLE w  (a INT) ENGINE=MyISAM;

CREATE TABLE mm (a INT) ENGINE=MERGE UNION=(m1) INSERT_METHOD=LAST;

INSERT INTO m1 VALUES (1),(2);

LOCK TABLES mm READ;

TRUNCATE TABLE m1;            # ERROR 1099 (correct)

DROP TABLE m1;                # ERROR 1099 (correct)

UNLOCK TABLES;

LOCK TABLES mm READ, w WRITE;  # w is unrelated to mm

SELECT COUNT(*) FROM mm;      # 2

INSERT INTO m1 VALUES (3);    # ERROR 1099 (correct)

TRUNCATE TABLE m1;            # succeeds  <-- expected ERROR 1099

SELECT COUNT(*) FROM mm;      # 0         <-- READ-locked data removed

ALTER TABLE m1 ADD COLUMN b INT;  # succeeds  <-- expected ERROR 1099

DROP TABLE m1;                # succeeds  <-- expected ERROR 1099

UNLOCK TABLES;

SELECT * FROM mm;             # ERROR 1168

13.2.0-opt>CREATE TABLE m1 (a INT) ENGINE=MyISAM;

Query OK, 0 rows affected (0.007 sec)

13.2.0-opt>CREATE TABLE w  (a INT) ENGINE=MyISAM;

Query OK, 0 rows affected (0.003 sec)

13.2.0-opt>CREATE TABLE mm (a INT) ENGINE=MERGE UNION=(m1) INSERT_METHOD=LAST;

Query OK, 0 rows affected (0.003 sec)

13.2.0-opt>INSERT INTO m1 VALUES (1),(2);

Query OK, 2 rows affected (0.001 sec)

Records: 2  Duplicates: 0  Warnings: 0

13.2.0-opt>LOCK TABLES mm READ;

Query OK, 0 rows affected (0.000 sec)

13.2.0-opt>TRUNCATE TABLE m1;            # ERROR 1099 (correct)

ERROR 1099 (HY000): Table 'm1' was locked with a READ lock and can't be updated

13.2.0-opt>DROP TABLE m1;                # ERROR 1099 (correct)

ERROR 1099 (HY000): Table 'm1' was locked with a READ lock and can't be updated

13.2.0-opt>UNLOCK TABLES;

Query OK, 0 rows affected (0.000 sec)

13.2.0-opt>LOCK TABLES mm READ, w WRITE;  # w is unrelated to mm

Query OK, 0 rows affected (0.000 sec)

13.2.0-opt>SELECT COUNT(*) FROM mm;      # 2

+----------+

| COUNT(*) |

+----------+

|        2 |

+----------+

1 row in set (0.000 sec)

13.2.0-opt>INSERT INTO m1 VALUES (3);    # ERROR 1099 (correct)

ERROR 1099 (HY000): Table 'm1' was locked with a READ lock and can't be updated

13.2.0-opt>TRUNCATE TABLE m1;            # succeeds  <-- expected ERROR 1099

Query OK, 0 rows affected (0.000 sec)

13.2.0-opt>SELECT COUNT(*) FROM mm;      # 0         <-- READ-locked data removed

+----------+

| COUNT(*) |

+----------+

|        0 |

+----------+

1 row in set (0.000 sec)

13.2.0-opt>ALTER TABLE m1 ADD COLUMN b INT;  # succeeds  <-- expected ERROR 1099

Query OK, 0 rows affected (0.008 sec)

Records: 0  Duplicates: 0  Warnings: 0

13.2.0-opt>DROP TABLE m1;                # succeeds  <-- expected ERROR 1099

Query OK, 0 rows affected (0.002 sec)

13.2.0-opt>UNLOCK TABLES;

Query OK, 0 rows affected (0.000 sec)

13.2.0-opt>SELECT * FROM mm;             # ERROR 1168

ERROR 1168 (HY000): Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist

13.2.0-opt>