MDEV-18160/MDEV-18162 Assertion failure or crash after DROP COLUMN

dict_table_t::instant_column(): Correctly compute the value of
metadata_changed. The original computation in
commit 0037207 would essentially
invoke memcmp(x,x,y), which can only return 0.

mysql-test/suite/innodb/r/instant_alter.result

@@ -638,6 +638,16 @@ INSERT INTO t1 VALUES (1);
 ALTER TABLE t1 ADD COLUMN b INT;
 ALTER TABLE t1 MODIFY COLUMN a INT NULL;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT, d INT) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1222,6 +1232,16 @@ INSERT INTO t1 VALUES (1);
 ALTER TABLE t1 ADD COLUMN b INT;
 ALTER TABLE t1 MODIFY COLUMN a INT NULL;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT, d INT) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1806,10 +1826,20 @@ INSERT INTO t1 VALUES (1);
 ALTER TABLE t1 ADD COLUMN b INT;
 ALTER TABLE t1 MODIFY COLUMN a INT NULL;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT, d INT) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-131
+143
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

mysql-test/suite/innodb/t/instant_alter.test

@@ -522,6 +522,19 @@ ALTER TABLE t1 ADD COLUMN b INT;
 ALTER TABLE t1 MODIFY COLUMN a INT NULL;
 DROP TABLE t1;
 
+# MDEV-18160/MDEV-18162 Failing assertion on ALTER
+eval CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT) $engine;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
+
+eval CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c INT, d INT) $engine;
+INSERT INTO t1 SET a=1;
+ALTER TABLE t1 DROP c;
+ALTER TABLE t1 DROP b, ADD v INT AS (a);
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/handler/handler0alter.cc

@@ -645,15 +645,18 @@ inline bool dict_table_t::instant_column(const dict_table_t& table,
 			       * sizeof *instant->dropped);
 		}
 
+		const field_map_element_t* field_map = old_instant
+			? old_instant->field_map : NULL;
+
 		init_instant<true>(table);
 
 		if (!metadata_changed) {
-			metadata_changed = !old_instant
-				|| memcmp(old_instant->field_map,
+			metadata_changed = !field_map
+				|| memcmp(field_map,
 					  instant->field_map,
 					  (index->n_fields
 					   - index->first_user_field())
-					  * sizeof *old_instant->field_map);
+					  * sizeof *field_map);
 		}
 	}