MDEV-26294 Duplicate entries in unique index not detected when changi…

…ng collation

Problem:
=======
ALTER TABLE in InnoDB fails to detect duplicate entries
for the unique index when the character set or collation of
an indexed column is changed in such a way that the character
encoding is compatible with the old table definition.
In this case, any secondary indexes on the changed columns
would be rebuilt (DROP INDEX, ADD INDEX).

Solution:
========
During ALTER TABLE, InnoDB keeps track of columns whose collation
changed, and will fill in the correct metadata when sorting the
index records, or applying changes from concurrent DML.
This metadata will be allocated in the dict_index_t::heap of
the being-created secondary indexes.

The fix was developed by Thirunarayanan Balathandayuthapani
and simplified by me.

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

@@ -9,11 +9,89 @@ msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_swedish_ci UNIQUE
 ) ENGINE=INNODB;
 INSERT INTO t1 VALUES (1, 'aaa');
 INSERT INTO t1 VALUES (2, 'ååå');
-ALTER TABLE t1
-MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci,
-ALGORITHM=NOCOPY;
-ERROR 0A000: ALGORITHM=NOCOPY is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
-ALTER TABLE t1 DROP INDEX msg,
-MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci,
-ALGORITHM=NOCOPY;
+ALTER TABLE t1 MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci, ALGORITHM=inplace;
+ERROR 23000: Duplicate entry 'NULL' for key 'msg'
+DROP TABLE t1;
+CREATE TABLE t1 (
+id INT PRIMARY KEY,
+msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_bin,
+id_2 INT not null,
+unique index(msg, id_2)
+) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1, 'aaa', 2);
+INSERT INTO t1 VALUES (2, 'AAA', 3);
+ALTER TABLE t1 MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci, ALGORITHM=inplace;
+DROP TABLE t1;
+CREATE TABLE t1 (
+id INT PRIMARY KEY,
+msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_bin,
+unique index(msg)
+) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1, 'aaa');
+INSERT INTO t1 VALUES (2, 'bbb');
+INSERT INTO t1 VALUES (3, 'ccc');
+SET DEBUG_SYNC = 'RESET';
+SET DEBUG_SYNC = 'row_log_apply_before SIGNAL before_apply WAIT_FOR go_ahead';
+ALTER TABLE t1 MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci, ALGORITHM=NOCOPY;
+connect  con1,localhost,root,,;
+connection con1;
+SET DEBUG_SYNC = 'now WAIT_FOR before_apply';
+INSERT INTO t1 VALUES (4, 'AAA');
+UPDATE t1 set msg = "ddd" where id = 2;
+DELETE FROM t1 WHERE id= 3;
+SET DEBUG_SYNC = 'now SIGNAL go_ahead';
+SET DEBUG_SYNC = 'RESET';
+connection default;
+ERROR 23000: Duplicate entry 'NULL' for key 'msg'
+SELECT * FROM t1;
+id	msg
+4	AAA
+1	aaa
+2	ddd
+DROP TABLE t1;
+CREATE TABLE t1 (
+id INT PRIMARY KEY,
+msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_bin,
+unique index(msg)
+) ENGINE=INNODB;
+INSERT INTO t1 VALUES (1, 'aaa');
+SET DEBUG_DBUG="+d,create_index_fail";
+SET DEBUG_SYNC="innodb_inplace_alter_table_enter SIGNAL con1_go WAIT_FOR alter_signal";
+ALTER TABLE t1 MODIFY msg VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci, ALGORITHM=NOCOPY;
+connection con1;
+SET DEBUG_SYNC="now WAIT_FOR con1_go";
+BEGIN;
+SELECT * FROM t1;
+id	msg
+1	aaa
+SET DEBUG_SYNC="now SIGNAL alter_signal";
+connection default;
+ERROR 23000: Duplicate entry '' for key '*UNKNOWN*'
+connection con1;
+rollback;
+INSERT INTO t1 VALUES(2, 'bbb');
+disconnect con1;
+connection default;
+SET DEBUG_SYNC=reset;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `id` int(11) NOT NULL,
+  `msg` varchar(100) CHARACTER SET utf8mb3 COLLATE utf8mb3_bin DEFAULT NULL,
+  PRIMARY KEY (`id`),
+  UNIQUE KEY `msg` (`msg`)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+INSERT INTO t1 VALUES(3, 'ccc');
+DROP TABLE t1;
+CREATE TABLE t1(id INT PRIMARY KEY, msg VARCHAR(100),
+msg_1 VARCHAR(100) AS (msg) VIRTUAL,
+msg_2 VARCHAR(100) AS (msg) STORED,
+UNIQUE(msg), UNIQUE(msg_1),
+UNIQUE(msg_2))ENGINE=InnoDB;
+ALTER TABLE t1 MODIFY msg_1 VARCHAR(100) CHARACTER SET utf8
+COLLATE utf8_unicode_ci, ALGORITHM=inplace;
+ERROR HY000: This is not yet supported for generated columns
+ALTER TABLE t1 MODIFY msg_2 VARCHAR(100) CHARACTER SET utf8
+COLLATE utf8_unicode_ci, ALGORITHM=inplace;
+ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
 DROP TABLE t1;

mysql-test/suite/innodb/r/instant_alter_charset,redundant.rdiff

@@ -1,12 +1,12 @@
-@@ -288,7 +288,6 @@
+@@ -279,7 +279,6 @@
  alter table boundary_255
  modify a varchar(70) charset utf8mb4,
  algorithm=instant;
 -ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  drop table boundary_255;
  create table t (
  a char(10) collate utf8mb3_general_ci,
-@@ -306,35 +305,21 @@
+@@ -297,32 +296,21 @@
  repeat('a', 10), repeat('a', 10)
  );
  alter table t modify a char(10) collate utf8mb4_general_ci, algorithm=instant;
@@ -25,30 +25,34 @@
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify aa char(10) collate utf8mb4_general_ci, algorithm=instant;
--ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 -check table t;
 -Table	Op	Msg_type	Msg_text
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify bb char(70) collate utf8mb4_general_ci, algorithm=instant;
--ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 -check table t;
 -Table	Op	Msg_type	Msg_text
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify cc char(100) collate utf8mb4_general_ci, algorithm=instant;
--ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
 -check table t;
 -Table	Op	Msg_type	Msg_text
 -test.t	check	status	OK
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify d char(10) collate utf8mb4_spanish_ci, algorithm=instant;
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  alter table t modify dd char(10) collate utf8mb4_spanish_ci, algorithm=instant;
--ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Collation change on an indexed column. Try ALGORITHM=COPY
+-ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
 +ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type. Try ALGORITHM=COPY
  select * from t;
  a	b	c	aa	bb	cc	d	dd
  aaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	aaaaaaaaaa	aaaaaaaaaa
-
-mysqltest: Result length mismatch
+@@ -2066,7 +2054,7 @@
+ ALTER TABLE t1
+ CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_520_ci,
+ ADD UNIQUE INDEX test_key (a);
+-ERROR 23000: Duplicate entry 'NULL' for key 'test_key'
++ERROR 23000: Duplicate entry 'a1' for key 'test_key'
+ ALTER TABLE t1 CONVERT TO CHARACTER SET UTF8MB4 COLLATE UTF8MB4_UNICODE_520_CI;
+ CHECK TABLE t1;
+ Table	Op	Msg_type	Msg_text