MDEV-26294 Duplicate entries in unique index not detected when changing collation

ha_innobase::check_if_supported_inplace_alter(): Refuse to change the
collation of a column that would become or remain indexed as part of
the ALTER TABLE operation.

In MariaDB Server 10.6, we will allow this type of operation;
that fix depends on MDEV-15250.

Author: dr-m

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

@@ -0,0 +1,19 @@
+#
+# MDEV-26294 Duplicate entries in unique index not detected when
+#	changing collation with INPLACE algorithm
+#
+SET NAMES utf8;
+CREATE TABLE t1 (
+id INT PRIMARY KEY,
+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;
+DROP TABLE t1;

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

@@ -1,14 +1,12 @@
---- instant_alter_charset.result	2019-06-17 14:36:02.311515062 +0300
-+++ instant_alter_charset,redundant.result	2019-06-17 14:50:11.888705725 +0300
-@@ -279,7 +279,6 @@
+@@ -288,7 +288,6 @@
  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,
-@@ -297,32 +296,21 @@
+@@ -306,35 +305,21 @@
  repeat('a', 10), repeat('a', 10)
  );
  alter table t modify a char(10) collate utf8mb4_general_ci, algorithm=instant;
@@ -27,25 +25,30 @@
 -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: ADD INDEX. Try ALGORITHM=NOCOPY
+-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: 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