MDEV-38698 mysql_upgrade does not fix charset and collation for mysql.user

In case if the view mysql.user was created (e.g. in 10.6) with a
pre- 18edb09 server with an unexpected
character_set_client and collation_connection, e.g.
utf8mb3 and utf8mb3_general_ci, mysql_upgrade did not fix it to the
expected latin1 and latin1_swedish_ci.

Since 11.8 this could often lead to "Illegax mix of collations" errors
when querying mysql.user, because since 11.8 the default collation for
utf8mb3 is utf8mb3_uca1400_ai_ci, according to the default
@@character_set_collations.

For example:

MariaDB [test]> select user,host,is_role from
    -> mysql.user where is_role='N';
ERROR 1267 (HY000): Illegal mix of collations (utf8mb3_general_ci,COERCIBLE)
                    and (utf8mb3_uca1400_ai_ci,COERCIBLE) for operation '='

Fixing mariadb_system_tables.sql to drop the view if it has non-standard
character set or collations, so it gets recreated again correctly.

Author: abarkov

mysql-test/main/mysql_upgrade.result

@@ -2453,3 +2453,18 @@ GRANT USAGE ON *.* TO `foo`@`bar`
 GRANT ALL PRIVILEGES ON `mysql`.* TO `foo`@`bar`
 drop user foo@bar;
 # End of 11.3 tests
+#
+# MDEV-38698 mysql_upgrade does not fix charset and collation for mysql.user
+#
+FLUSH TABLES mysql.user;
+FLUSH PRIVILEGES;
+SELECT CHARACTER_SET_CLIENT, COLLATION_CONNECTION FROM INFORMATION_SCHEMA.VIEWS
+WHERE TABLE_SCHEMA='mysql' AND TABLE_NAME='user';
+CHARACTER_SET_CLIENT	COLLATION_CONNECTION
+utf8mb3	utf8mb3_general_ci
+FLUSH TABLES mysql.user;
+FLUSH PRIVILEGES;
+SELECT CHARACTER_SET_CLIENT, COLLATION_CONNECTION FROM INFORMATION_SCHEMA.VIEWS
+WHERE TABLE_SCHEMA='mysql' AND TABLE_NAME='user';
+CHARACTER_SET_CLIENT	COLLATION_CONNECTION
+latin1	latin1_swedish_ci

mysql-test/main/mysql_upgrade.test

@@ -573,3 +573,25 @@ show grants for foo@bar;
 drop user foo@bar;
 
 --echo # End of 11.3 tests
+
+
+--echo #
+--echo # MDEV-38698 mysql_upgrade does not fix charset and collation for mysql.user
+--echo #
+
+--remove_file $MYSQLD_DATADIR/mysql/user.frm
+--copy_file std_data/upgrade/user-100625-utf8mb3.frm $MYSQLD_DATADIR/mysql/user.frm
+
+FLUSH TABLES mysql.user;
+FLUSH PRIVILEGES;
+SELECT CHARACTER_SET_CLIENT, COLLATION_CONNECTION FROM INFORMATION_SCHEMA.VIEWS
+WHERE TABLE_SCHEMA='mysql' AND TABLE_NAME='user';
+
+--replace_result $MYSQLTEST_VARDIR var
+--exec $MYSQL_UPGRADE --force --silent 2>&1
+--remove_file $MYSQLD_DATADIR/mariadb_upgrade_info
+
+FLUSH TABLES mysql.user;
+FLUSH PRIVILEGES;
+SELECT CHARACTER_SET_CLIENT, COLLATION_CONNECTION FROM INFORMATION_SCHEMA.VIEWS
+WHERE TABLE_SCHEMA='mysql' AND TABLE_NAME='user';

mysql-test/std_data/upgrade/user-100625-utf8mb3.frm

@@ -48,6 +48,28 @@ INSERT INTO tmp_user_sys (Host,User,Priv) VALUES ('localhost','mariadb.sys','{"a
 INSERT IGNORE INTO global_priv SELECT * FROM tmp_user_sys WHERE 0 <> @need_sys_user_creation;
 DROP TABLE tmp_user_sys;
 
+
+-- Drop mysql.user if it has wrong
+-- character_set_client or collation_connection
+DELIMITER $$
+BEGIN NOT ATOMIC
+  DECLARE v0 ROW TYPE OF INFORMATION_SCHEMA.VIEWS;
+  SET v0= (SELECT * FROM INFORMATION_SCHEMA.VIEWS
+           WHERE TABLE_CATALOG='def'
+             AND TABLE_SCHEMA='mysql'
+             AND TABLE_NAME='user');
+  -- Drop only if the DEFINER is standard
+  IF v0.DEFINER='mariadb.sys@localhost' AND
+     (v0.CHARACTER_SET_CLIENT<>'latin1' OR
+     v0.COLLATION_CONNECTION<>'latin1_swedish_ci')
+  THEN
+    DROP VIEW user;
+  END IF;
+END
+$$
+DELIMITER ;
+
+
 CREATE DEFINER='mariadb.sys'@'localhost' SQL SECURITY DEFINER VIEW IF NOT EXISTS user AS SELECT
   Host,
   User,