MDEV-38431: add COM_CHANGE_USER test cases

Add tests for mysql_change_user() with database parameter:
- Test 4: Short password - verifies basic COM_CHANGE_USER works
- Test 5: Long password (260 bytes) - verifies LENENC encoding path

Author: rophy

mysql-test/suite/plugins/r/mdev38431.result

@@ -29,7 +29,25 @@ GRANT ALL ON *.* TO verylonguser;
 db
 mdev38431_db
 #
+# Test 4: COM_CHANGE_USER with short password
+# Verify mysql_change_user() works correctly with database parameter
+#
+CREATE USER changeusertest IDENTIFIED VIA cleartext_plugin_server USING 'changepwd';
+GRANT ALL ON *.* TO changeusertest;
+SELECT DATABASE() AS db;
+db
+mdev38431_db
+#
+# Test 5: COM_CHANGE_USER with long password (260 bytes)
+# Verify mysql_change_user() works with LENENC-encoded password
+#
+CREATE USER changeuserlongpwd IDENTIFIED VIA cleartext_plugin_server USING 'cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc';
+GRANT ALL ON *.* TO changeuserlongpwd;
+SELECT DATABASE() AS db;
+db
+mdev38431_db
+#
 # Cleanup
 #
-DROP USER shortuser, longuser, verylonguser;
+DROP USER shortuser, longuser, verylonguser, changeusertest, changeuserlongpwd;
 DROP DATABASE mdev38431_db;

mysql-test/suite/plugins/t/mdev38431.test

@@ -51,9 +51,33 @@ GRANT ALL ON *.* TO verylonguser;
 
 --exec $MYSQL -h 127.0.0.1 -P $MASTER_MYPORT --default-auth=mysql_clear_password -u verylonguser -p"$very_long_pwd" --database=mdev38431_db -e "SELECT DATABASE() AS db"
 
+--echo #
+--echo # Test 4: COM_CHANGE_USER with short password
+--echo # Verify mysql_change_user() works correctly with database parameter
+--echo #
+CREATE USER changeusertest IDENTIFIED VIA cleartext_plugin_server USING 'changepwd';
+GRANT ALL ON *.* TO changeusertest;
+
+# Use change_user command to switch user with password and database
+change_user changeusertest,changepwd,mdev38431_db;
+SELECT DATABASE() AS db;
+change_user root;
+
+--echo #
+--echo # Test 5: COM_CHANGE_USER with long password (260 bytes)
+--echo # Verify mysql_change_user() works with LENENC-encoded password
+--echo #
+--let $change_pwd=`SELECT REPEAT('c', 260)`
+eval CREATE USER changeuserlongpwd IDENTIFIED VIA cleartext_plugin_server USING '$change_pwd';
+GRANT ALL ON *.* TO changeuserlongpwd;
+
+change_user changeuserlongpwd,$change_pwd,mdev38431_db;
+SELECT DATABASE() AS db;
+change_user root;
+
 --echo #
 --echo # Cleanup
 --echo #
-DROP USER shortuser, longuser, verylonguser;
+DROP USER shortuser, longuser, verylonguser, changeusertest, changeuserlongpwd;
 DROP DATABASE mdev38431_db;
 # Note: Do not uninstall cleartext_plugin_server as it was pre-loaded by MTR