MariaDB [test]> create table test (i int auto_increment primary key);

Query OK, 0 rows affected (0.002 sec)

MariaDB [test]> start transaction read only;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> insert into t() values ();

ERROR 1792 (25006): Cannot execute statement in a READ ONLY transaction

MariaDB [test]> select @@tx_read_only;

+----------------+

| @@tx_read_only |

+----------------+

|              0 |

+----------------+

1 row in set (0.000 sec)

MariaDB [test]> SELECT @@GLOBAL.tx_read_only, @@SESSION.tx_read_only;

+-----------------------+------------------------+

| @@GLOBAL.tx_read_only | @@SESSION.tx_read_only |

+-----------------------+------------------------+

|                     0 |                      0 |

+-----------------------+------------------------+

1 row in set (0.000 sec)

MariaDB [test]> rollback;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> start transaction

    -> ;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> SET TRANSACTION READ ONLY;

ERROR 1568 (25001): Transaction characteristics can't be changed while a transaction is in progress

MariaDB [test]> rollback;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> SET TRANSACTION READ ONLY;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> start transaction;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> SELECT @@GLOBAL.tx_read_only, @@SESSION.tx_read_only;

+-----------------------+------------------------+

| @@GLOBAL.tx_read_only | @@SESSION.tx_read_only |

+-----------------------+------------------------+

|                     0 |                      0 |

+-----------------------+------------------------+

1 row in set (0.000 sec)

MariaDB [test]> select version();

+--------------------+

| version()          |

+--------------------+

| 10.5.6-MariaDB-log |

+--------------------+

1 row in set (0.000 sec)