XMLISVALID should be 0 returns 1 for strings that are not a single well-formed XML document

set @s='<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

<xs:element name="r"><xs:complexType><xs:sequence><xs:element name="a" type="xs:int"/></xs:sequence></xs:complexType></xs:element></xs:schema>';

create table t1 (name varchar(20), doc text);

insert into t1 values

  ('empty', ''),

  ('whitespace only', '   '),

  ('text only', 'abc'),

  ('comment only', '<!--c-->'),

  ('two roots', '<r><a>1</a></r><r><a>1</a></r>'),

  ('leading text', 'x<r><a>1</a></r>'),

  ('trailing text', '<r><a>1</a></r>x'),

  ('unclosed', '<r><a>1</a>'),

  ('valid', '<r><a>1</a></r>');

select name, doc, XMLISVALID(doc, @s), CAST(doc AS XMLTYPE)  from t1;

MariaDB [test]> select name, doc, XMLISVALID(doc, @s), CAST(doc AS XMLTYPE)  from t1;

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

| name            | doc                            | XMLISVALID(doc, @s) | CAST(doc AS XMLTYPE) |

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

| empty           |                                |                   1 | NULL                 |

| whitespace only |                                |                   1 | NULL                 |

| text only       | abc                            |                   1 | NULL                 |

| comment only    | <!--c-->                       |                   1 | NULL                 |

| two roots       | <r><a>1</a></r><r><a>1</a></r> |                   1 | NULL                 |

| leading text    | x<r><a>1</a></r>               |                   1 | NULL                 |

| trailing text   | <r><a>1</a></r>x               |                   1 | NULL                 |

| unclosed        | <r><a>1</a>                    |                   0 | NULL                 |

| valid           | <r><a>1</a></r>                |                   1 | <r><a>1</a></r>      |

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

9 rows in set (0,002 sec)