[MDEV-12656] Crash in CREATE..SELECT..UNION with a ENUM column and NULL - Jira

This is a 10.3 specific problem.

This script makes the server crash:

CREATE OR REPLACE TABLE t1 (a ENUM('a1'));

CREATE OR REPLACE TABLE t2 AS SELECT (SELECT a FROM t1 UNION SELECT NULL);

with this stack trace:

#0  0x00007ffff5ddaa28 in raise () from /lib64/libc.so.6

#1  0x00007ffff5ddc62a in abort () from /lib64/libc.so.6

#2  0x00007ffff5dd3227 in __assert_fail_base () from /lib64/libc.so.6

#3  0x00007ffff5dd32d2 in __assert_fail () from /lib64/libc.so.6

#4  0x0000555555d13f72 in Type_handler_enum::make_table_field (

    this=0x5555571cd9c8 <type_handler_enum>, name=0x7ffe70013b90, addr=...,

    attr=..., table=0x7ffff4197260)

    at /home/bar/maria-git/server.10.2-ext/sql/sql_type.cc:1628

#5  0x0000555555d12cba in Type_handler::make_and_init_table_field (

    this=0x5555571cd9c8 <type_handler_enum>, name=0x7ffe70013b90, addr=...,

    attr=..., table=0x7ffff4197260)

    at /home/bar/maria-git/server.10.2-ext/sql/sql_type.cc:1218

#6  0x0000555555a7c89b in Item::tmp_table_field_from_field_type (

    this=0x7ffe70013b40, table=0x7ffff4197260)

    at /home/bar/maria-git/server.10.2-ext/sql/item.h:553

Note, if I change the CREATE..SELECT not to use UNION, it works fine:

CREATE OR REPLACE TABLE t1 (a ENUM('a1'));

CREATE OR REPLACE TABLE t2 AS SELECT (SELECT a FROM t1);

relates to

MDEV-12588 Add Type_handler::type_handler_for_tmp_table() and Type_handler::type_handler_for_union()

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