Server crashes in key_copy upon INSERT into table with long unique multi-part key and auto-increment

CREATE TABLE t1 (a INT AUTO_INCREMENT, b VARCHAR(1024), UNIQUE (b,a)) ENGINE=MyISAM;

INSERT INTO t1 () VALUES ();

# Cleanup

DROP TABLE t1;

#3  <signal handler called>

#4  0x0000560f03088abe in key_copy (to_key=0x7fbae4a47ce9 "\230\202\003\017V", from_record=0x7fbad412a0f8 "\377", key_info=0x7fbad400bc88, key_length=1016, with_zerofill=false) at /data/src/10.4/sql/key.cc:150

#5  0x0000560f036ed3e8 in ha_myisam::get_auto_increment (this=0x7fbad4132b78, offset=1, increment=1, nb_desired_values=1, first_value=0x7fbae4a48108, nb_reserved_values=0x7fbae4a48110) at /data/src/10.4/storage/myisam/ha_myisam.cc:2306

#6  0x0000560f02f727d1 in handler::update_auto_increment (this=0x7fbad4132b78) at /data/src/10.4/sql/handler.cc:3377

#7  0x0000560f036e8958 in ha_myisam::write_row (this=0x7fbad4132b78, buf=0x7fbad412a0f8 "\377") at /data/src/10.4/storage/myisam/ha_myisam.cc:946

#8  0x0000560f02f7bdaa in handler::ha_write_row (this=0x7fbad4132b78, buf=0x7fbad412a0f8 "\377") at /data/src/10.4/sql/handler.cc:6675

#9  0x0000560f02bc125c in write_record (thd=0x7fbad4000af0, table=0x7fbad400ae20, info=0x7fbae4a48490) at /data/src/10.4/sql/sql_insert.cc:2056

#10 0x0000560f02bbe1c7 in mysql_insert (thd=0x7fbad4000af0, table_list=0x7fbad4013280, fields=..., values_list=..., update_fields=..., update_values=..., duplic=DUP_ERROR, ignore=false) at /data/src/10.4/sql/sql_insert.cc:1078

#11 0x0000560f02c0c6dd in mysql_execute_command (thd=0x7fbad4000af0) at /data/src/10.4/sql/sql_parse.cc:4528

#12 0x0000560f02c18e67 in mysql_parse (thd=0x7fbad4000af0, rawbuf=0x7fbad4013198 "INSERT INTO t1 () VALUES ()", length=27, parser_state=0x7fbae4a49160, is_com_multi=false, is_next_command=false) at /data/src/10.4/sql/sql_parse.cc:7900

#13 0x0000560f02c0404a in dispatch_command (command=COM_QUERY, thd=0x7fbad4000af0, packet=0x7fbad4136221 "INSERT INTO t1 () VALUES ()", packet_length=27, is_com_multi=false, is_next_command=false) at /data/src/10.4/sql/sql_parse.cc:1842

#14 0x0000560f02c026d7 in do_command (thd=0x7fbad4000af0) at /data/src/10.4/sql/sql_parse.cc:1360

#15 0x0000560f02d8b943 in do_handle_one_connection (connect=0x560f05898ce0) at /data/src/10.4/sql/sql_connect.cc:1412

#16 0x0000560f02d8b692 in handle_one_connection (arg=0x560f05898ce0) at /data/src/10.4/sql/sql_connect.cc:1316

#17 0x0000560f03792585 in pfs_spawn_thread (arg=0x560f058b4c80) at /data/src/10.4/storage/perfschema/pfs.cc:1869

#18 0x00007fbaec8284a4 in start_thread (arg=0x7fbae4a4a700) at pthread_create.c:456

#19 0x00007fbaea95cd0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97

commit bd7f7b14163d0696aa92c90deeab16d63ae6bbd6

Author: Sergei Golubchik

Date:   Thu Feb 21 22:42:00 2019 +0100

    MDEV-371 Unique Index for long columns