MyISAM returns ERROR 1030 "Got error 0" instead of ERROR 1062 when adding a UNIQUE index with myisam_repair_threads=2

Creating a UNIQUE index on a MyISAM table whose column holds 100 identical values fails with an internal error instead of the duplicate-key error, when myisam_repair_threads is 2.

How to reproduce

CREATE OR REPLACE TABLE t(c INT) ENGINE=MyISAM;

SET SESSION myisam_repair_threads=2;

INSERT INTO t SELECT 1 FROM seq_1_to_100;

CREATE UNIQUE INDEX i ON t(c);

-- expected: ERROR 1062 (23000): Duplicate entry '1' for key 'i'

-- actual:   ERROR 1030 (HY000): Got error 0 "Internal error/check (Not system error)" from storage engine MyISAM

Notes

Controls on the same server: with myisam_repair_threads=1, or with 99 rows (seq_1_to_99), the statement fails with the expected 1062.

After the failed statement, CHECK TABLE t reports OK and SELECT COUNT FROM t returns 100. This report is about the misleading error code, not about table corruption.