ER_KEY_NOT_FOUND upon concurrent ALTER and transaction

--source include/have_debug_sync.inc

CREATE TABLE t (a int, b int) ENGINE=MyISAM;

INSERT INTO t VALUES (1,10),(2,20);

SET debug_sync= 'alter_table_online_progress WAIT_FOR goforit';

--send

  ALTER TABLE t ADD c INT, ALGORITHM=COPY, LOCK=NONE;

--connect (con1,localhost,root,,)

UPDATE t SET b = 100;

START TRANSACTION;

UPDATE t SET b = 200;

--connect (con2,localhost,root,,)

DELETE FROM t ORDER BY a LIMIT 1;

DELETE FROM t ORDER BY a LIMIT 1;

--connection con1

COMMIT;

SET debug_sync= 'now SIGNAL goforit';

--connection default

--reap

# Cleanup

DROP TABLE t;

--disconnect con2

--disconnect con1

mysqltest: At line 24: query 'reap' failed: ER_KEY_NOT_FOUND (1032): Can't find record in 't'

2023-04-13  1:51:44 4 [ERROR] mariadbd: Can't find record in 't'

2023-04-13  1:51:44 4 [ERROR] Slave SQL: Could not execute Delete_rows_v1 event on table test.t; Can't find record in 't', Error_code: 1032; handler error HA_ERR_END_OF_FILE; the event's master log FIRST, end_log_pos 112, Internal MariaDB error code: 1032