CREATE TABLE t1 (x INT NOT NULL UNIQUE KEY) ENGINE=InnoDB;
INSERT INTO t1 VALUES(5);
SET GLOBAL innodb_log_checkpoint_now=TRUE;
SET DEBUG_SYNC='commit_cache_rebuild SIGNAL ready WAIT_FOR finish';
ALTER TABLE t1 ADD PRIMARY KEY(x);
SET DEBUG_SYNC='now WAIT_FOR ready';
SET GLOBAL innodb_log_checkpoint_now=TRUE;
SET DEBUG_SYNC='now SIGNAL finish';
SHOW CREATE TABLE t1;
Table	Create Table
t1	CREATE TABLE `t1` (
  `x` int(11) NOT NULL,
  PRIMARY KEY (`x`),
  UNIQUE KEY `x` (`x`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
DROP TABLE t1;
SET DEBUG_SYNC='RESET';