create database tb;
create table t(id int primary key, c1 int, c2 int);
insert into t values(1,1,1),(2,1,2);
create table t1(id int primary key, c1 int, c2 int);
SET DEBUG_SYNC= 'RESET';
SET DEBUG_SYNC='before_commit_inplace_alter_table SIGNAL rollback WAIT_FOR commit';
#sending
alter table t1 add index(c1);;
SET DEBUG_SYNC='before_rollback_inplace_alter_table WAIT_FOR rollback';
SET DEBUG_SYNC='after_rollback_inplace_alter_table SIGNAL commit';
alter table t add unique index(c1);
ERROR 23000: Duplicate entry '1' for key 'c1'
# restart the server.
drop table t;
drop table t1;
drop database tb;