Unexpected error "DDL reorg element does not exist" when adding index

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(id int primary key, b int);
insert into t values (1, 1);
alter table t add unique index idx(b);

insert into t values (2, 1); -- write only
admin cancel ddl jobs X; -- the job must be rolling back because of the duplicated entry

It is not easy to reproduce with MySQL client because the state changes are usually fast.

2. What did you expect to see? (Required)

Cancelled DDL job

or

Duplicate entry '1' for key 't.idx'

3. What did you see instead (Required)

ERROR 8235 (HY000): DDL job rollback, error msg: DDL reorg element does not exist

4. What is your TiDB version? (Required)