How to produce “phantom read” in REPEATABLE READ? (MySQL)

The “phantom read” in MySQL on RR isolation level is hidden deep, but still can reproduce it. Here are the steps:

1. create table ab(a int primary key, b int);

2. Tx1:
   begin;
   select * from ab; // empty set

3. Tx2:
   begin;
   insert into ab values(1,1);
   commit;
4. Tx1:
   select * from ab; // empty set, expected phantom read missing.
   update ab set b = 2 where a = 1; // 1 row affected.
   select * from ab; // 1 row. phantom read here!!!!
   commit;