insert ... select ... can read latest data even under RR level

[zyguan]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

/* init */ drop table if exists t1, t2;
/* init */ create table t1 (id int primary key, v int);
/* init */ create table t2 (id int primary key, v int);
/* init */ insert into t1 values (1, 10), (2, 20);

/* t1 */ set session transaction isolation level repeatable read;
/* t1 */ begin;
/* t1 */ insert into t2 select * from t1;
/* t2 */ update t1 set id = id + 2;
/* t1 */ insert into t2 select * from t1; -- succeed
/* t1 */ select * from t2; -- (1, 10), (2, 20), (3, 10), (4, 20)
/* t1 */ commit;

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

Either t2 should be blocked or the second insert of t1 failed with duplicated error.

3. What did you see instead (Required)

The second insert of t1 succeed.

4. What is your TiDB version? (Required)

release-4.0 (2862cd7)

[cfzjywxk]

We need to discuss the expected behaviours for the insert select statement first.