BUG #17301: SELECT gets weird result while two transactions are submitted concurrently

The following bug has been logged on the website:

Bug reference:      17301
Logged by:          Dinary Dai
Email address:      dddinary@163.com
PostgreSQL version: 14.0
Operating system:   Ubuntu
Description:

/* init */ create table t(a int primary key, b int);
/* init */ insert into t values (1, 2), (2, 3)

/* t1 */ begin;
/* t1 */ set transaction isolation level repeatable read;
/* t1 */ select * from t where a = 1;
/* t2 */ begin;
/* t2 */ set transaction isolation level repeatable read;
/* t2 */ delete from t where a = 2;
/* t2 */ commit;
/* t1 */ update t set a = 2 where a = 1;
/* t1 */ select * from t where a = 2;  -- [(2, 3), (2, 2)]
/* t1 */ commit;

The final SELECT statement gets result [(2, 3), (2, 2)], which violates the
primary key constraint on column `a`.