可重复读隔离级别下，两个事务的写出现了丢失更新异常

<!-- #请认真填写以下信息，否则可能由于无法定位，导致issue无法解决而被取消 --> 【标题描述】: 【测试类型：SQL功能/并发】【测试版本：openGauss 6.0.0 build aee4abd5】 问题描述 当我在OpenGauss的可重复读隔离级别下中并发执行两个事务时，两个事务的写出现丢失更新的异常现象，符合R1[x0]W2[x1]C2W1[x2]C1的丢失更新模式，同时我测试了Postgres，PG没出现这个情况，测试用例如下。 ```sql CREATE TABLE t0(c0 INT primary key); INSERT INTO t0 VALUES (1); INSERT INTO t0 VALUES (-1); /* T1 */ SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; /* T2 */ SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; /* T1 */ BEGIN; /* T2 */ BEGIN; /* T1 */ SELECT * FROM t0; /* T2 */ SELECT * FROM t0; /* T1 */ UPDATE t0 SET c0 = 2 where c0 = 1; /* T1 */ SELECT * FROM t0; /* T1 */ COMMIT; /* T2 */ Delete FROM t0; /* T2 */ SELECT * FROM t0; /* T2 */ COMMIT; ``` 【操作系统和硬件信息】(查询命令: cat /etc/system-release, uname -a): Linux ubuntu 5.15.0-130-generic #140-Ubuntu SMP Wed Dec 18 17:59:53 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux 【测试环境】(单机/1主x备x级联备): 单机docker部署 【被测功能】: 事务和隔离级别 【测试类型】: 客户端 【数据库版本】(查询命令: gaussdb -V): ``` reproduce=> select version(); version ------------------------------------------------------------------------------------------------------------------------------------------------------- (openGauss 6.0.0 build aee4abd5) compiled at 2024-09-29 19:14:27 commit 0 last mr on x86_64-unknown-linux-gnu, compiled by g++ (GCC) 10.3.0, 64-bit (1 row) ``` 【预置条件】: 无 【操作步骤】(请填写详细的操作步骤): ```sql CREATE TABLE t0(c0 INT primary key); INSERT INTO t0 VALUES (1); INSERT INTO t0 VALUES (-1); /* T1 */ SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; /* T2 */ SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; /* T1 */ BEGIN; /* T2 */ BEGIN; /* T1 */ SELECT * FROM t0; c0 ---- 1 -1 (2 rows) /* T2 */ SELECT * FROM t0; c0 ---- 1 -1 (2 rows) /* T1 */ UPDATE t0 SET c0 = 2 where c0 = 1; UPDATE 1 /* T1 */ SELECT * FROM t0; c0 ---- -1 2 (2 rows) /* T1 */ COMMIT; /* T2 */ Delete FROM t0; DELETE 2 /* T2 */ SELECT …