回复：The same 2PC data maybe recovered twice

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发件人：蔡梦娟(玊于) <mengjuan.cmj@alibaba-inc.com>

发送时间：2023年7月6日(星期四) 10:02

收件人：pgsql-hackers <pgsql-hackers@postgresql.org>

抄　送：pgsql-bugs <pgsql-bugs@postgresql.org>

主　题：The same 2PC data maybe recovered twice

Hi, all. I want to report a bug about recovery of 2pc data, in current implementation of crash recovery, there are two ways to recover 2pc data:

1、before redo, func restoreTwoPhaseData() will restore 2pc data those xid < ShmemVariableCache->nextXid, which is initialized from checkPoint.nextXid;

2、during redo, func xact_redo() will add 2pc from wal;

The following scenario may cause the same 2pc to be added repeatedly：

1、start creating checkpoint_1, checkpoint_1.redo is set as curInsert;

2、before set checkPoint_1.nextXid, a new 2pc is prepared, suppose the xid of this 2pc is 100, and then ShmemVariableCache->nextXid will be advanced as 101;

3、checkPoint_1.nextXid is set as 101;

4、in CheckPointTwoPhase() of checkpoint_1, 2pc_100 won't be copied to disk because its prepare_end_lsn > checkpoint_1.redo;

5、checkPoint_1 is finished, after checkpoint_timeout, start creating checkpoint_2;

6、during checkpoint_2, data of 2pc_100 will be copied to disk;

7、before UpdateControlFile() of checkpoint_2, crash happened;

8、during crash recovery, redo will start from checkpoint_1, and 2pc_100 will be restored first by restoreTwoPhaseData() because xid_100 < checkPoint_1.nextXid, which is 101; 

9、because prepare_start_lsn of 2pc_100 > checkpoint_1.redo, 2pc_100 will be added again by xact_redo() during wal replay, resulting in the same 2pc data being added twice;

10、In RecoverPreparedTransactions() -> lock_twophase_recover(), lock the same 2pc will cause panic.

Is the above scenario reasonable, and do you have any good ideas for fixing this bug?

Thanks & Best Regard