On Tue, Jul 11, 2023 at 10:35:15AM +0800, suyu.cmj wrote:
> I want to report a bug about the recovery of two-phase transaction,
> 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 transaction to be
> added repeatedly, I have attached a patch for pg11 that reproduces
> the error:
> 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;
It looks like you have something here. I'll try to look at it. This
is a bug, so I have removed pgsql-hackers from the CC list keeping
only pgsql-bugs as cross-list posts are not encouraged.
--
Michael