Hi, all.
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;
10、In RecoverPreparedTransactions() -> lock_twophase_recover(), lock the same 2pc will cause FATAL.
After running the patch that reproduced the error, you will get the following error during crash recovery:
2023-07-10 13:04:30.670 UTC [11169] LOG: recovering prepared transaction 569 from shared memory2023-07-10 13:04:30.670 UTC [11169] LOG: recovering prepared transaction 569 from shared memory
2023-07-10 13:04:30.670 UTC [11169] FATAL: lock ExclusiveLock on object 569/0/0 is already held
2023-07-10 13:04:30.670 UTC [11168] LOG: startup process (PID 11169) exited with exit code 1
2023-07-10 13:04:30.670 UTC [11168] LOG: aborting startup due to startup process failure
I also added a patch for pg11 to fix this problem, hope you can check it when you have time.
Thanks & Best Regard