Re: out-of-order XID insertion in KnownAssignedXids

On 05.10.2018 11:04, Michael Paquier wrote:
> On Fri, Oct 05, 2018 at 10:06:45AM +0300, Konstantin Knizhnik wrote:
>> As you can notice, XID 2004495308 is encountered twice which cause error in
>> KnownAssignedXidsAdd:
>>
>>     if (head > tail &&
>>         TransactionIdFollowsOrEquals(KnownAssignedXids[head - 1], from_xid))
>>     {
>>         KnownAssignedXidsDisplay(LOG);
>>         elog(ERROR, "out-of-order XID insertion in KnownAssignedXids");
>>     }
>>
>> The probability of this error is very small but it can quite easily
>> reproduced: you should just set breakpoint in debugger after calling
>> MarkAsPrepared in twophase.c and then try to prepare any transaction.
>> MarkAsPrepared  will add GXACT to proc array and at this moment there will
>> be two entries in procarray with the same XID:
>>
>> [snip]
>>
>> Now generated RUNNING_XACTS record contains duplicated XIDs.
> So, I have been doing exactly that, and if you trigger a manual
> checkpoint then things happen quite correctly if you let the first
> session finish:
> rmgr: Standby len (rec/tot): 58/ 58, tx: 0, lsn:
> 0/016150F8, prev 0/01615088, desc: RUNNING_XACTS nextXid 608
> latestCompletedXid 605 oldestRunningXid 606; 2 xacts: 607 606
>
> If you still maintain the debugger after calling MarkAsPrepared, then
> the manual checkpoint would block. Now if you actually keep the
> debugger, and wait for a checkpoint timeout to happen, then I can see
> the incorrect record. It is impressive that your customer has been able
> to see that first, and then that you have been able to get into that
> state with simple steps.
>
>> I want to ask opinion of community about the best way of fixing this
>> problem. Should we avoid storing duplicated XIDs in procarray (by
>> invalidating XID in original pgaxct) or eliminate/change check for
>> duplicate in KnownAssignedXidsAdd (for example just ignore
>> duplicates)?
> Hmmmmm... Please let me think through that first. It seems to me that
> the record should not be generated to begin with. At least I am able to
> confirm what you see.

The simplest way to fix the problem is to ignore duplicates before
adding them to KnownAssignedXids.
We in any case perform sort i this place...

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company