From: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
To: Michael Paquier <michael@paquier.xyz>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: out-of-order XID insertion in KnownAssignedXids
Date: Mon, 8 Oct 2018 12:04:28 +0300
Message-ID: <581a80f7-3992-f319-302d-7c9a384d64f7@postgrespro.ru> (raw)
In-Reply-To: <20181005080422.GA7863@paquier.xyz>
References: <0c96b653-4696-d4b4-6b5d-78143175d113@postgrespro.ru>
<20181005080422.GA7863@paquier.xyz>
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
Attachments:runnings_xids.patch (text/x-patch, 656B) downloaddiff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.cindex bd20497..a2ea233 100644--- a/src/backend/storage/ipc/procarray.c+++ b/src/backend/storage/ipc/procarray.c@@ -807,9 +807,12 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
/*
* Add the sorted snapshot into KnownAssignedXids
*/
- for (i = 0; i < nxids; i++)- KnownAssignedXidsAdd(xids[i], xids[i], true);-+ KnownAssignedXidsAdd(xids[0], xids[0], true);+ for (i = 1; i < nxids; i++)+ {+ if (xids[i] != xids[i-1])+ KnownAssignedXidsAdd(xids[i], xids[i], true);+ }
KnownAssignedXidsDisplay(trace_recovery(DEBUG3));
}
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To: pgsql-hackers@postgresql.org
Cc: k.knizhnik@postgrespro.ru, michael@paquier.xyz
Subject: Re: out-of-order XID insertion in KnownAssignedXids
In-Reply-To: <581a80f7-3992-f319-302d-7c9a384d64f7@postgrespro.ru>
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