From: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
To: Andres Freund <andres@anarazel.de>
To: pgsql-hackers@lists.postgresql.org, 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 18:28:52 +0300
Message-ID: <fc51532c-dbf5-dce0-b31c-82c7a0b837ed@postgrespro.ru> (raw)
In-Reply-To: <2A819787-9E67-41EF-B89A-A906996924E5@anarazel.de>
References: <0c96b653-4696-d4b4-6b5d-78143175d113@postgrespro.ru>
<20181005080422.GA7863@paquier.xyz>
<581a80f7-3992-f319-302d-7c9a384d64f7@postgrespro.ru>
<2A819787-9E67-41EF-B89A-A906996924E5@anarazel.de>
On 08.10.2018 18:24, Andres Freund wrote:
>
> On October 8, 2018 2:04:28 AM PDT, Konstantin Knizhnik <k.knizhnik@postgrespro.ru> wrote:
>>
>> 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...
> I vehemently object to that as the proper course.
And what about adding qsort to GetRunningTransactionData or
LogCurrentRunningXacts and excluding duplicates here?
> Andres
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
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To: pgsql-hackers@postgresql.org
Cc: k.knizhnik@postgrespro.ru, andres@anarazel.de, michael@paquier.xyz
Subject: Re: out-of-order XID insertion in KnownAssignedXids
In-Reply-To: <fc51532c-dbf5-dce0-b31c-82c7a0b837ed@postgrespro.ru>
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