Re: [HACKERS] StandbyRecoverPreparedTransactions recovers subtranslinks incorrectly

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От Nikhil Sontakke
Тема Re: [HACKERS] StandbyRecoverPreparedTransactions recovers subtranslinks incorrectly
Дата
Msg-id CAMGcDxde4XjDyTjGvZCPVQROpXw1opfpC0vjpCkzc1pcQBqvrg@mail.gmail.com
обсуждение исходный текст
Ответ на Re: [HACKERS] StandbyRecoverPreparedTransactions recovers subtranslinks incorrectly  (Simon Riggs <simon@2ndquadrant.com>)
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>>>> Also, when I fix that, it gets further but still crashes at the same
>>>> Assert in SubTransSetParent.  The proximate cause this time seems to be
>>>> that RecoverPreparedTransactions's calculation of overwriteOK is wrong:
>>>> it's computing that as "false", but in reality the subtrans link in
>>>> question has already been set.
>>
>>> Not sure about that, investigating.
>>
>> As a quick hack, I just hotwired RecoverPreparedTransactions to set
>> overwriteOK = true always, and with that and the SubTransSetParent
>> argument-order fix, HEAD passes the recovery tests.  Maybe we can
>> be smarter than that, but this might be a good short-term fix to get
>> the buildfarm green again.
>
> That would work. I've been looking into a fix I can explain, but "do
> it always" may actually be it.


Here's what's happening:

On Master:

BEGIN;

INSERT INTO t_009_tbl VALUES (42);

SAVEPOINT s1;

INSERT INTO t_009_tbl VALUES (43);

PREPARE TRANSACTION 'xact_009_1';

On Master:

Do a fast shutdown

On Slave:

Restart the slave. This causes StandbyRecoverPreparedTransactions() to be invoked which causes ProcessTwoPhaseBuffer() to be invoked with setParent set to true. This sets up parent xid for the above subtransaction.

On Slave:

Promote the slave. This causes RecoverPreparedTransactions() to be invoked which ends up calling SubTransSetParent() for the above subxid. The overwriteOK bool remains false since we don't go over the PGPROC_MAX_CACHED_SUBXIDS limit. Since the parent xid was already set on restart above, we hit the assert. 
 
-------------------

I am wondering if StandbyRecoverPreparedTransactions() needs the parent linkage at all? I don't see SubTransGetParent() and related functions being called on the standby but we need to be careful here. As a quick test, If I don't call SubTransSetParent() as part of StandbyRecoverPreparedTransactions()  then all recovery tests pass ok. But the fact that StandbyRecoverPreparedTransactions() takes overwriteOK as a parameter means it might not be so simple.. 

Regards,
Nikhils
--
 Nikhil Sontakke                   http://www.2ndQuadrant.com/
 PostgreSQL/Postgres-XL Development, 24x7 Support, Training & Services

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