Re: StandbyRecoverPreparedTransactions recovers subtrans links incorrectly

>
> >>>> 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/
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