Date: Wed, 23 Apr 2014 14:28:04 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Andres Freund <andres@2ndquadrant.com>,
 Stephen Frost <sfrost@snowman.net>, pgsql-hackers@postgresql.org
Subject: Re: MultiXactId error after upgrade to 9.3.4
Message-ID: <20140423182804.GA21458@momjian.us>
References: <20140330040029.GY4582@tamriel.snowman.net>
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 <20140330185349.GD4582@tamriel.snowman.net>
 <20140331115452.GT9567@eldon.alvh.no-ip.org>
 <20140331115713.GA18358@alap3.anarazel.de>
 <20140423180102.GP25695@eldon.alvh.no-ip.org>
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On Wed, Apr 23, 2014 at 03:01:02PM -0300, Alvaro Herrera wrote:
> Andres Freund wrote:
> > On 2014-03-31 08:54:53 -0300, Alvaro Herrera wrote:
> > > My conclusion here is that some part of the code is failing to examine
> > > XMAX_INVALID before looking at the value stored in xmax itself.  There
> > > ought to be a short-circuit.  Fortunately, this bug should be pretty
> > > harmless.
> > > 
> > > .. and after looking, I'm fairly sure the bug is in
> > > heap_tuple_needs_freeze.
> > 
> > heap_tuple_needs_freeze() isn't *allowed* to look at
> > XMAX_INVALID. Otherwise it could miss freezing something still visible
> > on a standby or after an eventual crash.
> 
> I think what we should do here is that if we see that XMAX_INVALID is
> set, we just reset everything to zero without checking the multixact
> contents.  Something like the attached (warning: hand-edited, line
> numbers might be bogus)
> 
> I still don't know under what circumstances this situation could arise.
> This seems most strange to me.  I would wonder about this to be just
> papering over a different bug elsewhere, except that we know this tuple
> comes from a pg_upgraded table and so I think the only real solution is
> to cope.

Shouldn't we log something at least if we are unsure of the cause?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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