Date: Mon, 31 Mar 2014 14:30:30 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Stephen Frost <sfrost@snowman.net>, pgsql-hackers@postgresql.org
Subject: Re: MultiXactId error after upgrade to 9.3.4
Message-ID: <20140331123030.GB18358@alap3.anarazel.de>
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 <20140331115452.GT9567@eldon.alvh.no-ip.org>
 <20140331115713.GA18358@alap3.anarazel.de>
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On 2014-03-31 09:19:12 -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.
> 
> Ah, you're right.  It even says so on the comment at the top (no
> caffeine yet.)  But what it's doing is still buggy, per this report, so
> we need to do *something* ...

Are you sure needs_freeze() is the problem here?

IIRC it already does some checks for allow_old? Why is the check for
that not sufficient?

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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