Date: Mon, 31 Mar 2014 10:02:15 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Stephen Frost <sfrost@snowman.net>, pgsql-hackers@postgresql.org
Subject: Re: MultiXactId error after upgrade to 9.3.4
Message-ID: <20140331130215.GV9567@eldon.alvh.no-ip.org>
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 <20140330185349.GD4582@tamriel.snowman.net>
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 <20140331115713.GA18358@alap3.anarazel.de>
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Andres Freund wrote:
> 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?

GetMultiXactIdMembers has this:

	if (MultiXactIdPrecedes(multi, oldestMXact))
	{
		ereport(allow_old ? DEBUG1 : ERROR,
				(errcode(ERRCODE_INTERNAL_ERROR),
		 errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
				multi)));
		return -1;
	}

	if (!MultiXactIdPrecedes(multi, nextMXact))
		ereport(ERROR,
				(errcode(ERRCODE_INTERNAL_ERROR),
				 errmsg("MultiXactId %u has not been created yet -- apparent wraparound",
						multi)));

I guess I wasn't expecting that too-old values would last longer than a
full wraparound cycle.  Maybe the right fix is just to have the second
check also conditional on allow_old.

Anyway, it's not clear to me why this database has a multixact value of
6 million when the next multixact value is barely above one million.
Stephen said a wraparound here is not likely.

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


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