Date: Wed, 23 Apr 2014 15:01:02 -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: <20140423180102.GP25695@eldon.alvh.no-ip.org>
References: <20140330040029.GY4582@tamriel.snowman.net>
 <20140330141135.GA4582@tamriel.snowman.net>
 <20140330185349.GD4582@tamriel.snowman.net>
 <20140331115452.GT9567@eldon.alvh.no-ip.org>
 <20140331115713.GA18358@alap3.anarazel.de>
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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.

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

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diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 9283b70..72602fd 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -5585,7 +5602,12 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
 	 */
 	xid = HeapTupleHeaderGetRawXmax(tuple);
 
-	if (tuple->t_infomask & HEAP_XMAX_IS_MULTI)
+	if ((tuple->t_infomask & HEAP_XMAX_IS_MULTI) &&
+		(tuple->t_infomask & HEAP_XMAX_INVALID))
+	{
+		freeze_xmax = true;
+	}
+	else if (tuple->t_infomask & HEAP_XMAX_IS_MULTI)
 	{
 		TransactionId newxmax;
 		uint16		flags;

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