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
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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