Re: MultiXactId error after upgrade to 9.3.4

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 this rule is wrong. I think the rule ought to be something like
"if the XMAX_INVALID bit is set, then reset whatever is there if there
is something; if the bit is not set, proceed as today". Otherwise we
risk reading garbage, which is what is happening in this case.

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