Re: MultiXactId error after upgrade to 9.3.4

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.
>
> 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.

Andres asks on IM: How come there is garbage there in the first place?
I have to admit I have no idea.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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