Re: HeapTupleHeaderAdvanceLatestRemovedXid doing the wrong thing with multixacts

On Mon, Oct 17, 2011 at 8:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
>> I just noticed that HeapTupleHeaderAdvanceLatestRemovedXid is comparing Xmax as a TransactionId without verifying whether it is a multixact or not.  Since they advance separately, this could lead to bogus answers.  This probably needs to be fixed.  I didn't look into past releases to see if there's a live released bug here or not.
>
>> I think the fix is simply to ignore the Xmax if the HEAP_XMAX_IS_MULTI bit is set.
>
>> Additionally I think it should check HEAP_XMAX_INVALID before reading the Xmax at all.
>
> If it's failing to even check XMAX_INVALID, surely it's completely
> broken?  Perhaps it assumes its caller has checked all this?

HeapTupleHeaderAdvanceLatestRemovedXid() is only ever called when
HeapTupleSatisfiesVacuum() returns HEAPTUPLE_DEAD, which only happens
when HEAP_XMAX_IS_MULTI is not set.

I'll add an assert to check this and a comment to explain.

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