On 2013-11-25 17:10:39 -0300, Alvaro Herrera wrote:
> > I am not sure whether that's the origin of the problem but at the very
> > least it seems to me that heap_lock_updated_tuple_rec() is missing
> > several important pieces:
> > a) do the priorXmax==xmin dance to check we're still following the same
> >    ctid chain. Currently we could easily stumble across completely
> >    unrelated tuples if a chain element aborted and got vacuumed.
> This seems simple to handle by adding the check you propose to the loop.
> Basically if the xmax doesn't match the xmin, we reached the end,
> there's nothing more to lock and we can return success without any
> further work:

Right, that's what we do in other places (notably EvalPlanQualFetch()).

> > b) Check whether a chain element actually aborted - currently we're
> >    only doing that in the HEAP_KEYS_UPDATED updated case, but that seems
> >    wrong (we can't check for committed tho!).
> Let me point out that this is exactly the same code that would be
> affected by my proposed fix for #8434, which would have this check the
> updateXid in all cases, not only in KEYS_UPDATED as currently.

Hm. I don't see a patch in any of the mails about #8434 although I seem
to remember talking with you about one. Maybe that was on IM?

> I don't understand your comment about "can't check for committed".  I
> mean, if the updating transaction committed, then we need to return
> failure to caller, HeapTupleUpdated.

I mean that in the !KEYS_UPDATED case we don't need to abort if we're
only acquiring a key share...


Andres Freund

 Andres Freund                     http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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