Re: [HACKERS] pgsql: Fix freezing of a dead HOT-updated tuple

Peter Geoghegan wrote:
> Andres Freund <andres(at)anarazel(dot)de> wrote:

> > Staring at the vacuumlazy hunk I think I might have found a related bug:
> > heap_update_tuple() just copies the old xmax to the new tuple's xmax if
> > a multixact and still running. It does so without verifying liveliness
> > of members. Isn't that buggy? Consider what happens if we have three
> > blocks: 1 has free space, two is being vacuumed and is locked, three is
> > full and has a tuple that's key share locked by a live tuple and is
> > updated by a dead xmax from before the xmin horizon. In that case afaict
> > the multi will be copied from the third page to the first one. Which is
> > quite bad, because vacuum already processed it, and we'll set
> > relfrozenxid accordingly. I hope I'm missing something here?
>
> Can you be more specific about what you mean here? I think that I
> understand where you're going with this, but I'm not sure.

He means that the tuple that heap_update moves to page 1 (which will no
longer be processed by vacuum) will contain a multixact that's older
than relminmxid -- because it is copied unchanged by heap_update instead
of properly checking against age limit.

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