Re: SSI freezing bug

Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> wrote:

> So I don't think you can ever get a false conflict because of
> slot reuse.

I spent some time looking at this, and I now agree.

> And if there's a hole in that thinking I can't see right now, the
> worst that will happen is some unnecessary conflicts, ie. it's
> still correct.

That is definitely true; no doubt about that part.

> Summary: IMHO we should just remove xmin from the predicate lock
> tag.

I spent some time trying to see how the vacuum could happen at a
point that could cause a false positive, and was unable to do so.
Even if that is just a failure of imagination on my part, the above
argument that it doesn't produce incorrect results still holds.  I
think the fact that I couldn't find a sequence of events which
would trigger a false positive suggests it would be a rare case,
anyway.

I found the original bug report which led to the addition of xmin
to the tag, and it was because we were still carrying tuple locks
forward to new versions of those locks at the time.  This was later
proven to be unnecessary, which simplified other code; we
apparently missed a trick in not removing xmin from the lock tag at
that point.  Since leaving it there has now been shown to *cause* a
bug, I'm inclined to agree that we should simply remove it, and
backpatch that.

Patch attached.  Any objections to applying that Real Soon Now?
(When, exactly is the deadline to make today's minor release
cut-off?)

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company