Re: A design for amcheck heapam verification

On Mon, May 1, 2017 at 9:20 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> ISTM if you want to do that you have an inherent race condition.
> That is, no matter what you do, the moment after you look the currently
> oldest open transaction could commit, allowing some other session's
> view of RecentGlobalXmin to move past what you think it is, so that
> that session could start pruning stuff.

It can't prune the stuff we care about if we've got a shared content
lock on the target buffer. That's the trick pg_visibility uses:

/*
* Time has passed since we computed
OldestXmin, so it's
* possible that this tuple is
all-visible in reality even
* though it doesn't appear so based on our
* previously-computed value. Let's
compute a new value so we
* can be certain whether there is a problem.
*
* From a concurrency point of view,
it sort of sucks to
* retake ProcArrayLock here while
we're holding the buffer
* exclusively locked, but it should
be safe against
* deadlocks, because surely
GetOldestXmin() should never take
* a buffer lock. And this shouldn't
happen often, so it's
* worth being careful so as to avoid
false positives.
*/

--
Robert Haas
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