Re: Old row version in hot chain become visible after a freeze

On Mon, Sep 11, 2017 at 11:01 PM, Alvaro Herrera
<alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> (I also threw in a small sleep between heap_page_prune and
> HeapTupleSatisfiesVacuum while testing, just to widen the problem window
> to hopefully make any remaining problems more evident.)

I am understanding that you mean heap_prepare_freeze_tuple here
instead of heap_page_prune.

> This turned up a few different failure modes, which I fixed until no
> further problems arose. With the attached patch, I no longer see any
> failures (assertion failures) or misbehavior (additional rows), in a few
> dozen runs, which were easy to come by with the original code.

Well, you simply removed the assertion ;), and my tests don't show
additional rows as well, which is nice.

> The
> resulting patch, which I like better than the previously proposed idea
> of skipping the freeze, takes the approach of handling freeze correctly
> for the cases where the tuple still exists after pruning.

That's also something I was wondering when looking at the first patch.
I am unfortunately not as skilled as you are with this area of the
code (this thread has brought its quantity of study!), so I was not
able to draw a clear line with what needs to be done. But I am clearly
+1 with this approach.

> I also tweaked lazy_record_dead_tuple to fail with ERROR if the tuple
> cannot be recorded, as observed by Yi Wen. AFAICS that's not reachable
> because of the way the array is allocated, so an elog(ERROR) is
> sufficient.
>
> I regret my inability to turn the oneliner into a committable test case,
> but I think that's beyond what I can do for now.

Here are some comments about your last patch.

heap_tuple_needs_freeze looks to be still consistent with
heap_prepare_freeze_tuple even after what you have changed, which is
good.

Using again the test of Dan at the top of the thread, I am seeing from
time to time what looks like garbage data in xmax, like that:
ctid | xmin | xmax | id
-------+------+------+----
(0,1) | 620 | 0 | 1
(0,7) | 625 | 84 | 3
(2 rows)
[...]
ctid | xmin | xmax | id
-------+------+------+----
(0,1) | 656 | 0 | 1
(0,6) | 661 | 128 | 3
(2 rows)
Putting manual sleeps in lazy_scan_heap does not change the frequency
of their appearances.
--
Michael