Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae

Hi,

On 2024-04-15 13:52:04 -0700, Noah Misch wrote:
> On Mon, Apr 15, 2024 at 12:35:59PM -0400, Robert Haas wrote:
> > I propose to remove this open item from
> > https://wiki.postgresql.org/wiki/PostgreSQL_17_Open_Items
> >
> > On the original thread (BUG #17257), Alexander Lakhin says that he
> > can't reproduce this after dad1539ae/18b87b201. Based on my analysis
>
> I have observed the infinite loop in production with v15.5, so that
> non-reproduce outcome is a limitation in the test procedure. (v14.2 added
> those two commits.)

How closely have you analyzed those production occurences? It's not too hard
to imagine some form of corruption that leads to such a loop, but which isn't
related to the horizon going backwards? E.g. a corrupted HOT chain can lead
to heap_page_prune() not acting on a DEAD tuple, but lazy_scan_prune() would
then encounter a DEAD tuple.

> > of the code, I suspect that there is a residual bug, or at least that
> > there was one prior to 6f47f6883151366c031cd6fd4011e66d2c702a90.
>
> Can you say more about how 6f47f6883151366c031cd6fd4011e66d2c702a90 mitigated
> the regression that 1ccc1e05ae introduced? Thanks for discovering that.

Which regression has 1ccc1e05ae actually introduced? As I pointed out
upthread, the proposed path to corruption doesn't seem to actually lead to
corruption, "just" an error? Which actually seems considerably better than an
endless retry loop that cannot be cancelled.

Greetings,

Andres Freund