Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae

Hi,

On 2024-04-15 12:35:59 -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
> of the code, I suspect that there is a residual bug, or at least that
> there was one prior to 6f47f6883151366c031cd6fd4011e66d2c702a90. (On
> the other thread, I cited 6dbb490261a6170a3fc3e326c6983ad63e795047,
> but that's not really what I meant.)

I assume the bug you suspect is that the horizon could go backwards during
pruning, which'd lead to a spurious "found xmin %u from before relfrozenxid
%u" error?


I think Matthias' observation of transaction aborts leading to the xmin
horizon going backward during transaction aborts needs to be fixed - it seems
quite problematic, regardless of whether it causes issues during VACUUM. Even
if it were perfectly safe today, it seems very likely to cause issues in the
future.


> But this code is too hairy for me to be certain whether there's a bug
> or not without some kind of a test case, and six weeks after the open
> item was added, we still don't have one that works on any v16 commit,
> either before or after the one named in the subject line.

Nor does any of the test cases fail on any of the other branches, right?
I.e. they're just testing a bug that's been fixed for ~3 years?

Greetings,

Andres Freund