Re: use `proc->pgxactoff` as the value of `index` in `ProcArrayRemove()`

Hi,

On 2021-05-06 15:27:29 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > If you do, I think it might be worthwhile to add an only-with-assertions
> > loop checking that there's no other entry with the same pgprocno in the
> > dense arrays.
>
> Hmm, I can definitely see keeping a check that the selected entry
> has the right PID and/or pgprocno, but making it search for duplicates
> seems a bit over the top.  The existing code isn't guarding against
> that, and I don't really see a reason why there's a meaningful risk
> of it.

The current code makes it at least more likely for things to fall over
badly if there's such an issue, because there's a 50/50 chance that the
wrong entry would be moved.  I do dimly remember hitting a nasty bug or
two during the development of 941697c3c where such a thing happened, but
I don't remember the details.

Greetings,

Andres Freund