Re: Clear padding in PgStat_HashKey keys

Hi,

On Mon, Nov 04, 2024 at 09:27:43AM +0100, Jelte Fennema-Nio wrote:
> On Mon, 4 Nov 2024 at 08:25, Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> > Perhaps it would be simpler to use a {0} like anywhere else for
> > PgStat_HashKey in pgstat_fetch_entry() and pgstat_drop_entry(), then
> > initialize the individual fields?
>
> {0} doesn't clear padding, it only sets all the fields to 0.

Thank you both to look at it!

>
> So in many places we use memset or MemSet to clear the padding already:
>
> rg 'memset.*key' -S | wc -l
> 31

Yeah, and 9fd45870c1 did not touch some of them (like the one I mentioned up-thread
in LOCALLOCKTAG localtag in LockHeldByMe()).

> And even using memset in this manner isn't a standards compliant way
> of handling this problem[0]. But it seems to have worked well enough
> in practice.
>
> Whether it's worth changing this now or only when we actually
> introduce padding is another question though.

I think it's worth to do it now:

1/ for example, it's done in LOCALLOCKTAG localtag in LockHeldByMe() while
LOCALLOCKTAG does not contain padding (see up-thread).

2/ the one that will add padding could miss this thread and spend some time
to figure out why his patch does break existing stats.

3/ when dealing with structs that are used as hash key I think we should not wait
for padding to be introduced.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com