Re: POC: make mxidoff 64 bits

On 27/12/2024 19:09, Maxim Orlov wrote:
>
> On Wed, 18 Dec 2024 at 13:21, Heikki Linnakangas <hlinnaka(at)iki(dot)fi
> <mailto:hlinnaka(at)iki(dot)fi>> wrote:
> Does the pg_upgrade code work though, if you have that buggy situation
> where oldestOffsetKnown == false ?

...

> >
> >               if (!TransactionIdIsValid(*xactptr))
> >               {
> >                       /* Corner case 3: we must be looking at
> unused slot zero */
> >                       Assert(offset == 0);
> >                       continue;
> >               }
>
> After upgrade, this corner case 3 would *not* happen on offset == 0. So
> looks like we're still missing test coverage for this upgrade corner
> case.
>
> Am I understanding correctly that you want to have a test corresponding
> to the buggy 9.3 and 9.4 era versions?

No, those were two different things. I think there might be two things
wrong here:

1. I suspect pg_upgrade might not correctly handle the situation where
oldestOffsetKnown==false, and

2. The above assertion in "corner case 3" would not hold. It seems that
we don't have a test case for it, or it would've hit the assertion.

Now that I think about it, yes, a test case for 1. would be good too.
But I was talking about 2.

> Do you think we could imitate this scenario on a current master branch
> like that:
> 1) generate a couple of offsets segments for the first table;
> 2) generate more segments for a second table;
> 3) drop first table;
> 4) stop pg cluster;
> 5) remove pg_multixact/offsets/0000
> 6) upgrade?

I don't remember off the top of my head.

It might be best to just refuse the upgrade if oldestOffsetKnown==false.
It's a very ancient corner case. It seems reasonable to require you to
upgrade to a newer minor version and run VACUUM before upgrading. IIRC
that sets oldestOffsetKnown.

--
Heikki Linnakangas
Neon (https://neon.tech)