Re: [RFC] Removing "magic" oids

On Tue, Nov 20, 2018 at 01:20:04AM -0800, Andres Freund wrote:
> On 2018-11-14 21:02:41 -0800, Andres Freund wrote:
> > On 2018-11-15 04:57:28 +0000, Noah Misch wrote:
> > > On Wed, Nov 14, 2018 at 12:01:52AM -0800, Andres Freund wrote:
> > > > - one pgbench test tested concurrent insertions into a table with
> > > >   oids, as some sort of stress test for lwlocks and spinlocks. I *think*
> > > >   this doesn't really have to be a system oid column, and this was just
> > > >   because that's how we triggered a bug on some machine. Noah, do I get
> > > >   this right?
> > > 
> > > The point of the test is to exercise OidGenLock by issuing many parallel
> > > GetNewOidWithIndex() and verifying absence of duplicates.  There's nothing
> > > special about OidGenLock, but it is important to use an operation that takes a
> > > particular LWLock many times, quickly.  If the test query spends too much time
> > > on things other than taking locks, it will catch locking races too rarely.
> > 
> > Sequences ought to do that, too. And if it's borked, we'd hopefully see
> > unique violations. But it's definitely not a 1:1 replacement.

> I've tested this on ppc.  Neither the old version nor the new version
> stress test spinlocks sufficiently to error out with weakened spinlocks
> (not that surprising, there are no spinlocks in any hot path of either
> workload). Both versions very reliably trigger on weakened lwlocks. So I
> think we're comparatively good on that front.

I tested this on xlc, the compiler that motivated the OID test, and the v12+
version of the test didn't catch the bug[1] with xlc 13.1.3.  CREATE TYPE
... AS ENUM generates an OID for each label, so the attached patch makes the
v12+ test have locking behavior similar to its v11 ancestor.

[1] https://postgr.es/m/flat/a72cfcb0-37d0-de2f-b3ec-f38ad8d6a8cc@postgrespro.ru