Re: Broken defenses against dropping a partitioning column

I wrote:
> So I think we're probably stuck with the approach of adding new internal
> dependencies. If we go that route, then our options for the released
> branches are (1) do nothing, or (2) back-patch the code that adds such
> dependencies, but without a catversion bump. That would mean that only
> tables created after the next minor releases would have protection against
> this problem. That's not ideal but maybe it's okay, considering that we
> haven't seen actual field reports of trouble of this kind.

Here's a proposed patch for that. It's mostly pretty straightforward,
except I had to add some recursion defenses in findDependentObjects that
weren't there before. But those seem like a good idea anyway to prevent
infinite recursion in case of bogus entries in pg_depend.

I also took the liberty of improving some related error messages that
I thought were unnecessarily vague and not up to project standards.

Per above, I'm envisioning applying this to HEAD and v12 with a catversion
bump, and to v11 and v10 with no catversion bump.

regards, tom lane