Re: Mark all GUC variable as PGDLLIMPORT

On Sun, Aug 22, 2021 at 09:29:01PM +0800, Julien Rouhaud wrote:
> On Sun, Aug 22, 2021 at 09:19:42AM -0400, Tom Lane wrote:
> > 
> > Uh, no, it's exactly *not* clear.  There are a lot of GUCs that are only
> > of interest to particular subsystems.  I do not see why being a GUC makes
> > something automatically more interesting than any other global variable.
> > Usually, the fact that one is global is only so the GUC machinery itself
> > can get at it, otherwise it'd be static in the owning module.
> > 
> > As for "extensions should be able to get at the values", the GUC machinery
> > already provides uniform mechanisms for doing that safely.  Direct access
> > to the variable's internal value would be unsafe in many cases.
> 
> Then shouldn't we try to prevent direct access on all platforms rather than
> only one?

So since the non currently explicitly exported GUC global variables shouldn't
be accessible by third-party code, I'm attaching a POC patch that does the
opposite of v1: enforce that restriction using a new pg_attribute_hidden()
macro, defined with GCC only, to start discussing that topic.

It would probably be better to have some other macro (e.g. PG_GLOBAL_PUBLIC and
PG_GLOBAL_PRIVATE or similar) to make declarations more consistent, but given
the amount of changes it would represent I prefer to have some feedback before
spending time on that.