Re: Mark all GUC variable as PGDLLIMPORT

On Mon, Aug 23, 2021 at 10:56:52AM -0400, Robert Haas wrote:
> On Mon, Aug 23, 2021 at 10:15 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > And yes, I absolutely would prohibit extensions from accessing many
> > of them, if there were a reasonable way to do it.  It would be a good
> > start towards establishing a defined API for extensions.
> 
> Mostly, it would make extension development more difficult for no
> discernable benefit to the project.
> 
> You've made this argument many times over the years ... but we're no
> closer to having an extension API than we ever were, and we continue
> to get complaints about breaking stuff on Windows on a pretty regular
> basis.
> 
> Honestly, it seems unimaginable that an API is ever really going to be
> possible. It would be a ton of work to maintain, and we'd just end up
> breaking it every time we discover that there's a new feature we want
> to implement which doesn't fit into the defined API now. That's what
> we do *now* with functions that third-party extensions actually call,
> and with variables that they access, and it's not something that, in
> my experience, is any great problem in maintaining an extension.
> You're running code that is running inside somebody else's executable
> and sometimes you have to adjust it for upstream changes. That's life,
> and I don't think that complaints about that topic are nearly as
> frequent as complaints about extensions breaking on Windows because of
> missing PGDLLIMPORT markings.
> 
> And more than that, I'm pretty sure that you've previously taken the
> view that we shouldn't document all the hook functions that only exist
> in the backend for the purpose of extension use.

As the person on the receiving end of that one, I was nonplussed, so I
took a step back to think it over.  I recognized at that time that I
didn't have a great answer for a legitimate concern, namely that any
change, however trivial, that goes into the core and doesn't go
directly to core database functionality, represents a long-term
maintenance burden.

The thing I'm coming to is that the key architectural feature
PostgreSQL has that other RDBMSs don't is its extensibility. Because
that's been a stable feature over time, I'm pretty sure we actually
need to see documenting that as a thing that does actually go to core
database functionality. Yes, there are resources involved with doing a
thing like this, but I don't think that they require constant or even
frequent attention from committers or even from seasoned DB hackers.

Best,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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