Re: Mark all GUC variable as PGDLLIMPORT

But ... what is the actual goal and expected outcome of such a hypothetical public/private API separation?

It won't help meaningfully with server maintenance: We already break extensions freely in major releases, and sometimes even minor releases. We don't make any stable API promise at all. So any argument that it would make maintenance of the core server easier is weak at best.

It won't help protect server runtime stability: The server is written in C, and makes heavy use of non-opaque / non-hidden types. Many of which would not be practical to hide without enormous refactoring if at all. Writing any sort of "safe" C API is very difficult even when the exposed functionality is very narrow and well defined. Even then such an API can only help prevent inadvertent mistakes, since C programs are free to grovel around in memory. Look at the mess with EXPORT_SYMBOL_GPL in the kernel for just how ugly that can get. So I don't think there's any realistic way to claim that narrowing the exposed API surface would make it safer to load and run extensions that the user has not separately checked and reviewed or obtained from a trusted source with robust testing practices. Certainly it offers no benefit at all against a bad actor.

It won't make it safer to use untrusted extensions.

What will it do? Not much, in the short term, except cripple existing extensions or add a pile of questionably useful code annotations. The only real benefits I see are some improvement in link-time optimization and export symbol table size. Both of which are nice, but IMO not worth the pain by themselves for a pure C program. In C++, with its enormous symbol tables it's absolutely worth it. But not really for Pg.

To be clear, I actually love the idea of starting to define a solid public API, with API, ABI and semantic promises and associated tests. But to say it's a nontrivial amount of work is an enormous understatement. And unless done by an existing committer who is trusted to largely define a provisional API without bike-shedding and arguing the merits of every single part of it, it's nearly impossible to do with the way Pg is currently developed.

It's completely beyond me why it's OK to export all function symbols on Windows, but not all data symbols. Or why it's OK to export all data symbols on *nix, but not on Windows.