Re: Mark all GUC variable as PGDLLIMPORT

On Mon, 23 Aug 2021 at 22:15, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > On Sun, Aug 22, 2021 at 09:29:01PM +0800, Julien Rouhaud wrote:
> >> Then shouldn't we try to prevent direct access on all platforms rather
> than
> >> only one?
>
> > Agreed. If Julian says 99% of the non-export problems are GUCs, and we
> > can just export them all, why not do it? We already export every global
> > variable on Unix-like systems, and we have seen no downsides.
>
> By that argument, *every* globally-visible variable should be marked
> PGDLLIMPORT. But the mere fact that two backend .c files need to access
> some variable doesn't mean that we want any random bit of code doing so.
>
> 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.
>

There is: -fvisibility=hidden and __attribute__((visibility("default"))) .
Or if you prefer to explicitly mark private symbols, use
__attribute__((visiblity("hidden"))) .

In addition to API surface control this also gives you a smaller export
symbol table for faster dynamic linking, and it improves link-time
optimization.

I could've sworn I proposed its use in the past but I can't find a relevant
list thread except quite a recent one. All I can find is [1] . But that is
where we should start: switch from a linker script for libpq to using
PGDLLIMPORT (actually it'd be LIBPQDLLIMPORT) in libpq. When
-DBUILDING_LIBPQ this would expand to __declspec("dllexport") on Windows
and __attribute__((visibility("default"))) on gcc/clang. Otherwise it
expands to __declspec("dllimport") on Windows and empty on other targets.
This would also be a good time to rename the confusingly named BUILDING_DLL
macro to BUILDING_POSTGRES .

The next step would be to have PGDLLIMPORT expand to
__attribute__((visibility("default"))) on gcc/clang when building the
server itself. This won't do anything by itself since all symbols are
already default visibility. But once the "public API" of both function and
data symbol is so-annotated, we could switch to building Pg with
-fvisibility=hidden by default, and on Windows, we'd disable the linker
script that exports all functions using a generated .DEF file.

[1]
https://www.postgresql.org/message-id/CAMsr%2BYHa3TfA4rKtnZuzurwCSmxxXNQHFE3UE29BoDEQcwfuxA%40mail.gmail.com