Re: BUG #15367: Crash in pg_fe_scram_free when using foreign tables

I wrote:
> On the other front of developing a test case to detect this problem,
> I did not have much luck with mechanizing this specific test: it
> requires some functionality we have in the TAP tests, like setting
> up an installation with SCRAM password authentication enabled, as
> well as other functionality we have in the isolation tester, like
> doing things in two different sessions concurrently. But we don't
> really need to test this *exact* scenario; we just need something
> that will behave differently if libpq links to backend versions of
> any of the problematic functions. I'm a bit tempted to add
> something like this to saslprep.c:

> bool
> saslprep_is_frontend()
> {
> #ifdef FRONTEND
> return true;
> #else
> return false;
> #endif
> }

On reflection, saslprep.c is not the right place for this: if some
other shlib needs a similar test, it might be forced to import SASL
support it has no need for. So I put the test function into its
own new .c file, which increases the footprint of the patch a bit
but seems much less ad-hoc.

I verified that this patch causes a failure in the postgres_fdw
regression test on affected platforms, even my FreeBSD box where
the original bug doesn't cause an easily-detectable failure.

I propose this for HEAD only; the main point is just to detect
which platforms have the issue, and HEAD seems like enough for that.

regards, tom lane