Re: pgsql: pg_logicalinspect: Fix possible crash when passing a directory p

On Thu, 13 Mar 2025 at 21:33, Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
> Ok, this is weird, because we have pg_unreachable() support for MSVC:
>
> #if defined(HAVE__BUILTIN_UNREACHABLE) && !defined(USE_ASSERT_CHECKING)
> #define pg_unreachable() __builtin_unreachable()
> #elif defined(_MSC_VER) && !defined(USE_ASSERT_CHECKING)
> #define pg_unreachable() __assume(0)
> #else
> #define pg_unreachable() abort()
> #endif
>
> Is there a way to reshuffle those conditionals to make this actually do
> something useful on MSVC?

I've just been experimenting with this and it seems the problem isn't
with pg_unreachable(), it's with the compiler not understanding that
the particular pg_unreachable() is always reached.

What's happening is down to the multi-eval protection code for elevel
in ereport_domain(). Because elevel is assigned to the variable
"elevel_" the compiler seems to lose its proof that the
pg_unreachable() is always reached. Adjusting that condition to use
the elevel parameter directly makes the warning disappear.

I looked around to see if MSVC might have something to allow us to fix
this, but didn't find anything. There does not seem to be any sort of
__builtin_constant_p with MSVC, otherwise we could've done something
similar to the HAVE__BUILTIN_CONSTANT_P version of ereport_domain just
above.

> Are you compiling with assertions on in this case? Does anything change
> about this if you don't use assertions (or vice versa)?

It happens with both.

David