Re: `pg_ctl init` crashes when run concurrently; semget(2) suspected

I wrote:
> This is from current macOS, but equivalent text appears on Linux and
> in the POSIX spec. So it's just luck that nobody has reported the
> same problem elsewhere --- unless maybe there is some macOS-specific
> behavior making it more likely that different installs would try the
> same key.

Hmm, no, there is a platform dependency here. I made the attached
test program to see what happens when there's a key collision,
and on Linux I get

semget(SEMAS_PER_SET) failed: File exists
semget(SEMAS_PER_SET + 1) failed: File exists

but macOS and NetBSD give

semget(SEMAS_PER_SET) failed: File exists
semget(SEMAS_PER_SET + 1) failed: Invalid argument

I didn't try other BSDen; this might be a NetBSD-ism that Apple
inherited, or maybe it's common among the BSDen. I don't see any
text in POSIX specifying which errno is to be returned in this case,
so we can't really argue that the behavior is wrong.

regards, tom lane