Fixing posix_spawn_file_actions_addopen with O_CLOEXEC

(continuation from cygwin-patches thread)

On Tue, 8 Jul 2025, Corinna Vinschen wrote:

> On Jul  7 14:43, Jeremy Drake via Cygwin-patches wrote:
> > I noticed a bit that you quoted that I hadn't noticed before
> > additionally, when the new process image is executed, any
> >   file descriptor (from this new set) which has its FD_CLOEXEC flag set
> >   shall be closed (see posix_spawn()).
> >
> > The "from this new set" is not handled properly in my implementation, and
> > I'm pretty sure not from the existing newlib implementation: I copied what
> > it was doing, which was to clear the FD_SETFD flag after open.  That
> > would be wrong, if faced with an addopen with the O_CLOEXEC flag set.
> > This is obviously a stupid thing for a caller to do...
>
> the POSIX posix_spawn man page has a 4 steps list how to process
> the file actions with step 4: close all FD_CLOEXEC descriptors.
>
> I wonder if that doesn't make some sense, e.g.
>
>   addopen (&fact, 42, "somedir", O_CLOEXEC);
>   addfchdir (42);
>
> but then again, you could just add an addclose(42) and you would have the
> same effect.
>
> Well, *shrug*, as long as we can do it right.
>
> But yeah, I don't see anywhere in the POSIX docs that the addopen
> descriptors have the FD_CLOEXEC flag removed automatically.  They
> are supposed to be closed instead, FWIW.
>
> A fix to newlib's posix_spawn might make sense.

OK.  I'm not sure about what functions are allowed to  be used there,
though (fcntl is in a preprocessor check, so I guess not all posix funcs
can be assumed to be present).  Can I use dup3 or do I need to fcntl
F_SETFD after the dup2?  Should I use the O_CLOEXEC flag to determine this
or use F_GETFD on the open-ed fd to F_SETFD on the dup2-ed one?

Also, I noticed a new spawnattr flag for calling setsid () in the child, I
could do a patch to add that, but is that function allowed to be used or
does it need a preprocessor check?