Re: [musl] Feature request: posix_spawnattr_setrlimit_np()

[Tavian Barnes <tavianator@tavianator.com>]

On Wed, Nov 15, 2023 at 5:38 PM Rich Felker <dalias@libc.org> wrote:
>
> On Wed, Nov 15, 2023 at 05:00:51PM -0500, Tavian Barnes wrote:
> > I have a program that raises its soft RLIMIT_NOFILE, but wants to
> > spawn processes with the original value (in case they use select(),
> > for example).  There seems to be no nice way to do this with
> > posix_spawn().  I can temporarily lower the rlimit in the parent, but
> > that interferes with other threads, and can make posix_spawn() fail
> > with EMFILE.
> >
> > Corresponding glibc feature request:
> > https://sourceware.org/bugzilla/show_bug.cgi?id=31049
>
> I don't have any objection to this as long as it's coordinated and
> there's agreement from other implementors, but there *is* a way to do
> it already. You posix_spawnp:
>
> sh -c 'ulimit -n whatever && exec "$0" "$@"' your_program args...

True!  Except that ulimit -n is not POSIX.  There are ways around
that, like making a dedicated shim binary that just does

setrlimit(RLIMIT_NOFILE, make_rlimit(argv[1]));
execv(argv[2], argv + 2);

(or making it a special mode of your_program).  But I think it would
be better to have a convenient interface for it.  And the
double-exec() is not free either.

I'm hoping if some libcs implement this, POSIX will reconsider
https://www.austingroupbugs.net/view.php?id=603.  Alternatively I
suppose POSIX could standardize ulimit -n, which is already existing
practice ~everywhere.

> This is the general solution to doing all sorts of "child process
> state setup" things that posix_spawn doesn't have a dedicated
> attribute for.
>
> Note that for a proposal for setting rlimits via an attribute, one
> complication that needs to be specified is whether the limits take
> place before or after file actions, since they could change the
> outcome of file actions. I'm not sure what the answer is, but just
> YOLO'ing an implementation without thinking about that is a bad idea.

True!  Actually when I implemented my own posix_spawn()-like
interface, I had setrlimit() as a file_action.  Then at least the
order is unambiguous.

> Rich

-- 
Tavian Barnes