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Date: Wed, 15 Nov 2023 19:29:56 -0500
From: Tavian Barnes <tavianator@...ianator.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Feature request: posix_spawnattr_setrlimit_np()
On Wed, Nov 15, 2023 at 5:38 PM Rich Felker <dalias@...c.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
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