Re: RLIMIT_NOFILE fallback

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 18, 2013, 19:50 UTC
Message-ID: <xmqq61qmhrb3.fsf@gitster.dls.corp.google.com>
In-Reply-To: <20131218191702.GA9083@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

> That is, does sysconf actually work on such a system (or does it need a
> similar run-time fallback)? And either way, we should try falling back
> to OPEN_MAX rather than 1 if we have it.

Interesting.

> As far as the warning, I am not sure I see a point. The user does not
> have any useful recourse, and git should continue to operate as normal.
> Having every single git invocation print "by the way, RLIMIT_NOFILE does
> not work on your system" seems like it would get annoying.

Very true.  That makes the resulting function look like this:

-------------------------------- 8< ------------------------------

static unsigned int get_max_fd_limit(void)
{
#ifdef RLIMIT_NOFILE
	struct rlimit lim;

	if (!getrlimit(RLIMIT_NOFILE, &lim))
		return lim.rlim_cur;
#endif

#if defined(_SC_OPEN_MAX)
	{
		long sc_open_max = sysconf(_SC_OPEN_MAX);
		if (0 < sc_open_max)
			return sc_open_max;
	}

#if defined(OPEN_MAX)
	return OPEN_MAX;
#else
	return 1; /* see the caller ;-) */
#endif
}

-------------------------------- >8 ------------------------------

But the sysconf part makes me wonder; here is what we see in http://pubs.opengroup.org/onlinepubs/9699919799/functions/sysconf.html

    If name is an invalid value, sysconf() shall return -1 and set errno
    to indicate the error. If the variable corresponding to name is
    described in <limits.h> as a maximum or minimum value and the
    variable has no limit, sysconf() shall return -1 without changing
    the value of errno. Note that indefinite limits do not imply
    infinite limits; see <limits.h>.

For a broken system (like RLIMIT_NOFILE defined for the compiler, but the actual call returns a bogus error), the compiler may see the _SC_OPEN_MAX defined, while sysconf() may say "I've never heard of such a name" and return -1, or the system, whether broken or not, may want to say "Unlimited" and return -1. The caller takes anything unreasonable as a positive value capped to 25 or something, so there isn't a real harm if we returned a bogus value from here, but I am not sure what the safe default behaviour of this function should be to help such a broken system while not harming systems that are functioning correctly.