This is xnu-8019. See this file in:
#include <stdio.h>
#include <mach/mach.h>
#include <mach/message.h>
#include <unistd.h>
#include <assert.h>
#include <string.h>
#include <stdlib.h>
#include <bootstrap.h>
#include <fcntl.h>
#include <sys/errno.h>
#include <sys/resource.h>

static int
connect_to_server(void);

typedef struct {
	mach_msg_header_t   header;
	mach_msg_body_t     body;
	mach_msg_port_descriptor_t port_descriptor;
	mach_msg_trailer_t  trailer;            // subtract this when sending
} ipc_complex_message;

static ipc_complex_message icm_request = {};

struct args {
	const char *progname;
	int verbose;
	int voucher;
	int num_msgs;
	const char *server_port_name;
	mach_port_t server_port;
	mach_port_t reply_port;
	int request_msg_size;
	void *request_msg;
	int reply_msg_size;
	void *reply_msg;
	uint32_t persona_id;
	long client_pid;
};

static void
parse_args(struct args *args)
{
	args->verbose = 0;
	args->voucher = 0;
	args->server_port_name = "TEST_FD_TABLE_LIMITS";
	args->server_port = MACH_PORT_NULL;
	args->reply_port = MACH_PORT_NULL;
	args->num_msgs = 1;
	args->request_msg_size = sizeof(ipc_complex_message) - sizeof(mach_msg_trailer_t);
	args->reply_msg_size = sizeof(ipc_complex_message) - sizeof(mach_msg_trailer_t);
	args->request_msg = &icm_request;
	args->reply_msg = NULL;
	args->client_pid = getpid();
}

static int
connect_to_server(void)
{
	struct args client_args = {};
	parse_args(&client_args);

	/* Find the bootstrap port */
	mach_port_t bsport;
	kern_return_t ret = task_get_bootstrap_port(mach_task_self(), &bsport);
	if (ret) {
		mach_error("client: task_get_bootstrap_port()", ret);
		exit(1);
	}

	printf("client: Look up bootstrap service port\n");
	ret = bootstrap_look_up(bsport, client_args.server_port_name,
	    &client_args.server_port);
	if (ret) {
		mach_error("client: bootstrap_look_up()", ret);
		exit(1);
	}

	printf("client: Set service port as the resource notify port\n");
	ret = task_set_special_port(mach_task_self(), TASK_RESOURCE_NOTIFY_PORT, client_args.server_port);
	if (ret) {
		mach_error("client: task_set_special_port()", ret);
		exit(1);
	}

	return 0;
}

int
main(int argc, char *argv[])
{
	int fd = 0;
	int soft_limit = 0;
	int hard_limit = 0;
	int test_num = 0;
	int ret;
	if (argc == 4) {
		soft_limit = atoi(argv[1]);
		hard_limit = atoi(argv[2]);
		test_num = atoi(argv[3]);
	} else {
		printf("Usage: ./fd_table_limits_client <soft limit> <hard limit> <test_num>\n");
		goto fail_and_exit;
	}

	printf("client: soft limit = %d, hard limit = %d, test_num = %d\n", soft_limit, hard_limit, test_num);
	if (test_num == 2) {
		ret = connect_to_server();
		if (ret) {
			goto fail_and_exit;
		}
	}

	struct rlimit rl;

	ret = getrlimit(RLIMIT_NOFILE, &rl);
	if (ret < 0) {
		printf("client: getrlimit failed with err = %d\n", errno);
		goto fail_and_exit;
	}

	int new_rlim_max = 2000;
	if (rl.rlim_cur < new_rlim_max) {
		printf("client: raising file open limit from %llu (max %llu) to %d\n",
		    rl.rlim_cur, rl.rlim_max, new_rlim_max);
		rl.rlim_cur = new_rlim_max;
		rl.rlim_max = new_rlim_max;
		ret = setrlimit(RLIMIT_NOFILE, &rl);
		if (ret < 0) {
			printf("client: setrlimit failed with err = %d\n", errno);
			goto fail_and_exit;
		}
	}


	printf("client: Starting the fd allocation loop\n");
	int i = 0;
	while (i < new_rlim_max) {
		fd = open("/bin/ls", O_RDONLY | 0x08000000 /* O_CLOFORK */);
		if (fd < 0) {
			printf("open(/bin/ls) FD #%d failed with err = %d ", i, errno);
			goto fail_and_exit;
		}

		if ((i % 20) == 0) {
			/* Print every port in the multiple of 20 */
			printf("client: FD #%d\n", i);
			sleep(1);
		}

		if (i == soft_limit && !hard_limit) {
			printf("client: Hit the soft limit \n");
			exit(0);
		}

		if (i == hard_limit) {
			printf("client: Hit the hard limit\n");
		}

		i++;
	}

fail_and_exit:
	exit(91);
}