#include <stdio.h>
#include <stdlib.h>
#include <err.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

/**
 * NOTE: requires fdescfs to be mounted to run interpreted files (starting with #!)
 */

#define DEFAULT_FD 3
#define BIN_HDR_LEN 2

char *args[] = { "batchrun", NULL };

void
set_async(int fd, int on)
{
	int flags;

	if ((flags = fcntl(fd, F_GETFL)) == -1)
		err(1, "fcntl()");

	if (on) {
		flags |= O_NONBLOCK;
	} else {
		flags &= ~O_NONBLOCK;
	}

	if (fcntl(fd, F_SETFL, flags) == -1)
		err(1, "fcntl()");
}

void
set_cloexec(int fd, int on)
{
	if (fcntl(fd, F_SETFD, on ? FD_CLOEXEC : 0) == -1)
		err(1, "fcntl()");
}

int
main(int argc, char **argv)
{
	char buf[BIN_HDR_LEN], *end = NULL;
	ssize_t ret;
	int tries = 0, fd = DEFAULT_FD;

	if (argc > 1) {
		fd = strtol(argv[1], &end, 10);

		if (end == argv[1])
			errx(1, "Filedescriptor supplied (\"%s\") is not a number", argv[1]);
	}

	set_cloexec(fd, 1);
	set_async(fd, 1);

retry:
	if (tries++ >= 3)
		errx(1, "Failed to read file header");

	ret = pread(fd, buf, sizeof(buf), 0);
	if (ret == sizeof(buf)) {
		if (strncmp(buf, "#!", sizeof(buf)) == 0) {
			set_cloexec(fd, 0);
		}
	} else if (ret == -1) {
		if (errno == EINTR)
			goto retry;
		else if (errno != EAGAIN)
			err(1, "pread()");
	} else if (ret != 0) {
		/**
		 * We take a lazy approach here and only want to know if the
		 * file has more contents (and don't care about errors).
		 */
		ret = pread(fd, buf + ret, sizeof(buf) - ret, ret);
		if (ret != 0) {
			goto retry;
		}
	}

	set_async(fd, 0);

	fexecve(fd, args, NULL);
	err(1, "fexecve()");
}