/* detach - detach the current process group from its controlling tty
 *
 * According to POSIX 3.2.2.2, orphaned process groups (becoming orphaned due to
 * the exit of its process group leader) that do have a controlling tty receive
 * a SIGHUP (or SIGCONT respectively).  'detach' can be used to circumvent the
 * signal sending.
 *
 *
 * Copyright (C) 2016 AVM Audiovisuelles Marketing und Computersysteme GmbH, nsc
 *
 * This program is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License as published by the Free Software
 * Foundation; either version 2 of the License, or (at your option) any later
 * version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 59 Temple
 * Place, Suite 330, Boston, MA  02111-1307  USA
 */

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/wait.h>


static void die(const char *format, ...)
	__attribute__((noreturn,format(printf,1,2)));

static void die(const char *format, ...)
{
	va_list ap;
	char *fmt;

	va_start(ap, format);
	asprintf(&fmt, "%s: %s%s\n",
		 program_invocation_short_name, format, errno ? ": %m" : "");
	vfprintf(stderr, fmt, ap);
	free(fmt);
	va_end(ap);
	exit(EXIT_FAILURE);
}


static int detach_controlling_tty(void)
{
	int fd;

	fd = open("/dev/tty", O_RDWR);

	if (fd < 0)
		return 0;

	if (ioctl(fd, TIOCNOTTY, 0))
		return -1;

	close(fd);

	return 0;
}


static int steal_controlling_tty(void)
{
	return ioctl(STDIN_FILENO, TIOCSCTTY, 1);
}


static int do_fork(int wait_for_child_exit)
{
	siginfo_t info;
	pid_t pid = fork();
	int ret;

	if (!pid)
		/* child */
		return 0;

	if (pid < 0)
		/* error, not forked */
		return errno;

	/* parent */

	ret = wait_for_child_exit ?
		waitid(P_PID, pid, &info, WEXITED), info.si_status : 0;

	_exit(ret);
}


int main(int argc, char **argv)
{
	if (sigignore(SIGTTOU))
		die("sigignore(SIGTTOU)");

	if (detach_controlling_tty())
		die("Unable to detach controlling tty");

	if (do_fork(1))
		die("fork");

	if (setsid() < 0)
		die("setsid");

	if (steal_controlling_tty())
		die("Unable to steal controlling tty");

	if (argc < 2)
		exit(0);

	if (do_fork(0))
		die("fork");

	execvp(argv[1], argv+1);
	die("Unable to exec %s", argv[1]);
}