#define _GNU_SOURCE

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

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/wait.h>
#include <sys/epoll.h>
#include <sys/signalfd.h>

/* TTY data */
typedef struct tty_t {
	char         *path;
	int           fd;
	int           pid;
	struct tty_t *next;
} tty_t;

/* Local Data */
static int running;
static int epoll;
static int sigs;
static tty_t *ttys;

/* Helper functions */
void error(const char *fmt, ...)
{
	va_list ap;
	va_start(ap, fmt);
	fprintf(stderr, "Error ");
	vfprintf(stderr, fmt, ap);
	fprintf(stderr, ": %s\n", strerror(errno));
	va_end(ap);
	if (!running)
		exit(1);
}

int add_poll(int fd, void *ptr)
{
	struct epoll_event ctl = {
		.events = EPOLLIN,
		.data.ptr = ptr,
	};
	return epoll_ctl(epoll, EPOLL_CTL_ADD, fd, &ctl);
}

int del_poll(int fd)
{
	return epoll_ctl(epoll, EPOLL_CTL_DEL, fd, NULL);
}

static int start_tty(tty_t *tty)
{
	const char *prompt = "[Press enter to login]";
	struct termios attr;

	if (chown(tty->path, 0, 0) < 0)
		return -1;
	if (chmod(tty->path, 0600) < 0)
		return -1;
	if ((tty->fd = open(tty->path, O_RDWR|O_NOCTTY|O_NONBLOCK|O_CLOEXEC, 0)) < 0)
		return -1;
	if (add_poll(tty->fd, tty) < 0)
		return -1;

	tcgetattr(tty->fd, &attr);
	attr.c_lflag &= ~ECHO;
	tcsetattr(tty->fd, TCSANOW, &attr);

	write(tty->fd, "\033c", 2);
	write(tty->fd, "\033[?1c", 5);
	write(tty->fd, prompt, strlen(prompt));

	return 0;
}

static void read_tty(tty_t *tty)
{
	int flags;
	char ch, login;
	while (read(tty->fd, &ch, 1) == 1)
		if (ch == '\n' || ch == '\r')
			login = 1;
	if (!login)
		return;
	if ((tty->pid = fork()) < 0)
		return;
	if (tty->pid == 0) {
		if (putenv("TERM=linux"))
			error("setting environment");
		if (setsid() < 0)
			error("setting sid");
		if (write(tty->fd, "\033c", 2) < 0)
			error("resetting tty");
		if ((flags = fcntl(tty->fd, F_GETFL)) < 0)
			error("getting fd flags");
		if (fcntl(tty->fd, F_SETFL, flags&~O_NONBLOCK) < 0)
			error("setting blocking flags");
		if (ioctl(tty->fd, TIOCSCTTY, 0) < 0)
			error("setting ctty");
		if (dup2(tty->fd, 0) != 0)
			error("setting stdin");
		if (dup2(tty->fd, 1) != 1)
			error("setting stdout");
		if (dup2(tty->fd, 2) != 2)
			error("setting stderr");
		if (execl("/bin/login", "login", NULL) < 0)
			error("execing login program");
	}
	del_poll(tty->fd);
	close(tty->fd);
}

void on_child(void)
{
	struct signalfd_siginfo info;
	if (read(sigs, &info, sizeof(info)) != sizeof(info))
		return;
	if (info.ssi_signo != SIGCHLD)
		return;

	int status;
	pid_t pid;
	tty_t *tty;
	while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
		for (tty = ttys; tty; tty = tty->next) {
			if (pid == tty->pid) {
				tty->pid = 0;
				start_tty(tty);
			}
		}
	}
}

/* Main */
int main(int argc, char **argv)
{
	int i, count;
	tty_t *tty;
	sigset_t mask;

	/* Check arguments */
	if (argc <= 1) {
		printf("usage: gettyd <tty> ...\n");
		return 0;
	}

	/* Setup */
	sigemptyset(&mask);
	sigaddset(&mask, SIGCHLD);
	sigaddset(&mask, SIGHUP);
	if (sigprocmask(SIG_BLOCK, &mask, NULL) < 0)
		error("blocking signals");
	if ((sigs = signalfd(-1, &mask, SFD_CLOEXEC)) < 0)
		error("creating signal fd");
	if ((epoll = epoll_create1(EPOLL_CLOEXEC)) < 0)
		error("creating epoll");
	if (add_poll(sigs, &sigs) < 0)
		error("adding signal epoll");

	/* Open TTYs */
	for (i = 1; i < argc; i++) {
		if (!(tty = malloc(sizeof(tty_t))))
			error("allocating memory");
		if (asprintf(&tty->path, "/dev/%s", argv[i]) < 0)
			error("allocating path name");
		if (start_tty(tty) < 0)
			error("starting tty '%s'", tty->path);
		tty->next = ttys;
		ttys = tty;
	}

	/* Main loop */
	running = 1;
	while (1) {
		struct epoll_event event;
		errno = 0;
		count = epoll_wait(epoll, &event, 1, -1);
		if (errno == EINTR)
			continue;
		if (count < 0)
			continue;
		if (event.data.ptr == &sigs)
			on_child();
		else
			read_tty(event.data.ptr);
	}

	return 0;
}