dtch.c in git
DTCH(1)                 FreeBSD General Commands Manual                DTCH(1)

NAME
     dtch, atch – detached sessions

SYNOPSIS
     dtch [name [command ...]]
     atch [name]

DESCRIPTION
     dtch spawns the command (by default, the value of SHELL) inside a new
     PTY.  It then lends control of the detached PTY to atch over a UNIX-
     domain socket.  dtch should be run as a background job or with nohup(1).

     atch attaches to the dtch session with the corresponding name.  To detach
     from a session, type ^Q.

ENVIRONMENT
     HOME   Location of .dtch directory for UNIX-domain sockets.

     SHELL  The default command to run, or /bin/sh if unset.

EXAMPLES
           dtch foo vim &
           atch foo

SEE ALSO
     nohup(1), screen(1), forkpty(3)

FreeBSD 12.0-RELEASE-p4        September 7, 2018       FreeBSD 12.0-RELEASE-p4
/* Copyright (C) 2017, 2018  C. McEnroe <june@causal.agency>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/stat.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <sysexits.h>
#include <termios.h>
#include <unistd.h>

#if defined __FreeBSD__
#include <libutil.h>
#elif defined __linux__
#include <pty.h>
#else
#include <util.h>
#endif

typedef unsigned char byte;

static struct sockaddr_un sockAddr(const char *home, const char *name) {
	struct sockaddr_un addr = { .sun_family = AF_UNIX };
	snprintf(addr.sun_path, sizeof(addr.sun_path), "%s/.dtch/%s", home, name);
	return addr;
}

static byte z;
static struct iovec iov = { .iov_base = &z, .iov_len = 1 };

static ssize_t sendFD(int sock, int fd) {
	size_t size = CMSG_SPACE(sizeof(int));
	byte buf[size];
	struct msghdr msg = {
		.msg_iov = &iov,
		.msg_iovlen = 1,
		.msg_control = buf,
		.msg_controllen = size,
	};

	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
	cmsg->cmsg_level = SOL_SOCKET;
	cmsg->cmsg_type = SCM_RIGHTS;
	*(int *)CMSG_DATA(cmsg) = fd;

	return sendmsg(sock, &msg, 0);
}

static int recvFD(int sock) {
	size_t size = CMSG_SPACE(sizeof(int));
	char buf[size];
	struct msghdr msg = {
		.msg_iov = &iov,
		.msg_iovlen = 1,
		.msg_control = buf,
		.msg_controllen = size,
	};

	ssize_t n =  recvmsg(sock, &msg, 0);
	if (n < 0) return -1;

	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
	if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS) {
		errno = ENOMSG;
		return -1;
	}

	return *(int *)CMSG_DATA(cmsg);
}

static struct sockaddr_un addr;

static void sigExit(int sig) {
	unlink(addr.sun_path);
	_exit(128 + sig);
}

static int dtch(int argc, char *argv[]) {
	int error;

	const char *name = "dtch";
	if (argc > 1) {
		name = argv[1];
		argv++;
		argc--;
	}
	if (argc > 1) {
		argv++;
	} else {
		argv[0] = getenv("SHELL");
		if (!argv[0]) argv[0] = "/bin/sh";
	}

	const char *home = getenv("HOME");
	if (!home) errx(EX_CONFIG, "HOME unset");

	int fd = open(home, 0);
	if (fd < 0) err(EX_CANTCREAT, "%s", home);

	error = mkdirat(fd, ".dtch", 0700);
	if (error && errno != EEXIST) err(EX_CANTCREAT, "%s/.dtch", home);

	close(fd);

	int server = socket(PF_UNIX, SOCK_STREAM, 0);
	if (server < 0) err(EX_OSERR, "socket");

	addr = sockAddr(home, name);
	error = bind(server, (struct sockaddr *)&addr, sizeof(addr));
	if (error) err(EX_CANTCREAT, "%s", addr.sun_path);
	fcntl(server, F_SETFD, FD_CLOEXEC);

	int pty;
	pid_t pid = forkpty(&pty, NULL, NULL, NULL);
	if (pid < 0) err(EX_OSERR, "forkpty");

	if (!pid) {
		execvp(argv[0], argv);
		err(EX_NOINPUT, "%s", argv[0]);
	}

	signal(SIGINT, sigExit);
	signal(SIGTERM, sigExit);

	error = listen(server, 0);
	if (error) err(EX_OSERR, "listen");

	for (;;) {
		int client = accept(server, NULL, NULL);
		if (client < 0) err(EX_IOERR, "accept");

		ssize_t size = sendFD(client, pty);
		if (size < 0) warn("sendmsg");

		size = recv(client, &z, sizeof(z), 0);
		if (size < 0) warn("recv");

		close(client);

		int status;
		pid_t dead = waitpid(pid, &status, WNOHANG);
		if (dead < 0) err(EX_OSERR, "waitpid");
		if (dead) {
			unlink(addr.sun_path);
			if (WIFEXITED(status)) return WEXITSTATUS(status);
			else return 128 + WTERMSIG(status);
		}
	}
}

static struct termios saveTerm;
static void restoreTerm(void) {
	tcsetattr(STDIN_FILENO, TCSADRAIN, &saveTerm);
	printf("\x1B\x63"); // rs1
}

static void sigNop(int sig) {
	(void)sig;
}

static int atch(int argc, char *argv[]) {
	int error;

	const char *home = getenv("HOME");
	if (!home) errx(EX_CONFIG, "HOME unset");

	const char *name = (argc > 1) ? argv[1] : "dtch";

	int client = socket(PF_UNIX, SOCK_STREAM, 0);
	if (client < 0) err(EX_OSERR, "socket");

	struct sockaddr_un addr = sockAddr(home, name);
	error = connect(client, (struct sockaddr *)&addr, sizeof(addr));
	if (error) err(EX_NOINPUT, "%s", addr.sun_path);

	int pty = recvFD(client);
	if (pty < 0) err(EX_IOERR, "recvmsg");

	struct winsize window;
	error = ioctl(STDIN_FILENO, TIOCGWINSZ, &window);
	if (error) err(EX_IOERR, "ioctl");

	struct winsize redraw = { .ws_row = 1, .ws_col = 1 };
	error = ioctl(pty, TIOCSWINSZ, &redraw);
	if (error) err(EX_IOERR, "ioctl");

	error = ioctl(pty, TIOCSWINSZ, &window);
	if (error) err(EX_IOERR, "ioctl");

	error = tcgetattr(STDIN_FILENO, &saveTerm);
	if (error) err(EX_IOERR, "tcgetattr");
	atexit(restoreTerm);

	struct termios raw = saveTerm;
	cfmakeraw(&raw);
	error = tcsetattr(STDIN_FILENO, TCSADRAIN, &raw);
	if (error) err(EX_IOERR, "tcsetattr");

	signal(SIGWINCH, sigNop);

	byte buf[4096];
	struct pollfd fds[2] = {
		{ .events = POLLIN, .fd = STDIN_FILENO },
		{ .events = POLLIN, .fd = pty },
	};
	for (;;) {
		int n = poll(fds, 2, -1);
		if (n < 0) {
			if (errno != EINTR) err(EX_IOERR, "poll");

			error = ioctl(STDIN_FILENO, TIOCGWINSZ, &window);
			if (error) err(EX_IOERR, "ioctl");

			error = ioctl(pty, TIOCSWINSZ, &window);
			if (error) err(EX_IOERR, "ioctl");

			continue;
		}

		if (fds[0].revents) {
			ssize_t size = read(STDIN_FILENO, buf, sizeof(buf));
			if (size < 0) err(EX_IOERR, "read(%d)", STDIN_FILENO);

			if (size == 1 && buf[0] == CTRL('Q')) return EX_OK;

			size = write(pty, buf, size);
			if (size < 0) err(EX_IOERR, "write(%d)", pty);
		}

		if (fds[1].revents) {
			ssize_t size = read(pty, buf, sizeof(buf));
			if (size < 0) err(EX_IOERR, "read(%d)", pty);

			size = write(STDOUT_FILENO, buf, size);
			if (size < 0) err(EX_IOERR, "write(%d)", STDOUT_FILENO);
		}
	}
}

int main(int argc, char *argv[]) {
	switch (argv[0][0]) {
		case 'd': return dtch(argc, argv);
		case 'a': return atch(argc, argv);
		default:  return EX_USAGE;
	}
}