forkoff()

[Nicolas Williams]

Please review the function below, forkoff(), meant to be used in
clientloop.c instead of daemon() and the code in process_escapes().

The intention is to make ~D ( like ~& but also detach) possible and to
make it possible for ssh -f (or ssh -f -f - see other thread on this)
to detach, not just forkoff().

I also intend to use the same detach technique in a feature patch for
the hang-on-exit behaviour that's been hashed to death. I.e., I might
like to add an option to control ssh's behaviour when the remote session
exits, the behaviours being: wait for the channel to be closed [current,
default], close all channels and exit, background, detach and force
exit.

To make detaching and some of the hang-on-exit options work correctly I
need a way to force a channel closed and to do it in way that is
correct wrt to the SSHv2 protocol. The forkoff() function below does
that, I think, but it would be nice to have confirmation.

Comments?

Nico

static
void
forkoff(int stop_listening, int detach)
{
	pid_t pid;
	int fd;
	Channel *c;

	/* Restore tty modes. */
	leave_raw_mode();

	/* Stop listening for new connections. */
	if (stop_listening)
		channel_stop_listening();

	fprintf(stderr, "Forking off into the background - %s",
		stop_listening ? "no longer listening" : "still listening");

	/* Fork into background. */
	pid = fork();
	if (pid < 0) {
		error("fork: %.100s", strerror(errno));
		return;
	}
	if (pid != 0) {	/* This is the parent. */
		/* The parent just exits. */
		exit(0);
	}

	c = channel_lookup(session_ident);
	if (c == NULL)
		error("couldn't lookup session channel");

	/* The child continues serving connections. */
	/* fake EOF on stdin */
	if (compat20) {
		buffer_append(&stdin_buffer, "\004", 1);
	} else if (!stdin_eof) {
		/*
		 * Sending SSH_CMSG_EOF alone does not always appear
		 * to be enough.  So we try to send an EOF character
		 * first.
		 */
		packet_start(SSH_CMSG_STDIN_DATA);
		packet_put_string("\004", 1);
		packet_send();
		/* Close stdin. */
		stdin_eof = 1;
		if (buffer_len(&stdin_buffer) == 0) {
			packet_start(SSH_CMSG_EOF);
			packet_send();
		}
	}

	if (detach) {
		/*
		 * There should be a chan_wont_read()/chan_wont_write()
		 * API, differing only in the debug messages used.
		 */
		chan_read_failed(c);
		chan_write_failed(c);
		channel_close_fds(c);
		fd = open(_PATH_DEVNULL, O_RDWR, 0);
		if (fd < 0)
			return;
		(void) dup2(fd, STDIN_FILENO);
		(void) dup2(fd, STDOUT_FILENO);
		(void) dup2(fd, STDERR_FILENO);
		if (fd > 2)
			(void) close(fd);
		(void) setsid();
	}
}
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