tty: handle SIGHUP when allocating a tty (#104)

When a session is hung up and a tty is allocated SIGHUP will be sent
to all processes in the session group.  Since we were ignoring SIGHUP
this led to bst processes hanging around.

Add signal handling for SIGHUP to close the tty master.  This will lead
to cleanup of the child session.

Fixes: 1047634

enter.c

@@ -392,8 +392,9 @@ int enter(struct entry_settings *opts)
 		sigfillset(&mask);
 
 		if (opts->tty) {
-			/* tty_parent_setup handles SIGWINCH to resize the pty */
+			/* tty_parent_setup handles SIGWINCH to resize the pty and SIGHUP to close the pty master */
 			sigdelset(&mask, SIGWINCH);
+			sigdelset(&mask, SIGHUP);
 			tty_parent_setup(&opts->ttyopts, epollfd, socket_fdpass[SOCKET_PARENT]);
 		}
 		sig_setup(epollfd, &mask, outer_helper.pid, sig_handler);

tty.c

@@ -189,8 +189,18 @@ static int tty_handle_sig(int epollfd, const struct epoll_event *ev, int fd, pid
 	siginfo_t siginfo;
 	sig_read(fd, &siginfo);
 
-	assert(siginfo.si_signo == SIGWINCH && "tty_handle_sig can only handle SIGWINCH");
-	tty_set_winsize();
+	assert((siginfo.si_signo == SIGWINCH || siginfo.si_signo == SIGHUP) && "tty_handle_sig can only handle SIGWINCH and SIGHUP");
+	switch (siginfo.si_signo) {
+	case SIGWINCH:
+		tty_set_winsize();
+		break;
+	case SIGHUP:
+		if (info.termfd > 0) {
+			close(info.termfd);
+			info.termfd = -1;
+		}
+		break;
+	}
 	return EPOLL_HANDLER_CONTINUE;
 }
 
@@ -398,6 +408,7 @@ void tty_parent_setup(struct tty_opts *opts, int epollfd, int socket)
 	sigset_t sigmask;
 	sigemptyset(&sigmask);
 	sigaddset(&sigmask, SIGWINCH);
+	sigaddset(&sigmask, SIGHUP);
 
 	int sigfd = signalfd(-1, &sigmask, 0);
 	if (sigfd == -1) {