It seems when su executes in a shell scripts, there is a timing race,

sometimes, su will receive a SIGTTOU when parent su tries to set child
su's process group as foreground group, and su will be stopped unexpectly,
ignoring SIGTTOU fixes the problem.

Noticed by: fjoe

usr.bin/su/su.c

@@ -359,6 +359,8 @@ main(int argc, char *argv[])
 	child_pid = fork();
 	switch (child_pid) {
 	default:
+		sa.sa_handler = SIG_IGN;
+		sigaction(SIGTTOU, &sa, NULL);
 		close(fds[0]);
 		setpgid(child_pid, child_pid);
 		tcsetpgrp(1, child_pid);