terminal: teach save_term to fail when not foreground

e22b245 (terminal: teach git how to save/restore its terminal
settings, 2021-10-05) allows external calls to the termios code,
but kept the assumption that all operations were done with
foreground processes, which was proven incorrect.

Add a check to validate that the current process is indeed in the
foreground and in control of the terminal and fail early if not the
case.

To avoid changing behaviour from the other users of save_term() the
full_duplex parameter has been overloaded to restrict the new check
to only future callers, as it is set to 0 for all current users.

The detection is done in a helper function so it can be reused by
all other functions that might benefit from it later, and once that
is done that overloading might be unnecessary and cleaned up, but
doing so has been punted from this series as it is not needed and
might require backward incompatible changes.

Helped-by: Phillip Wood <[email protected]>
Signed-off-by: Carlo Marcelo Arenas Belón <[email protected]>

compat/terminal.c

@@ -29,16 +29,31 @@ void restore_term(void)
 		return;
 
 	tcsetattr(term_fd, TCSAFLUSH, &old_term);
+
 	close(term_fd);
 	term_fd = -1;
 }
 
+static int is_controlling_terminal(int fd)
+{
+	return (getpgid(0) == tcgetpgrp(fd));
+}
+
 int save_term(int full_duplex)
 {
 	if (term_fd < 0)
 		term_fd = open("/dev/tty", O_RDWR);
 
-	return (term_fd < 0) ? -1 : tcgetattr(term_fd, &old_term);
+	if (term_fd < 0)
+		return -1;
+
+	if (full_duplex && !is_controlling_terminal(term_fd)) {
+		close(term_fd);
+		term_fd = -1;
+		return -1;
+	}
+
+	return tcgetattr(term_fd, &old_term);
 }
 
 static int disable_bits(tcflag_t bits)