terminal: don't assume stdin is /dev/tty

read_key_without_echo() reads from stdin but uses /dev/tty when it
disables echo. This is unfortunate as there no guarantee that stdin is
the same device as /dev/tty. The perl version of "add -p" uses stdin
when it sets the terminal mode, this commit does the same for the
builtin version. There is still a difference between the perl and
builtin versions though - the perl version will ignore any errors when
setting the terminal mode[1] and will still read single bytes when
stdin is not a terminal. The builtin version displays a warning if
setting the terminal mode fails and switches to reading a line at a
time.

[1] b061c913bb/ReadKey.xs (L1090)

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/terminal.c

@@ -20,28 +20,41 @@ static void restore_term_on_signal(int sig)
 #define INPUT_PATH "/dev/tty"
 #define OUTPUT_PATH "/dev/tty"
 
+static volatile sig_atomic_t term_fd_needs_closing;
 static int term_fd = -1;
 static struct termios old_term;
 
+static void close_term_fd(void)
+{
+	if (term_fd_needs_closing)
+		close(term_fd);
+	term_fd_needs_closing = 0;
+	term_fd = -1;
+}
+
 void restore_term(void)
 {
 	if (term_fd < 0)
 		return;
 
 	tcsetattr(term_fd, TCSAFLUSH, &old_term);
-	close(term_fd);
-	term_fd = -1;
+	close_term_fd();
 	sigchain_pop_common();
 }
 
 int save_term(enum save_term_flags flags)
 {
 	if (term_fd < 0)
-		term_fd = open("/dev/tty", O_RDWR);
+		term_fd = ((flags & SAVE_TERM_STDIN)
+			   ? 0
+			   : open("/dev/tty", O_RDWR));
 	if (term_fd < 0)
 		return -1;
-	if (tcgetattr(term_fd, &old_term) < 0)
+	term_fd_needs_closing = !(flags & SAVE_TERM_STDIN);
+	if (tcgetattr(term_fd, &old_term) < 0) {
+		close_term_fd();
 		return -1;
+	}
 	sigchain_push_common(restore_term_on_signal);
 
 	return 0;
@@ -52,7 +65,7 @@ static int disable_bits(enum save_term_flags flags, tcflag_t bits)
 	struct termios t;
 
 	if (save_term(flags) < 0)
-		goto error;
+		return -1;
 
 	t = old_term;
 
@@ -65,9 +78,7 @@ static int disable_bits(enum save_term_flags flags, tcflag_t bits)
 		return 0;
 
 	sigchain_pop_common();
-error:
-	close(term_fd);
-	term_fd = -1;
+	close_term_fd();
 	return -1;
 }
 
@@ -362,7 +373,7 @@ int read_key_without_echo(struct strbuf *buf)
 	static int warning_displayed;
 	int ch;
 
-	if (warning_displayed || enable_non_canonical(0) < 0) {
+	if (warning_displayed || enable_non_canonical(SAVE_TERM_STDIN) < 0) {
 		if (!warning_displayed) {
 			warning("reading single keystrokes not supported on "
 				"this platform; reading line instead");

compat/terminal.h

@@ -4,6 +4,8 @@
 enum save_term_flags {
 	/* Save input and output settings */
 	SAVE_TERM_DUPLEX = 1 << 0,
+	/* Save stdin rather than /dev/tty (fails if stdin is not a terminal) */
+	SAVE_TERM_STDIN  = 1 << 1,
 };
 
 /*