[PATCH v3 2/4] terminal: don't assume stdin is /dev/tty

[Phillip Wood <phillip.wood123@gmail.com>]

From: Phillip Wood <phillip.wood@dunelm.org.uk>

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] https://github.com/jonathanstowe/TermReadKey/blob/b061c913bbf7ff9bad9b4eea6caae189eacd6063/ReadKey.xs#L1090

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 compat/terminal.c | 18 ++++++++++++------
 compat/terminal.h |  2 ++
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/compat/terminal.c b/compat/terminal.c
index da2f788137..bfbde3c1af 100644
--- a/compat/terminal.c
+++ b/compat/terminal.c
@@ -23,21 +23,28 @@ static void restore_term_on_signal(int sig)
 static int term_fd = -1;
 static struct termios old_term;
 
+static void close_term_fd(void)
+{
+	if (term_fd)
+		close(term_fd);
+	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)
@@ -66,8 +73,7 @@ static int disable_bits(enum save_term_flags flags, tcflag_t bits)
 
 	sigchain_pop_common();
 error:
-	close(term_fd);
-	term_fd = -1;
+	close_term_fd();
 	return -1;
 }
 
@@ -362,7 +368,7 @@ 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");
diff --git a/compat/terminal.h b/compat/terminal.h
index aeb24c9478..79ed00cf61 100644
--- a/compat/terminal.h
+++ b/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,
 };
 
 /*
-- 
2.35.1