libcontainer: io: stop screwing with \n in console output

The default terminal setting for a new pty on Linux (unix98) has +ONLCR,
resulting in '\n' writes by a container process to be converted to
'\r\n' reads by the managing process. This is quite unexpected, and
causes multiple issues with things like bats testing. To fix it, make
the terminal sane after opening it by setting -ONLCR.

This patch might need to be rewritten after the console rewrite patchset
is merged.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

libcontainer/console_linux.go

@@ -17,6 +17,9 @@ func NewConsole(uid, gid int) (Console, error) {
 	if err != nil {
 		return nil, err
 	}
+	if err := saneTerminal(master); err != nil {
+		return nil, err
+	}
 	console, err := ptsname(master)
 	if err != nil {
 		return nil, err
@@ -143,3 +146,26 @@ func ptsname(f *os.File) (string, error) {
 	}
 	return fmt.Sprintf("/dev/pts/%d", n), nil
 }
+
+// saneTerminal sets the necessary tty_ioctl(4)s to ensure that a pty pair
+// created by us acts normally. In particular, a not-very-well-known default of
+// Linux unix98 ptys is that they have +onlcr by default. While this isn't a
+// problem for terminal emulators, because we relay data from the terminal we
+// also relay that funky line discipline.
+func saneTerminal(terminal *os.File) error {
+	// Go doesn't have a wrapper for any of the termios ioctls.
+	var termios syscall.Termios
+
+	if err := ioctl(terminal.Fd(), syscall.TCGETS, uintptr(unsafe.Pointer(&termios))); err != nil {
+		return fmt.Errorf("ioctl(tty, tcgets): %s", err.Error())
+	}
+
+	// Set -onlcr so we don't have to deal with \r.
+	termios.Oflag &^= syscall.ONLCR
+
+	if err := ioctl(terminal.Fd(), syscall.TCSETS, uintptr(unsafe.Pointer(&termios))); err != nil {
+		return fmt.Errorf("ioctl(tty, tcsets): %s", err.Error())
+	}
+
+	return nil
+}