[PATCH 1/1] Cygwin: console: Maintain EXTENDED_FLAGS state

[Mitchell Hentges <mhentges@mozilla.com>]

As well-described over in this post [1], it's possible for
the active console mode to be impossible to correctly determine.
Specifically, if ENABLE_EXTENDED_FLAGS is at any point unset,
then the flags it's associated with (ENABLE_INSERT_MODE,
ENABLE_QUICK_EDIT_MODE) will no longer be discoverable - they'll
always show up as unset, regardless of real console state.

It's not possible to work around this by setting
ENABLE_EXTENDED_FLAGS once then re-querying, because setting
ENABLE_EXTENDED_FLAGS on it's own will *disable* its related
flags.

Anyways, to avoid this case, all programs doing SetConsoleMode()
must be good community citizens and carefully maintain its state.
Unfortunately, we're accidentally stepping on this in
fhandler_console::set_input_mode().

This patch solves this by carrying forward ENABLED_EXTENDED_FLAGS
(and friends) in the only place where it had been ignored: set_input_mode()
Since the previous behaviour of leaving all three flags unset would
essentially maintain their existing state (except for the footgun
being worked around here), *adding* the carry-over of the flags now
should not alter console behaviour.

[1] https://www.os2museum.com/wp/disabling-quick-edit-mode/
---
 winsup/cygwin/fhandler_console.cc | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/fhandler_console.cc
b/winsup/cygwin/fhandler_console.cc
index 7a1a45bc1..b2554c3ba 100644
--- a/winsup/cygwin/fhandler_console.cc
+++ b/winsup/cygwin/fhandler_console.cc
@@ -458,16 +458,18 @@ void
 fhandler_console::set_input_mode (tty::cons_mode m, const termios *t,
                                  const handle_set_t *p)
 {
-  DWORD flags = 0, oflags;
+  DWORD oflags;
   WaitForSingleObject (p->input_mutex, mutex_timeout);
   GetConsoleMode (p->input_handle, &oflags);
+  DWORD flags = oflags
+      & (ENABLE_EXTENDED_FLAGS | ENABLE_INSERT_MODE |
ENABLE_QUICK_EDIT_MODE);
   switch (m)
     {
     case tty::restore:
-      flags = ENABLE_ECHO_INPUT | ENABLE_LINE_INPUT |
ENABLE_PROCESSED_INPUT;
+      flags |= ENABLE_ECHO_INPUT | ENABLE_LINE_INPUT |
ENABLE_PROCESSED_INPUT;
       break;
     case tty::cygwin:
-      flags = ENABLE_WINDOW_INPUT;
+      flags |= ENABLE_WINDOW_INPUT;
       if (wincap.has_con_24bit_colors () && !con_is_legacy)
        flags |= ENABLE_VIRTUAL_TERMINAL_INPUT;
       else
--
2.35.1