Cygwin: console: re-enable the master thread before selecting cygwin input mode

When a cygwin program and a non-cygwin program run in the same foreground
process group (for example the pipeline `cat | ping`), Ctrl-C stopped
interrupting the cygwin program after "Cygwin: console: Ensure the master
thread runs only when it is supposed to".

The console only delivers Ctrl-C as a raw 0x03 byte (which the console
master thread reads and turns into a SIGINT for the foreground process
group) while that thread is live. When it is suspended or disabled,
set_input_mode (tty::cygwin) instead requests ENABLE_PROCESSED_INPUT,
so the console raises a CTRL_C_EVENT and the 0x03 byte never reaches
the master thread. The referenced commit reordered the two explicit
enable paths, bg_check () and post_open_setup (), and one path which
normally(*1) enables master thread, clearnup_for_non_cygwin_app(), so
that set_input_mode (tty::cygwin) runs while disable_master_thread is
still set; that leaves ENABLE_PROCESSED_INPUT on and the cygwin program
never receives its SIGINT.

(*1 ... except the process which calls exec() for non-cygwin app while
itself is the cons master. In this case, subsequent set_input_mode()
call sets the mode to tty::restore, and the master thread should be
kept 'disabled' until the process exits.)

Clear disable_master_thread before selecting cygwin input mode in those two
paths, so the mode is configured with the master thread already live and
ENABLE_PROCESSED_INPUT stays off. The disable paths and the synchronous
suspension that the referenced commit added are left unchanged, so
non-cygwin programs still get the master thread reliably suspended.

Fixes: 733d5a9 ("Cygwin: console: Ensure the master thread runs only when it is supposed to")
Assisted-by: Opus 4.8
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Co-Authored-by: Takashi Yano <takashi.yano@nifty.ne.jp>
Reviewed-by: Takashi Yano <takashi.yano@nifty.ne.jp>

Author: dscho

winsup/cygwin/fhandler/console.cc

@@ -991,14 +991,14 @@ fhandler_console::cleanup_for_non_cygwin_app (handle_set_t *p)
   termios *ti = shared_console_info[unit] ?
     &(shared_console_info[unit]->tty_min_state.ti) : &dummy;
   /* Cleaning-up console mode for non-cygwin app. */
+  set_disable_master_thread (con.owner == GetCurrentProcessId ());
   /* conmode can be tty::restore when non-cygwin app is
      exec'ed from login shell. */
   tty::cons_mode conmode = cons_mode_on_close (p);
   if (con.curr_output_mode != conmode)
     set_output_mode (conmode, ti, p);
   if (con.curr_input_mode != conmode)
     set_input_mode (conmode, ti, p);
-  set_disable_master_thread (con.owner == GetCurrentProcessId ());
 }
 
 /* Return the tty structure associated with a given tty number.  If the
@@ -1191,8 +1191,8 @@ fhandler_console::bg_check (int sig, bool dontsignal)
      in the same process group. */
   if (sig == SIGTTIN && con.curr_input_mode != tty::cygwin)
     {
-      set_input_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
       set_disable_master_thread (false, this);
+      set_input_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
     }
   if (sig == SIGTTOU && con.curr_output_mode != tty::cygwin)
     set_output_mode (tty::cygwin, &tc ()->ti, get_handle_set ());
@@ -2111,8 +2111,8 @@ fhandler_console::post_open_setup (int fd)
   /* Setting-up console mode for cygwin app started from non-cygwin app. */
   if (fd == 0)
     {
-      set_input_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);
       set_disable_master_thread (false, this);
+      set_input_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);
     }
   else if (fd == 1 || fd == 2)
     set_output_mode (tty::cygwin, &get_ttyp ()->ti, &handle_set);