Re: poor handling of multiple C-g with multi-tty (apparent hangs)

[Chong Yidong <cyd@stupidchicken.com>]

"Ami Fischman" <ami@fischman.org> writes:

> Emacs has code to deal with C-g being entered while a quit is already
> in progress, meant to suspend emacs and drop the user to the superior
> shell or debugger (see section 59.7 Emergency Escape of the emacs
> info).  This is done if emacs believes it's a good idea, which
> criteria includes "running in tty mode".  Unfortunately this was never
> updated to work correctly with the multi-tty patch.

Thanks for pointing this out.

IIUC, the "emergency C-g" behavior is geared towards the traditional
setup where there's only one text-only terminal available.  There, the
emergency C-g is the only way to drop back to the shell if Emacs gets
stuck.

It's disabled for graphical terminals because there, you can open up a
separate terminal to kill/suspend the Emacs process.

This implies that we should disable the emergency C-g if Emacs is
running on more than one terminal, as in the following patch.

*** trunk/src/keyboard.c.~1.969.~	2008-08-05 17:08:23.000000000 -0400
--- trunk/src/keyboard.c	2008-08-11 13:35:43.000000000 -0400
***************
*** 10953,10958 ****
--- 10953,10976 ----
    errno = old_errno;
  }
  
+ /* If there is exactly one terminal active, return it.  Otherwise,
+    return NULL.  */
+ 
+ static struct terminal *
+ just_one_tty_p ()
+ {
+   struct terminal *t, *found = NULL;
+   for (t = terminal_list; t; t = t->next_terminal)
+     if (TERMINAL_ACTIVE_P (t))
+       {
+ 	if (found)
+ 	  return NULL;
+ 	else
+ 	  found = t;
+       }
+   return found;
+ }
+ 
  /* This routine is called at interrupt level in response to C-g.
  
     It is called from the SIGINT handler or kbd_buffer_store_event.
***************
*** 10968,10980 ****
  handle_interrupt ()
  {
    char c;
  
    cancel_echoing ();
  
-   /* XXX This code needs to be revised for multi-tty support. */
    if (!NILP (Vquit_flag)
  #ifndef MSDOS
!       && get_named_tty ("/dev/tty")
  #endif
        )
      {
--- 10986,10999 ----
  handle_interrupt ()
  {
    char c;
+   struct terminal *t;
  
    cancel_echoing ();
  
    if (!NILP (Vquit_flag)
  #ifndef MSDOS
!       && (t = just_one_tty_p (), t)
!       && t->type == output_termcap
  #endif
        )
      {