bug#17170: 24.3.50; debug: Terminal 3 is locked, cannot read from it

[martin rudalics]

>> When it happens again please tell me the value of
>> `debugger-previous-window'.  AFAICT this can only happen when that
>> window was shown on the "initial frame F1" before.
>
> Yes it was a window on that frame (window 4 IIRC). I remember that
> because this is the reason I did
>      (setq debugger-previous-window (selected-window))
> to bring the debugger back to my frame.

I can't understand what happened here because the only way to set
`debugger-previous-window' is via invoking `debug' and then doing
unconditionally

(setq debugger-window (selected-window))

followed by

(setq debugger-previous-window debugger-window))

so the window must have been the selected window when `debug' was called
the last time and Emacs should have complained then already.

Anyway.  I checked in a fix for Emacs 24.5 which should avoid this
problem now.  If you want to try it immediately please apply:

--- a/lisp/emacs-lisp/debug.el
+++ b/lisp/emacs-lisp/debug.el
@@ -192,7 +192,9 @@ first will be printed into the backtrace buffer."
               debugger-buffer
               `((display-buffer-reuse-window
                  display-buffer-in-previous-window)
-                 . (,(when debugger-previous-window
+                . (,(when (and (window-live-p debugger-previous-window)
+                               (frame-visible-p
+                                (window-frame debugger-previous-window)))
                        `(previous-window . ,debugger-previous-window)))))
              (setq debugger-window (selected-window))
              (if (eq debugger-previous-window debugger-window)

martin