=== modified file 'doc/emacs/trouble.texi'
--- doc/emacs/trouble.texi	2011-01-25 04:08:28 +0000
+++ doc/emacs/trouble.texi	2011-03-29 17:47:26 +0000
@@ -812,6 +812,14 @@
 This backtrace is useful for debugging such long loops, so if you can
 produce it, copy it into the bug report.
 
address@hidden debug-on-special-event
+If the normal quit process is ineffective, you can try sending Emacs
+the special event given in @code{debug-on-special-event}, which by
+default corresponds to SIGUSR2.  When Emacs receives this event, it
+stops what it's doing, sets @code{debug-on-quit} to @code{t}, and
+tries to break into the debugger in a variety of ways.  This process
+happens even in places where quitting is normally not allowed.
+
 @item
 Check whether any programs you have loaded into the Lisp world,
 including your @file{.emacs} file, set any variables that may affect the

=== modified file 'doc/lispref/debugging.texi'
--- doc/lispref/debugging.texi	2011-01-25 04:08:28 +0000
+++ doc/lispref/debugging.texi	2011-03-29 17:46:44 +0000
@@ -185,6 +185,20 @@
 when you quit.  @xref{Quitting}.
 @end defopt
 
address@hidden debug-on-special-event
+When this variable contains a symbol matching one of the special
+events in @code{special-event-map} (@pxref{Active Keymaps}) and Emacs
+receives the corresponding event, Emacs will break into the debugger
+instead of processing the event normally, setting @code{debug-on-quit}
+to @code{t} as a side effect.  This feature is useful for terminating
+infinite loops in places where quits would normally be ignored, such
+as in code that runs during redisplay.
+
+Currently, the only events Emacs recognizes as valid values for
address@hidden are @code{sigusr1} and @code{sigusr2}.
+
address@hidden defopt
+
 @node Function Debugging
 @subsection Entering the Debugger on a Function Call
 @cindex function call debugging

=== modified file 'lisp/cus-start.el'
--- lisp/cus-start.el	2011-03-27 10:55:07 +0000
+++ lisp/cus-start.el	2011-03-29 17:36:51 +0000
@@ -259,6 +259,11 @@
 	     (suggest-key-bindings keyboard (choice (const :tag "off" nil)
 						    (integer :tag "time" 2)
 						    (other :tag "on")))
+             (debug-on-special-event debug
+			    (choice (const :tag "None" nil)
+                                    (const :tag "When sent SIGUSR1" sigusr1)
+                                    (const :tag "When sent SIGUSR2" sigusr2))
+                            "24.1")
 
 ;; This is not good news because it will use the wrong
 ;; version-specific directories when you upgrade.  We need

=== modified file 'src/keyboard.c'
--- src/keyboard.c	2011-03-27 02:27:11 +0000
+++ src/keyboard.c	2011-03-29 17:29:47 +0000
@@ -7165,12 +7165,33 @@
 {
   int old_errno = errno;
   struct user_signal_info *p;
+  const char* special_event_name = NULL;
 
   SIGNAL_THREAD_CHECK (sig);
-
+  
+  if (SYMBOLP (Vdebug_on_special_event) &&
+      STRINGP (XSYMBOL (Vdebug_on_special_event)->xname))
+    {
+      special_event_name =
+        SSDATA (XSYMBOL (Vdebug_on_special_event)->xname);
+    }
+  
   for (p = user_signals; p; p = p->next)
     if (p->sig == sig)
       {
+        if (special_event_name &&
+            strcmp (special_event_name, p->name) == 0)
+          {
+            /* Enter the debugger in many ways.  */
+            debug_on_next_call = 1;
+            debug_on_quit = 1;
+            Vquit_flag = Qt;
+            Vinhibit_quit = Qnil;
+
+            /* Eat the event.  */
+            break;
+          }
+        
 	p->npending++;
 #ifdef SIGIO
 	if (interrupt_input)
@@ -12178,6 +12199,17 @@
 `deactivate-mark' call uses this to set the window selection.  */);
   Vsaved_region_selection = Qnil;
 
+  DEFVAR_LISP ("debug-on-special-event",
+               Vdebug_on_special_event,
+               doc: /* Enter debugger on this special event.
+When Emacs receives the special event specifed by this variable,
+it will try to break into the debugger as soon as possible instead of
+processing the event normally through `special-event-map'.
+
+Currently, the only meaningful values for this
+variable are `sigusr1' and `sigusr2'.  */);
+  Vdebug_on_special_event = intern_c_string ("sigusr2");
+
   /* Create the initial keyboard. */
   initial_kboard = (KBOARD *) xmalloc (sizeof (KBOARD));
   init_kboard (initial_kboard);

=== modified file 'src/lisp.h'
--- src/lisp.h	2011-03-27 02:27:11 +0000
+++ src/lisp.h	2011-03-28 13:06:41 +0000
@@ -2814,7 +2814,7 @@
 
 /* Defined in eval.c */
 extern Lisp_Object Qautoload, Qexit, Qinteractive, Qcommandp, Qdefun, Qmacro;
-extern Lisp_Object Qinhibit_quit;
+extern Lisp_Object Qinhibit_quit, Qdebug;
 extern Lisp_Object Vautoload_queue;
 extern Lisp_Object Vsignaling_function;
 extern int handling_signal;