New debugging facility: backtraces from errors in Lisp called from redisplay

Setting backtrace-on-redisplay-error to non-nil enables the generation of a
Lisp backtrace in buffer *Redisplay-trace* following an error in Lisp called
from redisplay.

* doc/lispref/debugging.texi (Debugging Redisplay): New subsection.
(Error Debugging): Reference to the new subsection.

* etc/NEWS: New entry for the new facility.

* src/eval.c (redisplay_deep_handler): New variable.
(init_eval): Initialize redisplay_deep_handler.
(call_debugger): Don't throw to top-level after calling debug-early
(internal_condition_case_n): "Bind" redisplay_deep_handler to the current
handler.
(backtrace_yet): New boolean variable.
(signal_or_quit): New code section to handle Lisp errors occurring in
redisplay.
(syms_of_eval): New DEFVAR_BOOL backtrace-on-redisplay-error.

* src/keyboard.c (command_loop_1): Set backtrace_yet to false each time around
the loop.
(safe_run_hooks_error): Allow args to be up to four Lisp_Objects long.
(safe_run_hooks_2): New function.

* src/lisp.h (top level): declare as externs backtrace_yet and
safe_run_hooks_2.

* src/xdisp.c (run_window_scroll_functions): Replace a call to
run_hook_with_args_2 with one to safe_run_hooks_2.

1 files changed, 13 insertions, 1 deletions

diff --git a/src/keyboard.c b/src/keyboard.c
index 4ad6e4e6bd1..719226caedc 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1331,6 +1331,7 @@ command_loop_1 (void)
         display_malloc_warning ();
 
       Vdeactivate_mark = Qnil;
+      backtrace_yet = false;
 
       /* Don't ignore mouse movements for more than a single command
          loop.  (This flag is set in xdisp.c whenever the tool bar is
@@ -1841,7 +1842,7 @@ safe_run_hooks_1 (ptrdiff_t nargs, Lisp_Object *args)
 static Lisp_Object
 safe_run_hooks_error (Lisp_Object error, ptrdiff_t nargs, Lisp_Object *args)
 {
-  eassert (nargs == 2);
+  eassert (nargs >= 2 && nargs <= 4);
   AUTO_STRING (format, "Error in %s (%S): %S");
   Lisp_Object hook = args[0];
   Lisp_Object fun = args[1];
@@ -1915,6 +1916,17 @@ safe_run_hooks_maybe_narrowed (Lisp_Object hook, struct window *w)
   unbind_to (count, Qnil);
 }
 
+void
+safe_run_hooks_2 (Lisp_Object hook, Lisp_Object arg1, Lisp_Object arg2)
+{
+  specpdl_ref count = SPECPDL_INDEX ();
+
+  specbind (Qinhibit_quit, Qt);
+  run_hook_with_args (4, ((Lisp_Object []) {hook, hook, arg1, arg2}),
+                      safe_run_hook_funcall);
+  unbind_to (count, Qnil);
+}
+
 
 /* Nonzero means polling for input is temporarily suppressed.  */