bug#5662: flet not undone on lisp nesting error

[Jinseop Kim <vmfhrmfoaj@yahoo.com>]

You can see the bug of 'unwind-protect' using the following code.
We must see the message “finally”, but are not output.

```elisp
(defun endless-recursive ()
  (endless-recursive))

(unwind-protect
    (endless-recursive) ;; body
  (message "finally”)) ;; unwind-forms
```

The `unwind-protect’ will call the `eval_sub’ function.
And, the `eval_sub’ function check whether the infinite recursion as following code.

```c
/* ./src/eval.c:2086: */
if (++lisp_eval_depth > max_lisp_eval_depth)
    {
      if (max_lisp_eval_depth < 100)
	max_lisp_eval_depth = 100;
      if (lisp_eval_depth > max_lisp_eval_depth)
	error ("Lisp nesting exceeds `max-lisp-eval-depth'");
    }
```

The cause of problem is that `unwind-forms’ will call the `eval_sub’ function.
Of course, the `eval_sub’ function check for infinite recursion.
Consequently, the `unwind-protect’ wouldn't evaluate.

You must be noted step 7~10.
Problem's steps:
  1. eval.c:1208:Funwind_protect:  record_unwind_protect (unwind_body, XCDR (args)); 
  2. eval.c:1209:Funwind_protect:  val = eval_sub (XCAR (args)); 
  3. eval.c:2090:eval_sub:  if (lisp_eval_depth > max_lisp_eval_depth) error ("Lisp nesting exceeds `max-lisp-eval-depth'”); 
  4. eval.c:1582:xsignal:  Fsignal (error_symbol, data); 
  5. eval.c:1558:Fsignal:  unwind_to_catch (h, unwind_data); 
  6. eval.c:1161:unwind_to_catch:  unbind_to (handlerlist->pdlcount, Qnil); 
  7. eval.c:3305:unbind_to:  unwind_body // specpdl_ptr->unwind_ptr.func (specpdl_ptr->unwind_ptr.arg);
  8. eval.c:476:unwind_body:  Fprogn (body); 
  9. eval.c:462:Fprogn:  val = eval_sub (XCAR (body)); 
  10. eval.c:2090:eval_sub:  if (lisp_eval_depth > max_lisp_eval_depth) error ("Lisp nesting exceeds `max-lisp-eval-depth’”)


You can avoid this problem using `ignore-errors’ macro.
(eval.c:1175:unwind_to_catch:  lisp_eval_depth = catch->lisp_eval_depth;)

```elisp
(defun endless-recursive ()
  (endless-recursive))

(unwind-protect
    (ignore-errors (endless-recursive))
  (message "finally"))
```

> On Mar 6, 2010, at 5:30 AM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> If I trigger a lisp nesting error with an infinite recursion inside a
>> let and an flet binding, then the effects of the flet are not undone,
>> resulting in a change of binding at the top-level.
> 
>> (defun g () 'g-orig)
>> (setq a 'a-orig)
> 
>> (defun h ()
>>  (let ((a 'a-new))
>>    (flet ((g () 'g-new))
>>      (h))))
>> (h)   ;; <-- Lisp nesting exceeds `max-lisp-eval-depth'
>> (g)   ;; g-new ! 
>> a     ;; a-orig 
> 
> I can indeed reproduce it.  I'm not sure yet what's going on, but it
> might be due to the "Lisp nesting exceeds `max-lisp-eval-depth'" error
> happening in the unwind-protect code (i.e. while undoing the `g'
> binding).  The explanation can't be quite so simple, but my gut feeling
> tells me it's got to do with it.
> 
> 
>        Stefan
> 
> 
> 
>