Don't signal during backtrace unrewind (Bug#40088)

backtrace_eval_unrewind is used to temporarily reverse
let-bindings (it's called with a positive argument to reverse
bindings, and then a negative argument to re-apply them) by
backtrace--locals and backtrace-eval.  For the SPECPDL_LET_DEFAULT and
SPECPDL_LET_LOCAL cases (which occur for let-bindings on buffer-local
variables), the code calls Fdefault_value and Fbuffer_local_value on
the symbol.

For symbols which are unbound at top-level, the first (with positive
argument) call to backtrace_eval_unrewind will set the symbol's value
to unbound (putting the current value in the specpdl's "old value"
slot).  On the second (with negative argument) call,
backtrace_eval_unrewind attempts to retrieve the symbol's value with
Fdefault_value or Fbuffer_local_value, but that raises a void-variable
signal.  This interrupts the restoration of the let-bindings, so any
other variables more recent on the stack will now have the wrong
value.

* src/data.c (default_value): Make non-static.
* src/lisp.h: Declare it.
* src/eval.c (backtrace_eval_unrewind): Replace the calls to
Fdefault_value and Fbuffer_local_value with default_value and
buffer_local_value, respectively.  The latter do exactly the same as
the former, except if the symbol's value is Qunbound they just return
it instead of signaling void-variable.

Author: npostavs

src/data.c

@@ -1573,7 +1573,7 @@ notify_variable_watchers (Lisp_Object symbol,
 /* Return the default value of SYMBOL, but don't check for voidness.
    Return Qunbound if it is void.  */
 
-static Lisp_Object
+Lisp_Object
 default_value (Lisp_Object symbol)
 {
   struct Lisp_Symbol *sym;

src/eval.c

@@ -3816,7 +3816,7 @@ backtrace_eval_unrewind (int distance)
 	  {
 	    Lisp_Object sym = specpdl_symbol (tmp);
 	    Lisp_Object old_value = specpdl_old_value (tmp);
-	    set_specpdl_old_value (tmp, Fdefault_value (sym));
+	    set_specpdl_old_value (tmp, default_value (sym));
 	    Fset_default (sym, old_value);
 	  }
 	  break;
@@ -3832,7 +3832,7 @@ backtrace_eval_unrewind (int distance)
 	    if (!NILP (Flocal_variable_p (symbol, where)))
 	      {
 		set_specpdl_old_value
-		  (tmp, Fbuffer_local_value (symbol, where));
+		  (tmp, buffer_local_value (symbol, where));
                 set_internal (symbol, old_value, where, SET_INTERNAL_UNBIND);
 	      }
 	  }

src/lisp.h

@@ -594,6 +594,7 @@ extern void char_table_set (Lisp_Object, int, Lisp_Object);
 
 /* Defined in data.c.  */
 extern AVOID wrong_type_argument (Lisp_Object, Lisp_Object);
+extern Lisp_Object default_value (Lisp_Object symbol);
 
 
 /* Defined in emacs.c.  */