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

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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.
This commit is contained in:
Noam Postavsky 2020-03-20 04:07:39 -04:00
parent 8709aaddd8
commit 8944310d7c
3 changed files with 4 additions and 3 deletions
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2
src/data.c
View file

@ -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;

4
src/eval.c
View file

@ -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);
}
}

1
src/lisp.h
View file

@ -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. */