Let guard frames optionally pass through unwinding.

We have a bug in that when an exception occurs in a context
called from obj_print, the guard for stopping continuation
captures across obj_print also unintentially blocks the
unwinding. Let's make the unwinding blockage optional

* unwind.c (uw_unwind_to_exit_point): If a UW_GUARD is
encountered, do not abort if the uw_ok flag is set;
keep unwinding.
(uw_push_guard): New uw_ok argument, initializes the
uw_ok member of a guard frame.

* unwind.h (struct uw_guard): New struct type.
(union uw_frame): New member gu of type struct uw_guard.
(uw_push_guard): Declaration updated.

* ftw.c (ftw_callback): Pass zero as new uw_push_guard
argument: no unwinding across the POSIX library function ftw.

* glob.c (errfunc_thunk): Likewise, no unwinding across
the library function glob.

* lib.c (obj_print): Pass 1 as new uw_push_guard argument:
continuations can't be captured, but unwinding is okay.

1 files changed, 4 insertions, 1 deletions

diff --git a/unwind.c b/unwind.c
index 104bb11e..6fc2720e 100644
--- a/unwind.c
+++ b/unwind.c
@@ -95,6 +95,8 @@ static void uw_unwind_to_exit_point(void)
       uw_env_stack = uw_env_stack->ev.up_env;
       break;
     case UW_GUARD:
+      if (uw_stack->gu.uw_ok)
+        break;
       format(std_error, lit("~a: cannot unwind across foreign stack frames\n"),
              prog_string, nao);
       abort();
@@ -234,11 +236,12 @@ val uw_set_match_context(val context)
   return context;
 }
 
-void uw_push_guard(uw_frame_t *fr)
+void uw_push_guard(uw_frame_t *fr, int uw_ok)
 {
   memset(fr, 0, sizeof *fr);
   fr->uw.type = UW_GUARD;
   fr->uw.up = uw_stack;
+  fr->gu.uw_ok = uw_ok;
   uw_stack = fr;
 }