Should definitely fix #93

Our calced_stack_size should be reliable enough at this stage (I'm
getting some failures in 2.0 I think due to missing backports but it's
ok for now). It also has the advantage that we can easily "fake it" to
properly ignore byebug internal frames.

ext/byebug/byebug.c

@@ -675,6 +675,8 @@ bb_load(int argc, VALUE * argv, VALUE self)
   context = bb_current_context(self);
   Data_Get_Struct(context, debug_context_t, dc);
 
+  dc->calced_stack_size = 0;
+
   if (RTEST(stop))
     dc->steps = 1;
 

ext/byebug/context.c

@@ -48,7 +48,7 @@ dc_backtrace(const debug_context_t * context)
 static int
 dc_stack_size(const debug_context_t * context)
 {
-  return RARRAY_LENINT(dc_backtrace(context));
+  return context->calced_stack_size;
 }
 
 extern VALUE
@@ -64,9 +64,8 @@ context_create(VALUE thread)
   reset_stepping_stop_points(context);
   context->stop_reason = CTX_STOP_NONE;
 
-  rb_debug_inspector_open(context_backtrace_set, (void *)context);
-  context->calced_stack_size = dc_stack_size(context);
   context->backtrace = Qnil;
+  context->calced_stack_size = 0;
 
   if (rb_obj_class(thread) == cDebugThread)
     CTX_FL_SET(context, CTX_FL_IGNORE);