kernel.c: prepare for new recursive inspect check

Like previous recursive `<=>` check, scan call stack to detect recursive
`inspect` calls. We no longer need incomplete `_inspect` hack to pass
around a hash table to record objects currently inspecting.

include/mruby/internal.h

@@ -12,6 +12,8 @@ void mrb_ary_decref(mrb_state*, mrb_shared_array*);
 mrb_value mrb_ary_subseq(mrb_state *mrb, mrb_value ary, mrb_int beg, mrb_int len);
 #endif
 
+mrb_bool mrb_inspect_recursive_p(mrb_state *mrb, mrb_value self);
+
 #ifdef MRUBY_CLASS_H
 struct RClass *mrb_vm_define_class(mrb_state*, mrb_value, mrb_value, mrb_sym);
 struct RClass *mrb_vm_define_module(mrb_state*, mrb_value, mrb_sym);

src/kernel.c

@@ -98,6 +98,30 @@ mrb_cmp_m(mrb_state *mrb, mrb_value self)
   return mrb_nil_value();
 }
 
+static mrb_bool
+inspect_recursive_p(mrb_state *mrb, mrb_value obj, int n)
+{
+  for (mrb_callinfo *ci=&mrb->c->ci[-1-n]; ci>=mrb->c->cibase; ci--) {
+    if (ci->mid == MRB_SYM(inspect) &&
+        mrb_obj_eq(mrb, obj, ci->stack[0])) {
+      return TRUE;
+    }
+  }
+  return FALSE;
+}
+
+mrb_bool
+mrb_inspect_recursive_p(mrb_state *mrb, mrb_value obj)
+{
+  return inspect_recursive_p(mrb, obj, 0);
+}
+
+static mrb_value
+mrb_obj_inspect_recursive_p(mrb_state *mrb, mrb_value obj)
+{
+  return mrb_bool_value(inspect_recursive_p(mrb, obj, 1));
+}
+
 /* 15.3.1.3.3  */
 /* 15.3.1.3.33 */
 /*
@@ -571,6 +595,7 @@ mrb_init_kernel(mrb_state *mrb)
   mrb_define_method(mrb, krn, "__to_int",                   mrb_ensure_int_type,             MRB_ARGS_NONE());    /* internal */
   mrb_define_method(mrb, krn, "__ENCODING__",               mrb_encoding,                    MRB_ARGS_NONE());
   mrb_define_method(mrb, krn, "respond_to_missing?",        mrb_false,                       MRB_ARGS_ARG(1,1));
+  mrb_define_method(mrb, krn, "__inspect_recursive?",       mrb_obj_inspect_recursive_p,     MRB_ARGS_NONE());
 
   mrb_include_module(mrb, mrb->object_class, mrb->kernel_module);
 }