Multiple calls to mrb_gc_(un)register for the same object

[dabroz]

mrb_gc_register and mrb_gc_unregister make managing mruby objects on C side much easier. However, there is a problem if you call mrb_gc_register multiple times for the same object. The first call to mrb_gc_unregister will remove all registrations for this object.

I wrote a program illustrating this issue. Here we call a C function with two arguments, keeping only one of them from being collected. But if both arguments are actually the same object, the program will crash.

#include <mruby.h>
#include <mruby/proc.h>
#include <mruby/class.h>

mrb_value test;

mrb_value foo(mrb_state * mrb, mrb_value self)
{
  mrb_value a, b;
  mrb_get_args(mrb, "oo", &a, &b);
  mrb_gc_register(mrb, a);
  mrb_gc_register(mrb, b);
  // do something with a & b
  // a is no longer needed
  mrb_gc_unregister(mrb, a); // without this line the program will run correctly
  // only b will be kept
  test = b;
  return mrb_nil_value();
}

int main()
{
  mrb_state * mrb = mrb_open();
  RProc * proc = mrb_proc_new_cfunc(mrb, foo);
  mrb_define_method_raw(mrb, mrb->object_class, mrb_intern_cstr(mrb, "foo"), proc);
  mrb_load_string(mrb, "tap do ar = [1,2,3]; foo(ar, ar) end");
  mrb_garbage_collect(mrb);
  mrb_value state = mrb_funcall(mrb, test, "inspect", 0);
  mrb_funcall(mrb, mrb_top_self(mrb), "puts", 1, state);
  mrb_gc_unregister(mrb, test);
  mrb_close(mrb);
}

In theory I could ensure on C side that mrb_gc_register is only called once per object, but this is not as simple in general case.
So, could we modify behavior of mrb_gc_unregister so that it removes only one registration? Then it would be user responsibility to call mrb_gc_register and mrb_gc_unregister an equal number of times.