Ensure Generator::State is kept on the stack

[byroot]

Fix: #929

I don't know how it's possible, but somehow the self reference is optimized out of the stack. Perhaps because we inline so aggressively that the compiler end up spilling it on the heap.

Actually I get it now, the actual caller is:

static VALUE mHash_to_json(int argc, VALUE *argv, VALUE self)
{
    rb_check_arity(argc, 0, 1);
    VALUE Vstate = cState_from_state_s(cState, argc == 1 ? argv[0] : Qnil);
    return cState_partial_generate(Vstate, self, generate_json_object, Qfalse);
}

So the GCed variable is VState which indeed is no longer reachable rather quickly. So the fix does checks out.

Repro:

require 'json'

test_data = {
  "flag" => true,
  "data" => 10000.times.map { [1.0] },
  :flag => false,
}

10.times do
  test_data.to_json
end

But in practice the cause was just that the issued warning calls Hash#inspect on a big hash, which triggers GC.

So it can be triggered even more reliably with:

require 'json'

module JSON
  module Common
    def self.on_mixed_keys_hash(...)
      GC.start
    end
  end
end

test_data = {
  "flag" => true,
  "data" => 10000.times.map { [1.0] },
  :flag => false,
}

test_data.to_json