MonitorMixin#mon_initialize

# File tmp/rubies/ruby-3.0.5/ext/monitor/lib/monitor.rb, line 230
def mon_initialize
  if defined?(@mon_data)
    if defined?(@mon_initialized_by_new_cond)
      return # already initialized.
    elsif @mon_data_owner_object_id == self.object_id
      raise ThreadError, "already initialized"
    end
  end
  @mon_data = ::Monitor.new
  @mon_data_owner_object_id = self.object_id
end