code: func (r *Reference) resetValue() {
  	if r.finalizer != nil {
  		value, _ := r.get()
  		r.finalizer(value)
  	}
  	atomic.StorePointer(&r.ref, nil)
  }
docs: // resetValue is an expiration handler that calls the
  // finalizer and resets the reference to nil.
  // Note: This function is invoked from inside a write
  // lock so there's no need to lock