// SubnetStateRefreshFunc returns a resource.StateRefreshFunc that is used to watch
// a Subnet.
func SubnetStateRefreshFunc(conn *ec2.EC2, id string) resource.StateRefreshFunc {
	return func() (interface{}, string, error) {
		resp, err := conn.DescribeSubnets([]string{id}, ec2.NewFilter())
		if err != nil {
			if ec2err, ok := err.(*ec2.Error); ok && ec2err.Code == "InvalidSubnetID.NotFound" {
				resp = nil
			} else {
				log.Printf("Error on SubnetStateRefresh: %s", err)
				return nil, "", err
			}
		}

		if resp == nil {
			// Sometimes AWS just has consistency issues and doesn't see
			// our instance yet. Return an empty state.
			return nil, "", nil
		}

		subnet := &resp.Subnets[0]
		return subnet, subnet.State, nil
	}
}