package listenfd

import (
	"net"
	"os"
	"strconv"

	"github.com/pkg/errors"
)

const fdStart = 3

func GetListener(i uint64) (net.Listener, error) {
	lfds, present := os.LookupEnv("LISTEN_FDS")
	if !present {
		return nil, nil
	}

	fds, err := strconv.ParseUint(lfds, 10, 32)
	if err != nil {
		return nil, errors.Wrap(err, "could not parse LISTEN_FDS")
	}
	if i >= fds {
		return nil, errors.Errorf("only %d fds available, requested index %d", fds, i)
	}

	l, err := net.FileListener(os.NewFile(uintptr(i+fdStart), ""))
	if err != nil {
		return nil, errors.Wrap(err, "could not create listener")
	}

	return l, nil
}