pcmu sync.RWMutex
	PacketConn
	done chan bool // signal that indicates server stopped
}

func (ls *localPacketServer) buildup(handler func(*localPacketServer, PacketConn)) error {
	go func() {
		handler(ls, ls.PacketConn)
		close(ls.done)
	}()
	return nil
}

func (ls *localPacketServer) teardown() error {
	ls.pcmu.Lock()
	if ls.PacketConn != nil {

		return &TCPListener{fd: fd}, nil
	case *UnixAddr:
		return &UnixListener{fd: fd, path: laddr.Name, unlink: false}, nil
	}
	fd.Close()
	return nil, syscall.EINVAL
}

func filePacketConn(f *os.File) (PacketConn, error) {
	fd, err := newFileFD(f)
	if err != nil {
		return nil, err
	}
	switch fd.laddr.(type) {
	case *UDPAddr:
		return newUDPConn(fd), nil
	case *IPAddr:
		return newIPConn(fd), nil

	if err != nil {
		return nil, err
	}
	if s != "Listen" {
		return nil, errors.New("file does not represent a listener")
	}

	return &TCPListener{fd: fd}, nil
}

func filePacketConn(f *os.File) (PacketConn, error) {
	return nil, syscall.EPLAN9

	addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
	if err != nil {
		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
	}
	sl := &sysListener{
		ListenConfig: *lc,
		network:      network,
		address:      address,
	}
	var c PacketConn
	la := addrs.first(isIPv4)
	switch la := la.(type) {

	case <-up:
	}
	t.Cleanup(func() { _ = server.Shutdown() })
	server.Addr = server.PacketConn.LocalAddr().String()

	select {
	case <-time.After(time.Second * 10):
		t.Fatalf("setup timeout")
	case <-up:
	}
	t.Cleanup(func() { _ = tcp.Shutdown() })
	t.Cleanup(func() { _ = server.Shutdown() })
	tcp.Addr = server.PacketConn.LocalAddr().String()
	return server.Addr
}

	if err != nil {
		err = &OpError{Op: "file", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return
}

// PacketConn is a generic packet-oriented network connection.
//
// Multiple goroutines may invoke methods on a PacketConn simultaneously.
type PacketConn interface {
	// ReadFrom reads a packet from the connection,
	// copying the payload into p. It returns the number of

// loopback IP address or local file system path.
//
// The provided network must be "udp", "udp4", "udp6" or "unixgram".
func NewLocalPacketListener(network string) (net.PacketConn, error) {
	stackOnce.Do(probeStack)
	switch network {
	case "udp":
		if canListenTCP4OnLoopback {
			if c, err := net.ListenPacket("udp4", "127.0.0.1:0"); err == nil {
				return c, nil
			}
		}

			return dnsmessage.Parser{}, dnsmessage.Header{}, err
		}
		if d, ok := ctx.Deadline(); ok && !d.IsZero() {
			c.SetDeadline(d)
		}
		var p dnsmessage.Parser
		var h dnsmessage.Header
		if _, ok := c.(PacketConn); ok {
			p, h, err = dnsPacketRoundTrip(c, id, q, udpReq)
		} else {
			p, h, err = dnsStreamRoundTrip(c, id, q, tcpReq)
		}
		c.Close()
		if err != nil {

	// always be a literal IP address and not a host name, and the
	// port in the address parameter will be a literal port number
	// and not a service name.
	// If the Conn returned is also a PacketConn, sent and received DNS
	// messages must adhere to RFC 1035 section 4.2.1, "UDP usage".
	// Otherwise, DNS messages transmitted over Conn must adhere
	// to RFC 7766 section 5, "Transport Protocol Selection".

	}
	return len(b), nil
}

func (f *fakeDNSConn) SetDeadline(t time.Time) error {
	f.t = t
	return nil
}

type fakeDNSPacketConn struct {
	PacketConn
	fakeDNSConn
}

func (f *fakeDNSPacketConn) SetDeadline(t time.Time) error {
	return f.fakeDNSConn.SetDeadline(t)
}

func (f *fakeDNSPacketConn) Close() error {
	return f.fakeDNSConn.Close()
}