//go:build linux

package tun

import (
	"encoding/binary"
	"os"
	"sync"

	"golang.org/x/sys/unix"
)

// vnetSize is the size of the virtio_net_hdr that a tun with IFF_VNET_HDR has in front of every packet,
// unless it was told another one
const vnetSize = 10

// fdTun reads and writes the packets of a tun that is a file descriptor
type fdTun struct {
	fd int
	// file is what packets are read from: a read waits in the poller of the runtime and ends when file is closed
	file *os.File
	// vnet is the size of the virtio_net_hdr in front of every packet, zero if the tun has none.
	// With it the system does not split what it sends to the tun and takes what is not split from it
	vnet int

	writeLock sync.RWMutex
	closed    bool
}

// newFdTun takes a file descriptor that does not block
func newFdTun(fd, vnet int) *fdTun {
	return &fdTun{fd: fd, file: os.NewFile(uintptr(fd), "/dev/net/tun"), vnet: vnet}
}

func (t *fdTun) ReadPacket(b []byte) ([]byte, error) {
	n, err := t.file.Read(b)
	// what the virtio_net_hdr says is of no use here: no checksum is looked at, and a segment may have any size
	return b[min(t.vnet, n):n], err
}

func (t *fdTun) WritePacket(header, payload []byte) error {
	// a virtio_net_hdr of zeros leaves nothing to the system
	var vnet [packetRoom]byte
	return t.writev(vnet[:t.vnet], header, payload)
}

// Segments implements segmenter
func (t *fdTun) Segments() bool {
	return t.vnet != 0
}

// WriteSegment implements segmenter
func (t *fdTun) WriteSegment(header []byte, payload [][]byte, mss int) error {
	// the virtio_net_hdr and the headers go as one part
	var b [packetRoom + ipv6HeaderSize + 60]byte
	ip, gso := ipv4HeaderSize, byte(unix.VIRTIO_NET_HDR_GSO_TCPV4)
	if header[0]>>4 == 6 {
		ip, gso = ipv6HeaderSize, unix.VIRTIO_NET_HDR_GSO_TCPV6
	}
	size := 0
	for _, p := range payload {
		size += len(p)
	}
	b[0] = unix.VIRTIO_NET_HDR_F_NEEDS_CSUM
	if size > mss {
		b[1] = gso
		binary.NativeEndian.PutUint16(b[4:], uint16(mss))
	}
	binary.NativeEndian.PutUint16(b[2:], uint16(len(header)))
	// where the checksum starts, and where it is put from there
	binary.NativeEndian.PutUint16(b[6:], uint16(ip))
	binary.NativeEndian.PutUint16(b[8:], 16)
	var parts [8][]byte
	return t.writev(append(append(parts[:0], b[:t.vnet+copy(b[t.vnet:], header)]), payload...)...)
}

func (t *fdTun) writev(parts ...[]byte) error {
	t.writeLock.RLock()
	defer t.writeLock.RUnlock()
	if t.closed {
		return os.ErrClosed
	}
	for {
		_, err := unix.Writev(t.fd, parts)
		if err != unix.EINTR {
			return err
		}
	}
}

func (t *fdTun) closeFile() error {
	t.writeLock.Lock()
	defer t.writeLock.Unlock()
	t.closed = true
	return t.file.Close()
}