from __future__ import annotations

import fcntl
import os
import struct
import subprocess
import threading

TUNSETIFF = 0x400454CA
IFF_TAP = 0x0002
IFF_NO_PI = 0x1000


class TapDevice:
    def __init__(self, name: str, mtu: int = 9000) -> None:
        self.name = name
        self.mtu = mtu
        self.fd: int | None = None
        self._lock = threading.Lock()

    def open(self) -> None:
        if self.fd is not None:
            return
        fd = os.open("/dev/net/tun", os.O_RDWR)
        ifreq = struct.pack("16sH22x", self.name.encode("ascii"), IFF_TAP | IFF_NO_PI)
        fcntl.ioctl(fd, TUNSETIFF, ifreq)
        subprocess.run(["ip", "link", "set", "dev", self.name, "mtu", str(self.mtu)], check=True)
        subprocess.run(["ip", "link", "set", "dev", self.name, "up"], check=True)
        self.fd = fd

    def write(self, frame: bytes) -> int:
        if self.fd is None:
            raise RuntimeError("TAP is not open")
        with self._lock:
            return os.write(self.fd, frame)

    def close(self) -> None:
        if self.fd is not None:
            try:
                os.close(self.fd)
            finally:
                self.fd = None