#!/usr/bin/env python
"""Module to setup an RFC2136-capable DNS server"""
import os
import os.path
import shutil
import socket
import subprocess
import sys
import tempfile
import time
from types import TracebackType
from typing import Any, Sequence
from typing import Dict
from typing import Optional
from typing import Type

from pkg_resources import resource_filename

BIND_DOCKER_IMAGE = "internetsystemsconsortium/bind9:9.16"
BIND_BIND_ADDRESS = ("127.0.0.1", 45953)

# A TCP DNS message which is a query for '. CH A' transaction ID 0xcb37. This is used
# by _wait_until_ready to check that BIND is responding without depending on dnspython.
BIND_TEST_QUERY = bytearray.fromhex("0011cb37000000010000000000000000010003")


class DNSServer:
    """
    DNSServer configures and handles the lifetime of an RFC2136-capable server.
    DNServer provides access to the dns_xdist parameter, listing the address and port
    to use for each pytest node.

    At this time, DNSServer should only be used with a single node, but may be expanded in
    future to support parallelization (https://github.com/certbot/certbot/issues/8455).
    """

    def __init__(self, unused_nodes: Sequence[str], show_output: bool = False) -> None:
        """
        Create an DNSServer instance.
        :param list nodes: list of node names that will be setup by pytest xdist
        :param bool show_output: if True, print the output of the DNS server
        """

        self.bind_root = tempfile.mkdtemp()

        self.process: Optional[subprocess.Popen] = None

        self.dns_xdist = {"address": BIND_BIND_ADDRESS[0], "port": BIND_BIND_ADDRESS[1]}

        # Unfortunately the BIND9 image forces everything to stderr with -g and we can't
        # modify the verbosity.
        # pylint: disable=consider-using-with
        self._output = sys.stderr if show_output else open(os.devnull, "w")

    def start(self) -> None:
        """Start the DNS server"""
        try:
            self._configure_bind()
            self._start_bind()
        except:
            self.stop()
            raise

    def stop(self) -> None:
        """Stop the DNS server, and clean its resources"""
        if self.process:
            try:
                self.process.terminate()
                self.process.wait()
            except BaseException as e:
                print("BIND9 did not stop cleanly: {}".format(e), file=sys.stderr)

        shutil.rmtree(self.bind_root, ignore_errors=True)

        if self._output != sys.stderr:
            self._output.close()

    def _configure_bind(self) -> None:
        """Configure the BIND9 server based on the prebaked configuration"""
        bind_conf_src = resource_filename(
            "certbot_integration_tests", "assets/bind-config"
        )
        for directory in ("conf", "zones"):
            shutil.copytree(
                os.path.join(bind_conf_src, directory), os.path.join(self.bind_root, directory)
            )

    def _start_bind(self) -> None:
        """Launch the BIND9 server as a Docker container"""
        addr_str = "{}:{}".format(BIND_BIND_ADDRESS[0], BIND_BIND_ADDRESS[1])
        # pylint: disable=consider-using-with
        self.process = subprocess.Popen(
            [
                "docker",
                "run",
                "--rm",
                "-p",
                "{}:53/udp".format(addr_str),
                "-p",
                "{}:53/tcp".format(addr_str),
                "-v",
                "{}/conf:/etc/bind".format(self.bind_root),
                "-v",
                "{}/zones:/var/lib/bind".format(self.bind_root),
                BIND_DOCKER_IMAGE,
            ],
            stdout=self._output,
            stderr=self._output,
        )

        if self.process.poll():
            raise ValueError("BIND9 server stopped unexpectedly")

        try:
            self._wait_until_ready()
        except:
            # The container might be running even if we think it isn't
            self.stop()
            raise

    def _wait_until_ready(self, attempts: int = 30) -> None:
        """
        Polls the DNS server over TCP until it gets a response, or until
        it runs out of attempts and raises a ValueError.
        The DNS response message must match the txn_id of the DNS query message,
        but otherwise the contents are ignored.
        :param int attempts: The number of attempts to make.
        """
        if not self.process:
            raise ValueError("DNS server has not been started. Please run start() first.")

        for _ in range(attempts):
            if self.process.poll():
                raise ValueError("BIND9 server stopped unexpectedly")

            sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            sock.settimeout(5.0)
            try:
                sock.connect(BIND_BIND_ADDRESS)
                sock.sendall(BIND_TEST_QUERY)
                buf = sock.recv(1024)
                # We should receive a DNS message with the same tx_id
                if buf and len(buf) > 4 and buf[2:4] == BIND_TEST_QUERY[2:4]:
                    return
                # If we got a response but it wasn't the one we wanted, wait a little
                time.sleep(1)
            except:  # pylint: disable=bare-except
                # If there was a network error, wait a little
                time.sleep(1)
            finally:
                sock.close()

        raise ValueError(
            "Gave up waiting for DNS server {} to respond".format(BIND_BIND_ADDRESS)
        )

    def __start__(self) -> Dict[str, Any]:
        self.start()
        return self.dns_xdist

    def __exit__(self, exc_type: Optional[Type[BaseException]], exc: Optional[BaseException],
                 traceback: Optional[TracebackType]) -> None:
        self.stop()