#!/usr/bin/env python
"""Module to setup an RFC2136-capable DNS server"""
from __future__ import print_function

import os
import os.path
from pkg_resources import resource_filename
import shutil
import socket
import subprocess
import sys
import tempfile
import time


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(object):
    """
    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, nodes, show_output=False):
        """
        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 = 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.
        self._output = sys.stderr if show_output else open(os.devnull, 'w')

    def start(self):
        """Start the DNS server"""
        try:
            self._configure_bind()
            self._start_bind()
        except:
            self.stop()
            raise

    def stop(self):
        """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):
        """Configure the BIND9 server based on the prebaked configuration"""
        bind_conf_src = resource_filename('certbot_integration_tests', 'assets/bind-config')
        for dir in ('conf', 'zones'):
          shutil.copytree(os.path.join(bind_conf_src, dir), os.path.join(self.bind_root, dir))

    def _start_bind(self):
        """Launch the BIND9 server as a Docker container"""
        addr_str = '{}:{}'.format(BIND_BIND_ADDRESS[0], BIND_BIND_ADDRESS[1])
        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("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=30):
      # type: (int) -> 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.
      """
      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:
          # If there was a network error, wait a little
          time.sleep(1)
          pass
        finally:
          sock.close()

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

    def __enter__(self):
        self.start()
        return self.dns_xdist

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.stop()