text: # -*- coding: utf-8 -*-
  # Copyright 2014-2016 OpenMarket Ltd
  # Copyright 2019 New Vector Ltd
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  import logging
  import random
  import time
  from typing import List
  
  import attr
  
  from twisted.internet.error import ConnectError
  from twisted.names import client, dns
  from twisted.names.error import DNSNameError, DomainError
  
  from synapse.logging.context import make_deferred_yieldable
  
  logger = logging.getLogger(__name__)
  
  SERVER_CACHE = {}
  
  
  @attr.s(slots=True, frozen=True)
  class Server(object):
      """
      Our record of an individual server which can be tried to reach a destination.
  
      Attributes:
          host (bytes): target hostname
          port (int):
          priority (int):
          weight (int):
          expires (int): when the cache should expire this record - in *seconds* since
              the epoch
      """
  
      host = attr.ib()
      port = attr.ib()
      priority = attr.ib(default=0)
      weight = attr.ib(default=0)
      expires = attr.ib(default=0)
  
  
  def _sort_server_list(server_list):
      """Given a list of SRV records sort them into priority order and shuffle
      each priority with the given weight.
      """
      priority_map = {}
  
      for server in server_list:
          priority_map.setdefault(server.priority, []).append(server)
  
      results = []
      for priority in sorted(priority_map):
          servers = priority_map[priority]
  
          # This algorithms roughly follows the algorithm described in RFC2782,
          # changed to remove an off-by-one error.
          #
          # N.B. Weights can be zero, which means that they should be picked
          # rarely.
  
          total_weight = sum(s.weight for s in servers)
  
          # Total weight can become zero if there are only zero weight servers
          # left, which we handle by just shuffling and appending to the results.
          while servers and total_weight:
              target_weight = random.randint(1, total_weight)
  
              for s in servers:
                  target_weight -= s.weight
  
                  if target_weight <= 0:
                      break
  
              results.append(s)
              servers.remove(s)
              total_weight -= s.weight
  
          if servers:
              random.shuffle(servers)
              results.extend(servers)
  
      return results
  
  
  class SrvResolver(object):
      """Interface to the dns client to do SRV lookups, with result caching.
  
      The default resolver in twisted.names doesn't do any caching (it has a CacheResolver,
      but the cache never gets populated), so we add our own caching layer here.
  
      Args:
          dns_client (twisted.internet.interfaces.IResolver): twisted resolver impl
          cache (dict): cache object
          get_time (callable): clock implementation. Should return seconds since the epoch
      """
  
      def __init__(self, dns_client=client, cache=SERVER_CACHE, get_time=time.time):
          self._dns_client = dns_client
          self._cache = cache
          self._get_time = get_time
  
      async def resolve_service(self, service_name: bytes) -> List[Server]:
          """Look up a SRV record
  
          Args:
              service_name (bytes): record to look up
  
          Returns:
              a list of the SRV records, or an empty list if none found
          """
          now = int(self._get_time())
  
          if not isinstance(service_name, bytes):
              raise TypeError("%r is not a byte string" % (service_name,))
  
          cache_entry = self._cache.get(service_name, None)
          if cache_entry:
              if all(s.expires > now for s in cache_entry):
                  servers = list(cache_entry)
                  return _sort_server_list(servers)
  
          try:
              answers, _, _ = await make_deferred_yieldable(
                  self._dns_client.lookupService(service_name)
              )
          except DNSNameError:
              # TODO: cache this. We can get the SOA out of the exception, and use
              # the negative-TTL value.
              return []
          except DomainError as e:
              # We failed to resolve the name (other than a NameError)
              # Try something in the cache, else rereaise
              cache_entry = self._cache.get(service_name, None)
              if cache_entry:
                  logger.warning(
                      "Failed to resolve %r, falling back to cache. %r", service_name, e
                  )
                  return list(cache_entry)
              else:
                  raise e
  
          if (
              len(answers) == 1
              and answers[0].type == dns.SRV
              and answers[0].payload
              and answers[0].payload.target == dns.Name(b".")
          ):
              raise ConnectError("Service %s unavailable" % service_name)
  
          servers = []
  
          for answer in answers:
              if answer.type != dns.SRV or not answer.payload:
                  continue
  
              payload = answer.payload
  
              servers.append(
                  Server(
                      host=payload.target.name,
                      port=payload.port,
                      priority=payload.priority,
                      weight=payload.weight,
                      expires=now + answer.ttl,
                  )
              )
  
          self._cache[service_name] = list(servers)
          return _sort_server_list(servers)
  