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# 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 Callable, Dict, 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: Dict[bytes, List["Server"]] = {}
@attr.s(auto_attribs=True, slots=True, frozen=True)
class Server:
"""
Our record of an individual server which can be tried to reach a destination.
Attributes:
host: target hostname
port:
priority:
weight:
expires: when the cache should expire this record - in *seconds* since
the epoch
"""
host: bytes
port: int
priority: int = 0
weight: int = 0
expires: int = 0
def _sort_server_list(server_list: List[Server]) -> List[Server]:
"""Given a list of SRV records sort them into priority order and shuffle
each priority with the given weight.
"""
priority_map: Dict[int, List[Server]] = {}
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:
"""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: cache object
get_time: clock implementation. Should return seconds since the epoch
"""
def __init__(
self,
dns_client=client,
cache: Dict[bytes, List[Server]] = SERVER_CACHE,
get_time: Callable[[], float] = 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: 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(f"Service {service_name!r} unavailable")
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)
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