mirror of
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125 lines
4.0 KiB
Python
125 lines
4.0 KiB
Python
# -*- coding: utf-8 -*-
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# Copyright 2014-2016 OpenMarket Ltd
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# Copyright 2019 New Vector Ltd
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import logging
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import time
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import attr
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from twisted.internet import defer
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from twisted.internet.error import ConnectError
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from twisted.names import client, dns
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from twisted.names.error import DNSNameError, DomainError
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from synapse.util.logcontext import make_deferred_yieldable
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logger = logging.getLogger(__name__)
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SERVER_CACHE = {}
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@attr.s
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class Server(object):
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"""
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Our record of an individual server which can be tried to reach a destination.
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Attributes:
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host (bytes): target hostname
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port (int):
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priority (int):
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weight (int):
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expires (int): when the cache should expire this record - in *seconds* since
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the epoch
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"""
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host = attr.ib()
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port = attr.ib()
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priority = attr.ib(default=0)
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weight = attr.ib(default=0)
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expires = attr.ib(default=0)
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@defer.inlineCallbacks
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def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=time):
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"""Look up a SRV record, with caching
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The default resolver in twisted.names doesn't do any caching (it has a CacheResolver,
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but the cache never gets populated), so we add our own caching layer here.
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Args:
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service_name (unicode|bytes): record to look up
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dns_client (twisted.internet.interfaces.IResolver): twisted resolver impl
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cache (dict): cache object
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clock (object): clock implementation. must provide a time() method.
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Returns:
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Deferred[list[Server]]: a list of the SRV records, or an empty list if none found
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"""
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# TODO: the dns client handles both unicode names (encoding via idna) and pre-encoded
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# byteses; however they will obviously end up as separate entries in the cache. We
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# should pick one form and stick with it.
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cache_entry = cache.get(service_name, None)
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if cache_entry:
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if all(s.expires > int(clock.time()) for s in cache_entry):
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servers = list(cache_entry)
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defer.returnValue(servers)
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try:
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answers, _, _ = yield make_deferred_yieldable(
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dns_client.lookupService(service_name),
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)
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except DNSNameError:
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# TODO: cache this. We can get the SOA out of the exception, and use
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# the negative-TTL value.
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defer.returnValue([])
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except DomainError as e:
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# We failed to resolve the name (other than a NameError)
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# Try something in the cache, else rereaise
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cache_entry = cache.get(service_name, None)
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if cache_entry:
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logger.warn(
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"Failed to resolve %r, falling back to cache. %r",
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service_name, e
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)
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defer.returnValue(list(cache_entry))
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else:
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raise e
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if (len(answers) == 1
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and answers[0].type == dns.SRV
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and answers[0].payload
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and answers[0].payload.target == dns.Name(b'.')):
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raise ConnectError("Service %s unavailable" % service_name)
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servers = []
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for answer in answers:
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if answer.type != dns.SRV or not answer.payload:
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continue
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payload = answer.payload
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servers.append(Server(
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host=payload.target.name,
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port=payload.port,
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priority=payload.priority,
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weight=payload.weight,
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expires=int(clock.time()) + answer.ttl,
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))
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servers.sort() # FIXME: get rid of this (it's broken by the attrs change)
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cache[service_name] = list(servers)
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defer.returnValue(servers)
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