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17e1e80726
This gives a bit of a grace period where we can attempt to refetch a remote `well-known`, while still using the cached result if that fails. Hopefully this will make the well-known resolution a bit more torelant of failures, rather than it immediately treating failures as "no result" and caching that for an hour.
226 lines
7.7 KiB
Python
226 lines
7.7 KiB
Python
# -*- coding: utf-8 -*-
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# Copyright 2019 The Matrix.org Foundation C.I.C.
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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 json
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import logging
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import random
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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.web.client import RedirectAgent, readBody
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from twisted.web.http import stringToDatetime
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from synapse.logging.context import make_deferred_yieldable
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from synapse.util import Clock
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from synapse.util.caches.ttlcache import TTLCache
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from synapse.util.metrics import Measure
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# period to cache .well-known results for by default
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WELL_KNOWN_DEFAULT_CACHE_PERIOD = 24 * 3600
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# jitter to add to the .well-known default cache ttl
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WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER = 10 * 60
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# period to cache failure to fetch .well-known for
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WELL_KNOWN_INVALID_CACHE_PERIOD = 1 * 3600
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# cap for .well-known cache period
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WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600
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# lower bound for .well-known cache period
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WELL_KNOWN_MIN_CACHE_PERIOD = 5 * 60
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# Attempt to refetch a cached well-known N% of the TTL before it expires.
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# e.g. if set to 0.2 and we have a cached entry with a TTL of 5mins, then
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# we'll start trying to refetch 1 minute before it expires.
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WELL_KNOWN_GRACE_PERIOD_FACTOR = 0.2
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logger = logging.getLogger(__name__)
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_well_known_cache = TTLCache("well-known")
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@attr.s(slots=True, frozen=True)
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class WellKnownLookupResult(object):
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delegated_server = attr.ib()
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class WellKnownResolver(object):
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"""Handles well-known lookups for matrix servers.
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"""
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def __init__(self, reactor, agent, well_known_cache=None):
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self._reactor = reactor
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self._clock = Clock(reactor)
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if well_known_cache is None:
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well_known_cache = _well_known_cache
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self._well_known_cache = well_known_cache
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self._well_known_agent = RedirectAgent(agent)
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@defer.inlineCallbacks
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def get_well_known(self, server_name):
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"""Attempt to fetch and parse a .well-known file for the given server
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Args:
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server_name (bytes): name of the server, from the requested url
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Returns:
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Deferred[WellKnownLookupResult]: The result of the lookup
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"""
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try:
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prev_result, expiry, ttl = self._well_known_cache.get_with_expiry(
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server_name
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)
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now = self._clock.time()
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if now < expiry - WELL_KNOWN_GRACE_PERIOD_FACTOR * ttl:
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return WellKnownLookupResult(delegated_server=prev_result)
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except KeyError:
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prev_result = None
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# TODO: should we linearise so that we don't end up doing two .well-known
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# requests for the same server in parallel?
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try:
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with Measure(self._clock, "get_well_known"):
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result, cache_period = yield self._do_get_well_known(server_name)
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except _FetchWellKnownFailure as e:
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if prev_result and e.temporary:
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# This is a temporary failure and we have a still valid cached
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# result, so lets return that. Hopefully the next time we ask
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# the remote will be back up again.
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return WellKnownLookupResult(delegated_server=prev_result)
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result = None
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# add some randomness to the TTL to avoid a stampeding herd every hour
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# after startup
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cache_period = WELL_KNOWN_INVALID_CACHE_PERIOD
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cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
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if cache_period > 0:
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self._well_known_cache.set(server_name, result, cache_period)
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return WellKnownLookupResult(delegated_server=result)
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@defer.inlineCallbacks
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def _do_get_well_known(self, server_name):
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"""Actually fetch and parse a .well-known, without checking the cache
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Args:
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server_name (bytes): name of the server, from the requested url
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Raises:
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_FetchWellKnownFailure if we fail to lookup a result
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Returns:
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Deferred[Tuple[bytes,int]]: The lookup result and cache period.
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"""
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uri = b"https://%s/.well-known/matrix/server" % (server_name,)
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uri_str = uri.decode("ascii")
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logger.info("Fetching %s", uri_str)
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# We do this in two steps to differentiate between possibly transient
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# errors (e.g. can't connect to host, 503 response) and more permenant
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# errors (such as getting a 404 response).
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try:
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response = yield make_deferred_yieldable(
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self._well_known_agent.request(b"GET", uri)
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)
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body = yield make_deferred_yieldable(readBody(response))
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if 500 <= response.code < 600:
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raise Exception("Non-200 response %s" % (response.code,))
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except Exception as e:
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logger.info("Error fetching %s: %s", uri_str, e)
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raise _FetchWellKnownFailure(temporary=True)
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try:
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if response.code != 200:
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raise Exception("Non-200 response %s" % (response.code,))
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parsed_body = json.loads(body.decode("utf-8"))
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logger.info("Response from .well-known: %s", parsed_body)
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result = parsed_body["m.server"].encode("ascii")
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except Exception as e:
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logger.info("Error fetching %s: %s", uri_str, e)
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raise _FetchWellKnownFailure(temporary=False)
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cache_period = _cache_period_from_headers(
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response.headers, time_now=self._reactor.seconds
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)
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if cache_period is None:
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cache_period = WELL_KNOWN_DEFAULT_CACHE_PERIOD
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# add some randomness to the TTL to avoid a stampeding herd every 24 hours
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# after startup
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cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
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else:
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cache_period = min(cache_period, WELL_KNOWN_MAX_CACHE_PERIOD)
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cache_period = max(cache_period, WELL_KNOWN_MIN_CACHE_PERIOD)
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return (result, cache_period)
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def _cache_period_from_headers(headers, time_now=time.time):
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cache_controls = _parse_cache_control(headers)
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if b"no-store" in cache_controls:
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return 0
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if b"max-age" in cache_controls:
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try:
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max_age = int(cache_controls[b"max-age"])
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return max_age
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except ValueError:
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pass
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expires = headers.getRawHeaders(b"expires")
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if expires is not None:
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try:
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expires_date = stringToDatetime(expires[-1])
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return expires_date - time_now()
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except ValueError:
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# RFC7234 says 'A cache recipient MUST interpret invalid date formats,
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# especially the value "0", as representing a time in the past (i.e.,
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# "already expired").
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return 0
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return None
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def _parse_cache_control(headers):
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cache_controls = {}
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for hdr in headers.getRawHeaders(b"cache-control", []):
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for directive in hdr.split(b","):
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splits = [x.strip() for x in directive.split(b"=", 1)]
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k = splits[0].lower()
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v = splits[1] if len(splits) > 1 else None
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cache_controls[k] = v
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return cache_controls
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@attr.s()
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class _FetchWellKnownFailure(Exception):
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# True if we didn't get a non-5xx HTTP response, i.e. this may or may not be
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# a temporary failure.
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temporary = attr.ib()
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