forked-synapse/synapse/http/federation/well_known_resolver.py

314 lines
11 KiB
Python

# -*- coding: utf-8 -*-
# Copyright 2019 The Matrix.org Foundation C.I.C.
#
# 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
import attr
from twisted.internet import defer
from twisted.web.client import RedirectAgent, readBody
from twisted.web.http import stringToDatetime
from twisted.web.http_headers import Headers
from synapse.logging.context import make_deferred_yieldable
from synapse.util import Clock, json_decoder
from synapse.util.caches.ttlcache import TTLCache
from synapse.util.metrics import Measure
# period to cache .well-known results for by default
WELL_KNOWN_DEFAULT_CACHE_PERIOD = 24 * 3600
# jitter factor to add to the .well-known default cache ttls
WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER = 0.1
# period to cache failure to fetch .well-known for
WELL_KNOWN_INVALID_CACHE_PERIOD = 1 * 3600
# period to cache failure to fetch .well-known if there has recently been a
# valid well-known for that domain.
WELL_KNOWN_DOWN_CACHE_PERIOD = 2 * 60
# period to remember there was a valid well-known after valid record expires
WELL_KNOWN_REMEMBER_DOMAIN_HAD_VALID = 2 * 3600
# cap for .well-known cache period
WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600
# lower bound for .well-known cache period
WELL_KNOWN_MIN_CACHE_PERIOD = 5 * 60
# Attempt to refetch a cached well-known N% of the TTL before it expires.
# e.g. if set to 0.2 and we have a cached entry with a TTL of 5mins, then
# we'll start trying to refetch 1 minute before it expires.
WELL_KNOWN_GRACE_PERIOD_FACTOR = 0.2
# Number of times we retry fetching a well-known for a domain we know recently
# had a valid entry.
WELL_KNOWN_RETRY_ATTEMPTS = 3
logger = logging.getLogger(__name__)
_well_known_cache = TTLCache("well-known")
_had_valid_well_known_cache = TTLCache("had-valid-well-known")
@attr.s(slots=True, frozen=True)
class WellKnownLookupResult(object):
delegated_server = attr.ib()
class WellKnownResolver(object):
"""Handles well-known lookups for matrix servers.
"""
def __init__(
self,
reactor,
agent,
user_agent,
well_known_cache=None,
had_well_known_cache=None,
):
self._reactor = reactor
self._clock = Clock(reactor)
if well_known_cache is None:
well_known_cache = _well_known_cache
if had_well_known_cache is None:
had_well_known_cache = _had_valid_well_known_cache
self._well_known_cache = well_known_cache
self._had_valid_well_known_cache = had_well_known_cache
self._well_known_agent = RedirectAgent(agent)
self.user_agent = user_agent
@defer.inlineCallbacks
def get_well_known(self, server_name):
"""Attempt to fetch and parse a .well-known file for the given server
Args:
server_name (bytes): name of the server, from the requested url
Returns:
Deferred[WellKnownLookupResult]: The result of the lookup
"""
try:
prev_result, expiry, ttl = self._well_known_cache.get_with_expiry(
server_name
)
now = self._clock.time()
if now < expiry - WELL_KNOWN_GRACE_PERIOD_FACTOR * ttl:
return WellKnownLookupResult(delegated_server=prev_result)
except KeyError:
prev_result = None
# TODO: should we linearise so that we don't end up doing two .well-known
# requests for the same server in parallel?
try:
with Measure(self._clock, "get_well_known"):
result, cache_period = yield self._fetch_well_known(server_name)
except _FetchWellKnownFailure as e:
if prev_result and e.temporary:
# This is a temporary failure and we have a still valid cached
# result, so lets return that. Hopefully the next time we ask
# the remote will be back up again.
return WellKnownLookupResult(delegated_server=prev_result)
result = None
if self._had_valid_well_known_cache.get(server_name, False):
# We have recently seen a valid well-known record for this
# server, so we cache the lack of well-known for a shorter time.
cache_period = WELL_KNOWN_DOWN_CACHE_PERIOD
else:
cache_period = WELL_KNOWN_INVALID_CACHE_PERIOD
# add some randomness to the TTL to avoid a stampeding herd
cache_period *= random.uniform(
1 - WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
1 + WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
)
if cache_period > 0:
self._well_known_cache.set(server_name, result, cache_period)
return WellKnownLookupResult(delegated_server=result)
@defer.inlineCallbacks
def _fetch_well_known(self, server_name):
"""Actually fetch and parse a .well-known, without checking the cache
Args:
server_name (bytes): name of the server, from the requested url
Raises:
_FetchWellKnownFailure if we fail to lookup a result
Returns:
Deferred[Tuple[bytes,int]]: The lookup result and cache period.
"""
had_valid_well_known = self._had_valid_well_known_cache.get(server_name, False)
# We do this in two steps to differentiate between possibly transient
# errors (e.g. can't connect to host, 503 response) and more permenant
# errors (such as getting a 404 response).
response, body = yield self._make_well_known_request(
server_name, retry=had_valid_well_known
)
try:
if response.code != 200:
raise Exception("Non-200 response %s" % (response.code,))
parsed_body = json_decoder.decode(body.decode("utf-8"))
logger.info("Response from .well-known: %s", parsed_body)
result = parsed_body["m.server"].encode("ascii")
except defer.CancelledError:
# Bail if we've been cancelled
raise
except Exception as e:
logger.info("Error parsing well-known for %s: %s", server_name, e)
raise _FetchWellKnownFailure(temporary=False)
cache_period = _cache_period_from_headers(
response.headers, time_now=self._reactor.seconds
)
if cache_period is None:
cache_period = WELL_KNOWN_DEFAULT_CACHE_PERIOD
# add some randomness to the TTL to avoid a stampeding herd every 24 hours
# after startup
cache_period *= random.uniform(
1 - WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
1 + WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
)
else:
cache_period = min(cache_period, WELL_KNOWN_MAX_CACHE_PERIOD)
cache_period = max(cache_period, WELL_KNOWN_MIN_CACHE_PERIOD)
# We got a success, mark as such in the cache
self._had_valid_well_known_cache.set(
server_name,
bool(result),
cache_period + WELL_KNOWN_REMEMBER_DOMAIN_HAD_VALID,
)
return result, cache_period
@defer.inlineCallbacks
def _make_well_known_request(self, server_name, retry):
"""Make the well known request.
This will retry the request if requested and it fails (with unable
to connect or receives a 5xx error).
Args:
server_name (bytes)
retry (bool): Whether to retry the request if it fails.
Returns:
Deferred[tuple[IResponse, bytes]] Returns the response object and
body. Response may be a non-200 response.
"""
uri = b"https://%s/.well-known/matrix/server" % (server_name,)
uri_str = uri.decode("ascii")
headers = {
b"User-Agent": [self.user_agent],
}
i = 0
while True:
i += 1
logger.info("Fetching %s", uri_str)
try:
response = yield make_deferred_yieldable(
self._well_known_agent.request(
b"GET", uri, headers=Headers(headers)
)
)
body = yield make_deferred_yieldable(readBody(response))
if 500 <= response.code < 600:
raise Exception("Non-200 response %s" % (response.code,))
return response, body
except defer.CancelledError:
# Bail if we've been cancelled
raise
except Exception as e:
if not retry or i >= WELL_KNOWN_RETRY_ATTEMPTS:
logger.info("Error fetching %s: %s", uri_str, e)
raise _FetchWellKnownFailure(temporary=True)
logger.info("Error fetching %s: %s. Retrying", uri_str, e)
# Sleep briefly in the hopes that they come back up
yield self._clock.sleep(0.5)
def _cache_period_from_headers(headers, time_now=time.time):
cache_controls = _parse_cache_control(headers)
if b"no-store" in cache_controls:
return 0
if b"max-age" in cache_controls:
try:
max_age = int(cache_controls[b"max-age"])
return max_age
except ValueError:
pass
expires = headers.getRawHeaders(b"expires")
if expires is not None:
try:
expires_date = stringToDatetime(expires[-1])
return expires_date - time_now()
except ValueError:
# RFC7234 says 'A cache recipient MUST interpret invalid date formats,
# especially the value "0", as representing a time in the past (i.e.,
# "already expired").
return 0
return None
def _parse_cache_control(headers):
cache_controls = {}
for hdr in headers.getRawHeaders(b"cache-control", []):
for directive in hdr.split(b","):
splits = [x.strip() for x in directive.split(b"=", 1)]
k = splits[0].lower()
v = splits[1] if len(splits) > 1 else None
cache_controls[k] = v
return cache_controls
@attr.s()
class _FetchWellKnownFailure(Exception):
# True if we didn't get a non-5xx HTTP response, i.e. this may or may not be
# a temporary failure.
temporary = attr.ib()