340 lines
12 KiB
Python
340 lines
12 KiB
Python
# 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 logging
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import random
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import time
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from io import BytesIO
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from typing import Callable, Dict, Optional, Tuple
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import attr
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from twisted.internet import defer
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from twisted.internet.interfaces import IReactorTime
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from twisted.web.client import RedirectAgent
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from twisted.web.http import stringToDatetime
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from twisted.web.http_headers import Headers
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from twisted.web.iweb import IAgent, IResponse
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from synapse.http.client import BodyExceededMaxSize, read_body_with_max_size
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from synapse.logging.context import make_deferred_yieldable
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from synapse.util import Clock, json_decoder
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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 factor to add to the .well-known default cache ttls
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WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER = 0.1
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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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# period to cache failure to fetch .well-known if there has recently been a
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# valid well-known for that domain.
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WELL_KNOWN_DOWN_CACHE_PERIOD = 2 * 60
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# period to remember there was a valid well-known after valid record expires
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WELL_KNOWN_REMEMBER_DOMAIN_HAD_VALID = 2 * 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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# The maximum size (in bytes) to allow a well-known file to be.
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WELL_KNOWN_MAX_SIZE = 50 * 1024 # 50 KiB
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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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# Number of times we retry fetching a well-known for a domain we know recently
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# had a valid entry.
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WELL_KNOWN_RETRY_ATTEMPTS = 3
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logger = logging.getLogger(__name__)
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_well_known_cache: TTLCache[bytes, Optional[bytes]] = TTLCache("well-known")
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_had_valid_well_known_cache: TTLCache[bytes, bool] = TTLCache("had-valid-well-known")
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@attr.s(slots=True, frozen=True, auto_attribs=True)
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class WellKnownLookupResult:
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delegated_server: Optional[bytes]
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class WellKnownResolver:
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"""Handles well-known lookups for matrix servers."""
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def __init__(
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self,
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reactor: IReactorTime,
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agent: IAgent,
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user_agent: bytes,
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well_known_cache: Optional[TTLCache[bytes, Optional[bytes]]] = None,
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had_well_known_cache: Optional[TTLCache[bytes, bool]] = None,
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):
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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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if had_well_known_cache is None:
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had_well_known_cache = _had_valid_well_known_cache
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self._well_known_cache = well_known_cache
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self._had_valid_well_known_cache = had_well_known_cache
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self._well_known_agent = RedirectAgent(agent)
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self.user_agent = user_agent
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async def get_well_known(self, server_name: bytes) -> WellKnownLookupResult:
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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: name of the server, from the requested url
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Returns:
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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: Optional[bytes]
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cache_period: float
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result, cache_period = await self._fetch_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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if self._had_valid_well_known_cache.get(server_name, False):
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# We have recently seen a valid well-known record for this
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# server, so we cache the lack of well-known for a shorter time.
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cache_period = WELL_KNOWN_DOWN_CACHE_PERIOD
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else:
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cache_period = WELL_KNOWN_INVALID_CACHE_PERIOD
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# add some randomness to the TTL to avoid a stampeding herd
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cache_period *= random.uniform(
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1 - WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
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1 + WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
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)
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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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async def _fetch_well_known(self, server_name: bytes) -> Tuple[bytes, float]:
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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: 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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The lookup result and cache period.
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"""
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had_valid_well_known = self._had_valid_well_known_cache.get(server_name, False)
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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 permanent
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# errors (such as getting a 404 response).
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response, body = await self._make_well_known_request(
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server_name, retry=had_valid_well_known
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)
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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_decoder.decode(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 defer.CancelledError:
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# Bail if we've been cancelled
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raise
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except Exception as e:
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logger.info("Error parsing well-known for %s: %s", server_name, 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(
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1 - WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
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1 + WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER,
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)
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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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# We got a success, mark as such in the cache
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self._had_valid_well_known_cache.set(
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server_name,
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bool(result),
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cache_period + WELL_KNOWN_REMEMBER_DOMAIN_HAD_VALID,
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)
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return result, cache_period
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async def _make_well_known_request(
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self, server_name: bytes, retry: bool
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) -> Tuple[IResponse, bytes]:
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"""Make the well known request.
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This will retry the request if requested and it fails (with unable
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to connect or receives a 5xx error).
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Args:
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server_name: name of the server, from the requested url
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retry: Whether to retry the request if it fails.
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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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Returns the response object and body. Response may be a non-200 response.
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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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headers = {
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b"User-Agent": [self.user_agent],
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}
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i = 0
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while True:
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i += 1
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logger.info("Fetching %s", uri_str)
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try:
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response = await make_deferred_yieldable(
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self._well_known_agent.request(
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b"GET", uri, headers=Headers(headers)
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)
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)
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body_stream = BytesIO()
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await make_deferred_yieldable(
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read_body_with_max_size(response, body_stream, WELL_KNOWN_MAX_SIZE)
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)
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body = body_stream.getvalue()
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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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return response, body
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except defer.CancelledError:
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# Bail if we've been cancelled
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raise
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except BodyExceededMaxSize:
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# If the well-known file was too large, do not keep attempting
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# to download it, but consider it a temporary error.
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logger.warning(
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"Requested .well-known file for %s is too large > %r bytes",
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server_name.decode("ascii"),
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WELL_KNOWN_MAX_SIZE,
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)
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raise _FetchWellKnownFailure(temporary=True)
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except Exception as e:
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if not retry or i >= WELL_KNOWN_RETRY_ATTEMPTS:
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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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logger.info("Error fetching %s: %s. Retrying", uri_str, e)
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# Sleep briefly in the hopes that they come back up
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await self._clock.sleep(0.5)
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def _cache_period_from_headers(
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headers: Headers, time_now: Callable[[], float] = time.time
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) -> Optional[float]:
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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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max_age = cache_controls[b"max-age"]
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if max_age:
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try:
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return int(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: Headers) -> Dict[bytes, Optional[bytes]]:
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cache_controls = {}
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cache_control_headers = headers.getRawHeaders(b"cache-control") or []
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for hdr in cache_control_headers:
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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(slots=True)
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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: bool = attr.ib()
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