1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
|
# -*- coding: utf-8 -*-
# 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
import attr
from twisted.internet import defer
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 = {}
@attr.s(slots=True, frozen=True)
class Server(object):
"""
Our record of an individual server which can be tried to reach a destination.
Attributes:
host (bytes): target hostname
port (int):
priority (int):
weight (int):
expires (int): when the cache should expire this record - in *seconds* since
the epoch
"""
host = attr.ib()
port = attr.ib()
priority = attr.ib(default=0)
weight = attr.ib(default=0)
expires = attr.ib(default=0)
def pick_server_from_list(server_list):
"""Randomly choose a server from the server list
Args:
server_list (list[Server]): list of candidate servers
Returns:
Tuple[bytes, int]: (host, port) pair for the chosen server
"""
if not server_list:
raise RuntimeError("pick_server_from_list called with empty list")
# TODO: currently we only use the lowest-priority servers. We should maintain a
# cache of servers known to be "down" and filter them out
min_priority = min(s.priority for s in server_list)
eligible_servers = list(s for s in server_list if s.priority == min_priority)
total_weight = sum(s.weight for s in eligible_servers)
target_weight = random.randint(0, total_weight)
for s in eligible_servers:
target_weight -= s.weight
if target_weight <= 0:
return s.host, s.port
# this should be impossible.
raise RuntimeError("pick_server_from_list got to end of eligible server list.")
def _sort_server_list(server_list):
"""Given a list of SRV records sort them into priority order and shuffle
each priority with the given weight.
"""
priority_map = {}
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 follows the algorithm described in RFC2782.
#
# N.B. Weights can be zero, which means that you should pick that server
# last *or* that its the only server in this priority.
# We sort to ensure zero weighted items are first.
servers.sort(key=lambda s: s.weight)
total_weight = sum(s.weight for s in servers)
while servers:
target_weight = random.randint(0, 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
return results
class SrvResolver(object):
"""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 (dict): cache object
get_time (callable): clock implementation. Should return seconds since the epoch
"""
def __init__(self, dns_client=client, cache=SERVER_CACHE, get_time=time.time):
self._dns_client = dns_client
self._cache = cache
self._get_time = get_time
@defer.inlineCallbacks
def resolve_service(self, service_name):
"""Look up a SRV record
Args:
service_name (bytes): record to look up
Returns:
Deferred[list[Server]]:
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, _, _ = yield 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.warn(
"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("Service %s unavailable" % service_name)
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)
|
