Refactor and bugfix for resove_service (#4427)

synapse/http/federation/srv_resolver.py

@@ -0,0 +1,124 @@
+# -*- 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 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.util.logcontext import make_deferred_yieldable
+
+logger = logging.getLogger(__name__)
+
+SERVER_CACHE = {}
+
+
+@attr.s
+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)
+
+
+@defer.inlineCallbacks
+def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=time):
+    """Look up a SRV record, with 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:
+        service_name (unicode|bytes): record to look up
+        dns_client (twisted.internet.interfaces.IResolver): twisted resolver impl
+        cache (dict): cache object
+        clock (object): clock implementation. must provide a time() method.
+
+    Returns:
+        Deferred[list[Server]]: a list of the SRV records, or an empty list if none found
+    """
+    # TODO: the dns client handles both unicode names (encoding via idna) and pre-encoded
+    # byteses; however they will obviously end up as separate entries in the cache. We
+    # should pick one form and stick with it.
+    cache_entry = cache.get(service_name, None)
+    if cache_entry:
+        if all(s.expires > int(clock.time()) for s in cache_entry):
+            servers = list(cache_entry)
+            defer.returnValue(servers)
+
+    try:
+        answers, _, _ = yield make_deferred_yieldable(
+            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.
+        defer.returnValue([])
+    except DomainError as e:
+        # We failed to resolve the name (other than a NameError)
+        # Try something in the cache, else rereaise
+        cache_entry = cache.get(service_name, None)
+        if cache_entry:
+            logger.warn(
+                "Failed to resolve %r, falling back to cache. %r",
+                service_name, e
+            )
+            defer.returnValue(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=int(clock.time()) + answer.ttl,
+        ))
+
+    servers.sort()  # FIXME: get rid of this (it's broken by the attrs change)
+    cache[service_name] = list(servers)
+    defer.returnValue(servers)