Refactor and bugfix for resove_service (#4427)

6 files changed, 250 insertions, 86 deletions

diff --git a/changelog.d/4427.misc b/changelog.d/4427.misc
new file mode 100644
index 0000000000..75500bdbc2
--- /dev/null
+++ b/changelog.d/4427.misc
@@ -0,0 +1 @@
+Refactor and cleanup for SRV record lookup
diff --git a/synapse/http/endpoint.py b/synapse/http/endpoint.py
index 1c3b7ea28a..815f8ff2f7 100644
--- a/synapse/http/endpoint.py
+++ b/synapse/http/endpoint.py
@@ -12,29 +12,17 @@
 # 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 collections
 import logging
 import random
 import re
-import time
 
 from twisted.internet import defer
 from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
 from twisted.internet.error import ConnectError
-from twisted.names import client, dns
-from twisted.names.error import DNSNameError, DomainError
 
-logger = logging.getLogger(__name__)
-
-SERVER_CACHE = {}
+from synapse.http.federation.srv_resolver import Server, resolve_service
 
-# our record of an individual server which can be tried to reach a destination.
-#
-# "host" is the hostname acquired from the SRV record. Except when there's
-# no SRV record, in which case it is the original hostname.
-_Server = collections.namedtuple(
-    "_Server", "priority weight host port expires"
-)
+logger = logging.getLogger(__name__)
 
 
 def parse_server_name(server_name):
@@ -165,12 +153,9 @@ class SRVClientEndpoint(object):
         self.service_name = "_%s._%s.%s" % (service, protocol, domain)
 
         if default_port is not None:
-            self.default_server = _Server(
+            self.default_server = Server(
                 host=domain,
                 port=default_port,
-                priority=0,
-                weight=0,
-                expires=0,
             )
         else:
             self.default_server = None
@@ -240,57 +225,3 @@ class SRVClientEndpoint(object):
         )
         connection = yield endpoint.connect(protocolFactory)
         defer.returnValue(connection)
-
-
-@defer.inlineCallbacks
-def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=time):
-    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)
-
-    servers = []
-
-    try:
-        try:
-            answers, _, _ = yield dns_client.lookupService(service_name)
-        except DNSNameError:
-            defer.returnValue([])
-
-        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)
-
-        for answer in answers:
-            if answer.type != dns.SRV or not answer.payload:
-                continue
-
-            payload = answer.payload
-
-            servers.append(_Server(
-                host=str(payload.target),
-                port=int(payload.port),
-                priority=int(payload.priority),
-                weight=int(payload.weight),
-                expires=int(clock.time()) + answer.ttl,
-            ))
-
-        servers.sort()
-        cache[service_name] = list(servers)
-    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
-            )
-            servers = list(cache_entry)
-        else:
-            raise e
-
-    defer.returnValue(servers)
diff --git a/synapse/http/federation/__init__.py b/synapse/http/federation/__init__.py
new file mode 100644
index 0000000000..1453d04571
--- /dev/null
+++ b/synapse/http/federation/__init__.py
@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+# 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.
diff --git a/synapse/http/federation/srv_resolver.py b/synapse/http/federation/srv_resolver.py
new file mode 100644
index 0000000000..c49b82c394
--- /dev/null
+++ b/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)
diff --git a/tests/http/federation/__init__.py b/tests/http/federation/__init__.py
new file mode 100644
index 0000000000..1453d04571
--- /dev/null
+++ b/tests/http/federation/__init__.py
@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+# 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.
diff --git a/tests/test_dns.py b/tests/http/federation/test_srv_resolver.py
index 90bd34be34..1271a495e1 100644
--- a/tests/test_dns.py
+++ b/tests/http/federation/test_srv_resolver.py
@@ -1,5 +1,6 @@
 # -*- 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.
@@ -16,40 +17,63 @@
 from mock import Mock
 
 from twisted.internet import defer
+from twisted.internet.defer import Deferred
+from twisted.internet.error import ConnectError
 from twisted.names import dns, error
 
-from synapse.http.endpoint import resolve_service
+from synapse.http.federation.srv_resolver import resolve_service
+from synapse.util.logcontext import LoggingContext
 
+from tests import unittest
 from tests.utils import MockClock
 
-from . import unittest
 
-
-@unittest.DEBUG
-class DnsTestCase(unittest.TestCase):
-    @defer.inlineCallbacks
+class SrvResolverTestCase(unittest.TestCase):
     def test_resolve(self):
         dns_client_mock = Mock()
 
-        service_name = "test_service.example.com"
-        host_name = "example.com"
+        service_name = b"test_service.example.com"
+        host_name = b"example.com"
 
         answer_srv = dns.RRHeader(
             type=dns.SRV, payload=dns.Record_SRV(target=host_name)
         )
 
-        dns_client_mock.lookupService.return_value = defer.succeed(
-            ([answer_srv], None, None)
-        )
+        result_deferred = Deferred()
+        dns_client_mock.lookupService.return_value = result_deferred
 
         cache = {}
 
-        servers = yield resolve_service(
-            service_name, dns_client=dns_client_mock, cache=cache
-        )
+        @defer.inlineCallbacks
+        def do_lookup():
+            with LoggingContext("one") as ctx:
+                resolve_d = resolve_service(
+                    service_name, dns_client=dns_client_mock, cache=cache
+                )
+
+                self.assertNoResult(resolve_d)
+
+                # should have reset to the sentinel context
+                self.assertIs(LoggingContext.current_context(), LoggingContext.sentinel)
+
+                result = yield resolve_d
+
+                # should have restored our context
+                self.assertIs(LoggingContext.current_context(), ctx)
+
+                defer.returnValue(result)
+
+        test_d = do_lookup()
+        self.assertNoResult(test_d)
 
         dns_client_mock.lookupService.assert_called_once_with(service_name)
 
+        result_deferred.callback(
+            ([answer_srv], None, None)
+        )
+
+        servers = self.successResultOf(test_d)
+
         self.assertEquals(len(servers), 1)
         self.assertEquals(servers, cache[service_name])
         self.assertEquals(servers[0].host, host_name)
@@ -127,3 +151,59 @@ class DnsTestCase(unittest.TestCase):
 
         self.assertEquals(len(servers), 0)
         self.assertEquals(len(cache), 0)
+
+    def test_disabled_service(self):
+        """
+        test the behaviour when there is a single record which is ".".
+        """
+        service_name = b"test_service.example.com"
+
+        lookup_deferred = Deferred()
+        dns_client_mock = Mock()
+        dns_client_mock.lookupService.return_value = lookup_deferred
+        cache = {}
+
+        resolve_d = resolve_service(
+            service_name, dns_client=dns_client_mock, cache=cache
+        )
+        self.assertNoResult(resolve_d)
+
+        # returning a single "." should make the lookup fail with a ConenctError
+        lookup_deferred.callback((
+            [dns.RRHeader(type=dns.SRV, payload=dns.Record_SRV(target=b"."))],
+            None,
+            None,
+        ))
+
+        self.failureResultOf(resolve_d, ConnectError)
+
+    def test_non_srv_answer(self):
+        """
+        test the behaviour when the dns server gives us a spurious non-SRV response
+        """
+        service_name = b"test_service.example.com"
+
+        lookup_deferred = Deferred()
+        dns_client_mock = Mock()
+        dns_client_mock.lookupService.return_value = lookup_deferred
+        cache = {}
+
+        resolve_d = resolve_service(
+            service_name, dns_client=dns_client_mock, cache=cache
+        )
+        self.assertNoResult(resolve_d)
+
+        lookup_deferred.callback((
+            [
+                dns.RRHeader(type=dns.A, payload=dns.Record_A()),
+                dns.RRHeader(type=dns.SRV, payload=dns.Record_SRV(target=b"host")),
+            ],
+            None,
+            None,
+        ))
+
+        servers = self.successResultOf(resolve_d)
+
+        self.assertEquals(len(servers), 1)
+        self.assertEquals(servers, cache[service_name])
+        self.assertEquals(servers[0].host, b"host")