Cache dns lookups, and use the cache if we fail to lookup servers later

synapse/http/endpoint.py

@@ -17,7 +17,7 @@ from twisted.internet.endpoints import SSL4ClientEndpoint, TCP4ClientEndpoint
 from twisted.internet import defer
 from twisted.internet.error import ConnectError
 from twisted.names import client, dns
-from twisted.names.error import DNSNameError
+from twisted.names.error import DNSNameError, DomainError
 
 import collections
 import logging
@@ -27,6 +27,14 @@ import random
 logger = logging.getLogger(__name__)
 
 
+SERVER_CACHE = {}
+
+
+_Server = collections.namedtuple(
+    "_Server", "priority weight host port"
+)
+
+
 def matrix_federation_endpoint(reactor, destination, ssl_context_factory=None,
                                timeout=None):
     """Construct an endpoint for the given matrix destination.
@@ -73,10 +81,6 @@ class SRVClientEndpoint(object):
     Implements twisted.internet.interfaces.IStreamClientEndpoint.
     """
 
-    _Server = collections.namedtuple(
-        "_Server", "priority weight host port"
-    )
-
     def __init__(self, reactor, service, domain, protocol="tcp",
                  default_port=None, endpoint=TCP4ClientEndpoint,
                  endpoint_kw_args={}):
@@ -101,32 +105,8 @@ class SRVClientEndpoint(object):
 
     @defer.inlineCallbacks
     def fetch_servers(self):
-        try:
-            answers, auth, add = yield client.lookupService(self.service_name)
-        except DNSNameError:
-            answers = []
-
-        if (len(answers) == 1
-                and answers[0].type == dns.SRV
-                and answers[0].payload
-                and answers[0].payload.target == dns.Name('.')):
-            raise ConnectError("Service %s unavailable", self.service_name)
-
-        self.servers = []
         self.used_servers = []
-
-        for answer in answers:
-            if answer.type != dns.SRV or not answer.payload:
-                continue
-            payload = answer.payload
-            self.servers.append(self._Server(
-                host=str(payload.target),
-                port=int(payload.port),
-                priority=int(payload.priority),
-                weight=int(payload.weight)
-            ))
-
-        self.servers.sort()
+        self.servers = yield resolve_service(self.service_name)
 
     def pick_server(self):
         if not self.servers:
@@ -170,3 +150,64 @@ class SRVClientEndpoint(object):
         )
         connection = yield endpoint.connect(protocolFactory)
         defer.returnValue(connection)
+
+
+@defer.inlineCallbacks
+def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE):
+    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('.')):
+            raise ConnectError("Service %s unavailable", service_name)
+
+        for answer in answers:
+            if answer.type != dns.SRV or not answer.payload:
+                continue
+
+            payload = answer.payload
+
+            host = str(payload.target)
+
+            try:
+                answers, _, _ = yield dns_client.lookupAddress(host)
+            except DNSNameError:
+                continue
+
+            ips = [
+                answer.payload.dottedQuad()
+                for answer in answers
+                if answer.type == dns.A and answer.payload
+            ]
+
+            for ip in ips:
+                servers.append(_Server(
+                    host=ip,
+                    port=int(payload.port),
+                    priority=int(payload.priority),
+                    weight=int(payload.weight)
+                ))
+
+        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)