Read from DNS cache if within TTL

synapse/http/endpoint.py

@@ -22,6 +22,7 @@
 import collections
 import logging
 import random
+import time
 
 
 logger = logging.getLogger(__name__)
@@ -31,7 +32,7 @@
 
 
 _Server = collections.namedtuple(
-    "_Server", "priority weight host port"
+    "_Server", "priority weight host port expires"
 )
 
 
@@ -92,7 +93,8 @@ def __init__(self, reactor, service, domain, protocol="tcp",
                 host=domain,
                 port=default_port,
                 priority=0,
-                weight=0
+                weight=0,
+                expires=0,
             )
         else:
             self.default_server = None
@@ -153,7 +155,13 @@ def connect(self, protocolFactory):
 
 
 @defer.inlineCallbacks
-def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE):
+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:
@@ -173,27 +181,26 @@ def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE):
                 continue
 
             payload = answer.payload
-
             host = str(payload.target)
+            srv_ttl = answer.ttl
 
             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)
-                ))
+            for answer in answers:
+                if answer.type == dns.A and answer.payload:
+                    ip = answer.payload.dottedQuad()
+                    host_ttl = min(srv_ttl, answer.ttl)
+
+                    servers.append(_Server(
+                        host=ip,
+                        port=int(payload.port),
+                        priority=int(payload.priority),
+                        weight=int(payload.weight),
+                        expires=int(clock.time()) + host_ttl,
+                    ))
 
         servers.sort()
         cache[service_name] = list(servers)

tests/test_dns.py

@@ -69,8 +69,11 @@ def test_from_cache(self):
 
         service_name = "test_service.examle.com"
 
+        entry = Mock(spec_set=["expires"])
+        entry.expires = 999999999
+
         cache = {
-            service_name: [object()]
+            service_name: [entry]
         }
 
         servers = yield resolve_service(