Handle SERVFAILs when doing AAAA lookups for federation (#2477)

... to cope with people with broken dnssec setups, mostly

synapse/http/endpoint.py

@@ -354,16 +354,28 @@ def _get_hosts_for_srv_record(dns_client, host):
 
         return res[0]
 
-    def eb(res):
-        res.trap(DNSNameError)
-        return []
+    def eb(res, record_type):
+        if res.check(DNSNameError):
+            return []
+        logger.warn("Error looking up %s for %s: %s",
+                    record_type, host, res, res.value)
+        return res
 
     # no logcontexts here, so we can safely fire these off and gatherResults
     d1 = dns_client.lookupAddress(host).addCallbacks(cb, eb)
     d2 = dns_client.lookupIPV6Address(host).addCallbacks(cb, eb)
-    results = yield defer.gatherResults([d1, d2], consumeErrors=True)
+    results = yield defer.DeferredList(
+        [d1, d2], consumeErrors=True)
+
+    # if all of the lookups failed, raise an exception rather than blowing out
+    # the cache with an empty result.
+    if results and all(s == defer.FAILURE for (s, _) in results):
+        defer.returnValue(results[0][1])
+
+    for (success, result) in results:
+        if success == defer.FAILURE:
+            continue
 
-    for result in results:
         for answer in result:
             if not answer.payload:
                 continue