Linearize fetching of gaps on incoming events

This potentially stops the server from doing multiple requests for the
same data.

synapse/federation/federation_base.py

@@ -31,6 +31,9 @@ logger = logging.getLogger(__name__)
 
 
 class FederationBase(object):
+    def __init__(self, hs):
+        pass
+
     @defer.inlineCallbacks
     def _check_sigs_and_hash_and_fetch(self, origin, pdus, outlier=False,
                                        include_none=False):

synapse/federation/federation_client.py

@@ -52,6 +52,8 @@ sent_queries_counter = metrics.register_counter("sent_queries", labels=["type"])
 
 
 class FederationClient(FederationBase):
+    def __init__(self, hs):
+        super(FederationClient, self).__init__(hs)
 
     def start_get_pdu_cache(self):
         self._get_pdu_cache = ExpiringCache(

synapse/federation/federation_server.py

@@ -19,6 +19,7 @@ from twisted.internet import defer
 from .federation_base import FederationBase
 from .units import Transaction, Edu
 
+from synapse.util.async import Linearizer
 from synapse.util.logutils import log_function
 from synapse.events import FrozenEvent
 import synapse.metrics
@@ -44,6 +45,11 @@ received_queries_counter = metrics.register_counter("received_queries", labels=[
 
 
 class FederationServer(FederationBase):
+    def __init__(self, hs):
+        super(FederationServer, self).__init__(hs)
+
+        self._room_pdu_linearizer = Linearizer()
+
     def set_handler(self, handler):
         """Sets the handler that the replication layer will use to communicate
         receipt of new PDUs from other home servers. The required methods are
@@ -491,6 +497,14 @@ class FederationServer(FederationBase):
                 pdu.internal_metadata.outlier = True
             elif min_depth and pdu.depth > min_depth:
                 if get_missing and prevs - seen:
+                    # If we're missing stuff, ensure we only fetch stuff one
+                    # at a time.
+                    with (yield self._room_pdu_linearizer.queue(pdu.room_id)):
+                        # We recalculate seen, since it may have changed.
+                        have_seen = yield self.store.have_events(prevs)
+                        seen = set(have_seen.keys())
+
+                        if prevs - seen:
                             latest = yield self.store.get_latest_event_ids_in_room(
                                 pdu.room_id
                             )

synapse/federation/replication.py

@@ -72,5 +72,7 @@ class ReplicationLayer(FederationClient, FederationServer):
 
         self.hs = hs
 
+        super(ReplicationLayer, self).__init__(hs)
+
     def __str__(self):
         return "<ReplicationLayer(%s)>" % self.server_name