Filter the recent events before applying the limit when doing an incremental sync with a gap

synapse/handlers/sync.py

@@ -278,6 +278,40 @@ class SyncHandler(BaseHandler):
             next_batch=now_token,
         ))
 
+    @defer.inlineCallbacks
+    def load_filtered_recents(self, room_id, sync_config, since_token,
+                              now_token):
+        limited = True
+        recents = []
+        filtering_factor = 2
+        load_limit = max(sync_config.limit * filtering_factor, 100)
+        max_repeat = 3  # Only try a few times per room, otherwise
+        room_key = now_token.room_key
+
+        while limited and len(recents) < sync_config.limit and max_repeat:
+            events, room_key = yield self.store.get_recent_events_for_room(
+                room_id,
+                limit=load_limit + 1,
+                from_token=since_token.room_key,
+                end_token=room_key,
+            )
+            loaded_recents = sync_config.filter.filter_room_events(events)
+            loaded_recents.extend(recents)
+            recents = loaded_recents
+            if len(events) <= load_limit:
+                limited = False
+            max_repeat -= 1
+
+        if len(recents) > sync_config.limit:
+            recents = recents[-sync_config.limit:]
+            room_key = recents[0].internal_metadata.before
+
+        prev_batch_token = now_token.copy_and_replace(
+            "room_key", room_key
+        )
+
+        defer.returnValue((recents, prev_batch_token, limited))
+
     @defer.inlineCallbacks
     def incremental_sync_with_gap_for_room(self, room_id, sync_config,
                                            since_token, now_token,
@@ -288,28 +322,17 @@ class SyncHandler(BaseHandler):
         Returns:
             A Deferred RoomSyncResult
         """
+
         # TODO(mjark): Check if they have joined the room between
         # the previous sync and this one.
-        # TODO(mjark): Apply the event filter in sync_config taking care to get
-        # enough events to reach the limit
         # TODO(mjark): Check for redactions we might have missed.
-        recents, token = yield self.store.get_recent_events_for_room(
-            room_id,
-            limit=sync_config.limit + 1,
-            from_token=since_token.room_key,
-            end_token=now_token.room_key,
+
+        recents, prev_batch_token, limited = self.load_filtered_recents(
+            room_id, sync_config, since_token,
         )
 
         logging.debug("Recents %r", recents)
 
-        if len(recents) > sync_config.limit:
-            limited = True
-            recents = recents[1:]
-        else:
-            limited = False
-
-        prev_batch_token = now_token.copy_and_replace("room_key", token[0])
-
         # TODO(mjark): This seems racy since this isn't being passed a
         # token to indicate what point in the stream this is
         current_state_events = yield self.state_handler.get_current_state(