Return early on /sync code paths if a '*' filter is used

This is currently very conservative in that it only does this if there is no
`since` token. This limits the risk to clients likely to be doing one-off
syncs (like bridges), but does mean that normal human clients won't benefit
from the time savings here. If the savings are large enough, I would consider
generalising this to just check the filter.

synapse/api/filtering.py

@@ -202,6 +202,26 @@ def filter_room_ephemeral(self, events):
     def filter_room_account_data(self, events):
         return self._room_account_data.filter(self._room_filter.filter(events))
 
+    def blocks_all_presence(self):
+        return (
+            self._presence_filter.filters_all_types() or
+            self._presence_filter.filters_all_senders()
+        )
+
+    def blocks_all_room_ephemeral(self):
+        return (
+            self._room_ephemeral_filter.filters_all_types() or
+            self._room_ephemeral_filter.filters_all_senders() or
+            self._room_ephemeral_filter.filters_all_rooms()
+        )
+
+    def blocks_all_room_timeline(self):
+        return (
+            self._room_timeline_filter.filters_all_types() or
+            self._room_timeline_filter.filters_all_senders() or
+            self._room_timeline_filter.filters_all_rooms()
+        )
+
 
 class Filter(object):
     def __init__(self, filter_json):
@@ -218,6 +238,15 @@ def __init__(self, filter_json):
 
         self.contains_url = self.filter_json.get("contains_url", None)
 
+    def filters_all_types(self):
+        return "*" in self.not_types
+
+    def filters_all_senders(self):
+        return "*" in self.not_senders
+
+    def filters_all_rooms(self):
+        return "*" in self.not_rooms
+
     def check(self, event):
         """Checks whether the filter matches the given event.
 

synapse/handlers/sync.py

@@ -277,6 +277,7 @@ def _load_filtered_recents(self, room_id, sync_config, now_token,
         """
         with Measure(self.clock, "load_filtered_recents"):
             timeline_limit = sync_config.filter_collection.timeline_limit()
+            block_all_timeline = sync_config.filter_collection.blocks_all_room_timeline()
 
             if recents is None or newly_joined_room or timeline_limit < len(recents):
                 limited = True
@@ -293,7 +294,7 @@ def _load_filtered_recents(self, room_id, sync_config, now_token,
             else:
                 recents = []
 
-            if not limited:
+            if not limited or block_all_timeline:
                 defer.returnValue(TimelineBatch(
                     events=recents,
                     prev_batch=now_token,
@@ -531,9 +532,14 @@ def generate_sync_result(self, sync_config, since_token=None, full_state=False):
         )
         newly_joined_rooms, newly_joined_users = res
 
-        yield self._generate_sync_entry_for_presence(
-            sync_result_builder, newly_joined_rooms, newly_joined_users
+        block_all_presence_data = (
+            since_token is None and
+            sync_config.filter_collection.blocks_all_presence()
         )
+        if not block_all_presence_data:
+            yield self._generate_sync_entry_for_presence(
+                sync_result_builder, newly_joined_rooms, newly_joined_users
+            )
 
         yield self._generate_sync_entry_for_to_device(sync_result_builder)
 
@@ -709,13 +715,20 @@ def _generate_sync_entry_for_rooms(self, sync_result_builder, account_data_by_ro
             `(newly_joined_rooms, newly_joined_users)`
         """
         user_id = sync_result_builder.sync_config.user.to_string()
-
-        now_token, ephemeral_by_room = yield self.ephemeral_by_room(
-            sync_result_builder.sync_config,
-            now_token=sync_result_builder.now_token,
-            since_token=sync_result_builder.since_token,
+        block_all_room_ephemeral = (
+            sync_result_builder.since_token is None and
+            sync_result_builder.sync_config.filter_collection.blocks_all_room_ephemeral()
         )
-        sync_result_builder.now_token = now_token
+
+        if block_all_room_ephemeral:
+            ephemeral_by_room = {}
+        else:
+            now_token, ephemeral_by_room = yield self.ephemeral_by_room(
+                sync_result_builder.sync_config,
+                now_token=sync_result_builder.now_token,
+                since_token=sync_result_builder.since_token,
+            )
+            sync_result_builder.now_token = now_token
 
         ignored_account_data = yield self.store.get_global_account_data_by_type_for_user(
             "m.ignored_user_list", user_id=user_id,