Disable aggregation bundling on /sync responses (#11583)

* Disable aggregation bundling on `/sync` responses

A partial revert of #11478. This turns out to have had a significant CPU impact
on initial-sync handling. For now, let's disable it, until we find a more
efficient way of achieving this.

* Fix tests.

Co-authored-by: Patrick Cloke <patrickc@matrix.org>

changelog.d/11583.bugfix

@@ -0,0 +1 @@
+Fix a performance regression in `/sync` handling, introduced in 1.49.0.

synapse/rest/client/sync.py

@@ -522,7 +522,15 @@ def serialize(events: Iterable[EventBase]) -> Awaitable[List[JsonDict]]:
                 time_now=time_now,
                 # Don't bother to bundle aggregations if the timeline is unlimited,
                 # as clients will have all the necessary information.
-                bundle_aggregations=room.timeline.limited,
+                # bundle_aggregations=room.timeline.limited,
+                #
+                # richvdh 2021-12-15: disable this temporarily as it has too high an
+                # overhead for initialsyncs. We need to figure out a way that the
+                # bundling can be done *before* the events are stored in the
+                # SyncResponseCache so that this part can be synchronous.
+                #
+                # Ensure to re-enable the test at tests/rest/client/test_relations.py::RelationsTestCase.test_bundled_aggregations.
+                bundle_aggregations=False,
                 token_id=token_id,
                 event_format=event_formatter,
                 only_event_fields=only_fields,

tests/rest/client/test_relations.py

@@ -574,11 +574,11 @@ def _find_and_assert_event(events):
         assert_bundle(channel.json_body["event"]["unsigned"].get("m.relations"))
 
         # Request sync.
-        channel = self.make_request("GET", "/sync", access_token=self.user_token)
-        self.assertEquals(200, channel.code, channel.json_body)
-        room_timeline = channel.json_body["rooms"]["join"][self.room]["timeline"]
-        self.assertTrue(room_timeline["limited"])
-        _find_and_assert_event(room_timeline["events"])
+        # channel = self.make_request("GET", "/sync", access_token=self.user_token)
+        # self.assertEquals(200, channel.code, channel.json_body)
+        # room_timeline = channel.json_body["rooms"]["join"][self.room]["timeline"]
+        # self.assertTrue(room_timeline["limited"])
+        # _find_and_assert_event(room_timeline["events"])
 
         # Note that /relations is tested separately in test_aggregation_get_event_for_thread
         # since it needs different data configured.