Only send rooms with updates down sliding sync

3 files changed, 85 insertions, 15 deletions

diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index 69be113117..306c4286e7 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -617,6 +617,31 @@ class SlidingSyncHandler:
         # Fetch room data
         rooms: Dict[str, SlidingSyncResult.RoomResult] = {}
 
+        # Filter out rooms that haven't received updates and we've sent down
+        # previously.
+        if from_token:
+            rooms_should_send = set()
+            for room_id in relevant_room_map:
+                status = await self.connection_store.have_sent_room(
+                    sync_config,
+                    from_token.connection_position,
+                    room_id,
+                )
+                if status.status != HaveSentRoomFlag.LIVE:
+                    rooms_should_send.add(room_id)
+
+            # We only need to check for any new events and not state changes, as
+            # state changes can only happen if an event has also been sent.
+            rooms_that_have_updates = (
+                self.store._events_stream_cache.get_entities_changed(
+                    relevant_room_map, from_token.stream_token.room_key.stream
+                )
+            )
+            rooms_should_send.update(rooms_that_have_updates)
+            relevant_room_map = {
+                r: c for r, c in relevant_room_map.items() if r in rooms_should_send
+            }
+
         @trace
         @tag_args
         async def handle_room(room_id: str) -> None:
@@ -631,7 +656,9 @@ class SlidingSyncHandler:
                 to_token=to_token,
             )
 
-            rooms[room_id] = room_sync_result
+            # Filter out empty room results.
+            if room_sync_result:
+                rooms[room_id] = room_sync_result
 
         with start_active_span("sliding_sync.generate_room_entries"):
             await concurrently_execute(handle_room, relevant_room_map, 10)
diff --git a/synapse/types/handlers/__init__.py b/synapse/types/handlers/__init__.py
index 7c7fe130cb..58530bba70 100644
--- a/synapse/types/handlers/__init__.py
+++ b/synapse/types/handlers/__init__.py
@@ -238,6 +238,14 @@ class SlidingSyncResult:
         notification_count: int
         highlight_count: int
 
+        def __bool__(self) -> bool:
+            return (
+                self.initial
+                or bool(self.required_state)
+                or bool(self.timeline_events)
+                or bool(self.stripped_state)
+            )
+
     @attr.s(slots=True, frozen=True, auto_attribs=True)
     class SlidingWindowList:
         """
diff --git a/tests/rest/client/test_sync.py b/tests/rest/client/test_sync.py
index cbb673df2c..7f3d5907cf 100644
--- a/tests/rest/client/test_sync.py
+++ b/tests/rest/client/test_sync.py
@@ -3540,19 +3540,7 @@ class SlidingSyncTestCase(SlidingSyncBase):
         self.assertEqual(channel.code, 200, channel.json_body)
 
         # Nothing to see for this banned user in the room in the token range
-        self.assertIsNone(channel.json_body["rooms"][room_id1].get("timeline"))
-        # No events returned in the timeline so nothing is "live"
-        self.assertEqual(
-            channel.json_body["rooms"][room_id1]["num_live"],
-            0,
-            channel.json_body["rooms"][room_id1],
-        )
-        # There aren't anymore events to paginate to in this range
-        self.assertEqual(
-            channel.json_body["rooms"][room_id1]["limited"],
-            False,
-            channel.json_body["rooms"][room_id1],
-        )
+        self.assertIsNone(channel.json_body["rooms"].get(room_id1))
 
     def test_rooms_no_required_state(self) -> None:
         """
@@ -3662,12 +3650,15 @@ class SlidingSyncTestCase(SlidingSyncBase):
                         # This one doesn't exist in the room
                         [EventTypes.Tombstone, ""],
                     ],
-                    "timeline_limit": 0,
+                    "timeline_limit": 1,
                 }
             }
         }
         _, after_room_token = self.do_sync(sync_body, tok=user1_tok)
 
+        # Send a message so the room comes down sync.
+        self.helper.send(room_id1, "msg", tok=user1_tok)
+
         # Make the Sliding Sync request
         channel = self.make_request(
             "POST",
@@ -4745,6 +4736,50 @@ class SlidingSyncTestCase(SlidingSyncBase):
             exact=True,
         )
 
+    def test_rooms_with_no_updates_do_not_come_down_incremental_sync(self) -> None:
+        """
+        Test that rooms with no updates are returned in subsequent incremental
+        syncs.
+        """
+
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        self.helper.join(room_id1, user1_id, tok=user1_tok)
+
+        sync_body = {
+            "lists": {
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [
+                        [EventTypes.Create, ""],
+                        [EventTypes.RoomHistoryVisibility, ""],
+                        # This one doesn't exist in the room
+                        [EventTypes.Tombstone, ""],
+                    ],
+                    "timeline_limit": 0,
+                }
+            }
+        }
+
+        _, after_room_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # Make the Sliding Sync request
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint + f"?pos={after_room_token}",
+            content=sync_body,
+            access_token=user1_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # Nothing has happened in the room, so the room should not come down
+        # /sync.
+        self.assertIsNone(channel.json_body["rooms"].get(room_id1))
+
 
 class SlidingSyncToDeviceExtensionTestCase(SlidingSyncBase):
     """Tests for the to-device sliding sync extension"""