fixup: add comment

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/13749.bugfix

@@ -0,0 +1 @@
+Fix a long standing bug where device lists would remain cached when remote users left and rejoined the last room shared with the local homeserver.

synapse/handlers/device.py

@@ -45,7 +45,6 @@ from synapse.types import (
     JsonDict,
     StreamKeyType,
     StreamToken,
-    UserID,
     get_domain_from_id,
     get_verify_key_from_cross_signing_key,
 )
@@ -324,8 +323,6 @@ class DeviceHandler(DeviceWorkerHandler):
             self.device_list_updater.incoming_device_list_update,
         )
 
-        hs.get_distributor().observe("user_left_room", self.user_left_room)
-
         # Whether `_handle_new_device_update_async` is currently processing.
         self._handle_new_device_update_is_processing = False
 
@@ -569,14 +566,6 @@ class DeviceHandler(DeviceWorkerHandler):
             StreamKeyType.DEVICE_LIST, position, users=[from_user_id]
         )
 
-    async def user_left_room(self, user: UserID, room_id: str) -> None:
-        user_id = user.to_string()
-        room_ids = await self.store.get_rooms_for_user(user_id)
-        if not room_ids:
-            # We no longer share rooms with this user, so we'll no longer
-            # receive device updates. Mark this in DB.
-            await self.store.mark_remote_user_device_list_as_unsubscribed(user_id)
-
     async def store_dehydrated_device(
         self,
         user_id: str,

synapse/handlers/e2e_keys.py

@@ -175,6 +175,32 @@ class E2eKeysHandler:
                     user_ids_not_in_cache,
                     remote_results,
                 ) = await self.store.get_user_devices_from_cache(query_list)
+
+                # Check that the homeserver still shares a room with all cached users.
+                # Note that this check may be slightly racy when a remote user leaves a
+                # room after we have fetched their cached device list. In the worst case
+                # we will do extra federation queries for devices that we had cached.
+                cached_users = set(remote_results.keys())
+                valid_cached_users = (
+                    await self.store.get_users_server_still_shares_room_with(
+                        remote_results.keys()
+                    )
+                )
+                invalid_cached_users = cached_users - valid_cached_users
+                if invalid_cached_users:
+                    # Fix up results. If we get here, there is either a bug in device
+                    # list tracking, or we hit the race mentioned above.
+                    user_ids_not_in_cache.update(invalid_cached_users)
+                    for invalid_user_id in invalid_cached_users:
+                        remote_results.pop(invalid_user_id)
+                    # This log message may be removed if it turns out it's almost
+                    # entirely triggered by races.
+                    logger.error(
+                        "Devices for %s were cached, but the server no longer shares "
+                        "any rooms with them. The cached device lists are stale.",
+                        invalid_cached_users,
+                    )
+
                 for user_id, devices in remote_results.items():
                     user_devices = results.setdefault(user_id, {})
                     for device_id, device in devices.items():

synapse/storage/controllers/persist_events.py

@@ -598,9 +598,9 @@ class EventsPersistenceStorageController:
             # room
             state_delta_for_room: Dict[str, DeltaState] = {}
 
-            # Set of remote users which were in rooms the server has left. We
-            # should check if we still share any rooms and if not we mark their
-            # device lists as stale.
+            # Set of remote users which were in rooms the server has left or who may
+            # have left rooms the server is in. We should check if we still share any
+            # rooms and if not we mark their device lists as stale.
             potentially_left_users: Set[str] = set()
 
             if not backfilled:
@@ -725,6 +725,20 @@ class EventsPersistenceStorageController:
                                 current_state = {}
                                 delta.no_longer_in_room = True
 
+                            # Add all remote users that might have left rooms.
+                            potentially_left_users.update(
+                                user_id
+                                for event_type, user_id in delta.to_delete
+                                if event_type == EventTypes.Member
+                                and not self.is_mine_id(user_id)
+                            )
+                            potentially_left_users.update(
+                                user_id
+                                for event_type, user_id in delta.to_insert.keys()
+                                if event_type == EventTypes.Member
+                                and not self.is_mine_id(user_id)
+                            )
+
                             state_delta_for_room[room_id] = delta
 
             await self.persist_events_store._persist_events_and_state_updates(

tests/handlers/test_e2e_keys.py

@@ -891,6 +891,12 @@ class E2eKeysHandlerTestCase(unittest.HomeserverTestCase):
             new_callable=mock.MagicMock,
             return_value=make_awaitable(["some_room_id"]),
         )
+        mock_get_users = mock.patch.object(
+            self.store,
+            "get_users_server_still_shares_room_with",
+            new_callable=mock.MagicMock,
+            return_value=make_awaitable({remote_user_id}),
+        )
         mock_request = mock.patch.object(
             self.hs.get_federation_client(),
             "query_user_devices",
@@ -898,7 +904,7 @@ class E2eKeysHandlerTestCase(unittest.HomeserverTestCase):
             return_value=make_awaitable(response_body),
         )
 
-        with mock_get_rooms, mock_request as mocked_federation_request:
+        with mock_get_rooms, mock_get_users, mock_request as mocked_federation_request:
             # Make the first query and sanity check it succeeds.
             response_1 = self.get_success(
                 e2e_handler.query_devices(