Properly handle unknown results for the stream change cache. (#14592)

StreamChangeCache.get_all_changed_entities can return None to signify
it does not have information at the given stream position. Two callers (related
to device lists and presence) were treating this response the same as an empty
list (i.e. there being no updates).

changelog.d/14592.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where a device list update might not be sent to clients in certain circumstances.

synapse/handlers/presence.py

@@ -1764,14 +1764,14 @@ class PresenceEventSource(EventSource[int, UserPresenceState]):
         Returns:
             A list of presence states for the given user to receive.
         """
+        updated_users = None
         if from_key:
             # Only return updates since the last sync
             updated_users = self.store.presence_stream_cache.get_all_entities_changed(
                 from_key
             )
-            if not updated_users:
-                updated_users = []
 
+        if updated_users is not None:
             # Get the actual presence update for each change
             users_to_state = await self.get_presence_handler().current_state_for_users(
                 updated_users

synapse/storage/databases/main/devices.py

@@ -842,12 +842,11 @@ class DeviceWorkerStore(RoomMemberWorkerStore, EndToEndKeyWorkerStore):
                 user_ids, from_key
             )
 
-        if not user_ids_to_check:
+        # If an empty set was returned, there's nothing to do.
+        if user_ids_to_check is not None and not user_ids_to_check:
             return set()
 
         def _get_users_whose_devices_changed_txn(txn: LoggingTransaction) -> Set[str]:
-            changes: Set[str] = set()
-
             stream_id_where_clause = "stream_id > ?"
             sql_args = [from_key]
 
@@ -858,19 +857,25 @@ class DeviceWorkerStore(RoomMemberWorkerStore, EndToEndKeyWorkerStore):
             sql = f"""
                 SELECT DISTINCT user_id FROM device_lists_stream
                 WHERE {stream_id_where_clause}
-                AND
             """
 
-            # Query device changes with a batch of users at a time
-            # Assertion for mypy's benefit; see also
-            # https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions
-            assert user_ids_to_check is not None
-            for chunk in batch_iter(user_ids_to_check, 100):
-                clause, args = make_in_list_sql_clause(
-                    txn.database_engine, "user_id", chunk
-                )
-                txn.execute(sql + clause, sql_args + args)
-                changes.update(user_id for user_id, in txn)
+            # If the stream change cache gave us no information, fetch *all*
+            # users between the stream IDs.
+            if user_ids_to_check is None:
+                txn.execute(sql, sql_args)
+                return {user_id for user_id, in txn}
+
+            # Otherwise, fetch changes for the given users.
+            else:
+                changes: Set[str] = set()
+
+                # Query device changes with a batch of users at a time
+                for chunk in batch_iter(user_ids_to_check, 100):
+                    clause, args = make_in_list_sql_clause(
+                        txn.database_engine, "user_id", chunk
+                    )
+                    txn.execute(sql + " AND " + clause, sql_args + args)
+                    changes.update(user_id for user_id, in txn)
 
             return changes