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).

1 files changed, 19 insertions, 14 deletions

diff --git a/synapse/storage/databases/main/devices.py b/synapse/storage/databases/main/devices.py
index 534f7fc04a..8ba995df3b 100644
--- a/synapse/storage/databases/main/devices.py
+++ b/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