1 files changed, 11 insertions, 2 deletions
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 3c13859faa..2dfe4c0b66 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -460,8 +460,17 @@ class MultiWriterIdGenerator(AbstractStreamIdGenerator):
                 # Cast safety: this corresponds to the types returned by the query above.
                 rows.extend(cast(Iterable[Tuple[str, int]], cur))
 
-            # Sort so that we handle rows in order for each instance.
-            rows.sort()
+            # Sort by stream_id (ascending, lowest -> highest) so that we handle
+            # rows in order for each instance because we don't want to overwrite
+            # the current_position of an instance to a lower stream ID than
+            # we're actually at.
+            def sort_by_stream_id_key_func(row: Tuple[str, int]) -> int:
+                (instance, stream_id) = row
+                # If `stream_id` is ever `None`, we will see a `TypeError: '<'
+                # not supported between instances of 'NoneType' and 'X'` error.
+                return stream_id
+
+            rows.sort(key=sort_by_stream_id_key_func)
 
             with self._lock:
                 for (