1 files changed, 7 insertions, 0 deletions
diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py
index 0ab7cb8dbd..964f41ca57 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -941,6 +941,12 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         Returns:
             All membership changes to the current state in the token range. Events are
             sorted by `stream_ordering` ascending.
+
+            `event_id`/`sender` can be `None` when the server leaves a room (meaning
+            everyone locally left) or a state reset which removed the person from the
+            room. We can't tell the difference between the two cases with what's
+            available in the `current_state_delta_stream` table. To actually check for a
+            state reset, you need to check if a membership still exists in the room.
         """
         # Start by ruling out cases where a DB query is not necessary.
         if from_key == to_key:
@@ -1052,6 +1058,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                         membership=(
                             membership if membership is not None else Membership.LEAVE
                         ),
+                        # This will also be null for the same reasons if `s.event_id = null`
                         sender=sender,
                         # Prev event
                         prev_event_id=prev_event_id,