Check redaction state when event is pulled out of the database

synapse/storage/events_worker.py

@@ -466,6 +466,19 @@ class EventsWorkerStore(SQLBaseStore):
                     # will serialise this field correctly
                     redacted_event.unsigned["redacted_because"] = because
 
+                    # Starting in room version v3, some redactions need to be
+                    # rechecked if we didn't have the redacted event at the
+                    # time, so we recheck on read instead.
+                    if because.internal_metadata.need_to_check_redaction():
+                        expected_domain = get_domain_from_id(original_ev.sender)
+                        if get_domain_from_id(because.sender) == expected_domain:
+                            # This redaction event is allowed. Mark as not needing a
+                            # recheck.
+                            because.internal_metadata.recheck_redaction = False
+                        else:
+                            # Senders don't match, so the event is actually redacted
+                            redacted_event = None
+
             cache_entry = _EventCacheEntry(
                 event=original_ev,
                 redacted_event=redacted_event,