Fix fetching censored redactions from DB

Fetching a censored redactions caused an exception due to the code
expecting redactions to have a `redact` key, which redacted redactions
don't have.

synapse/storage/events_worker.py

@@ -238,6 +238,20 @@ class EventsWorkerStore(SQLBaseStore):
             # we have to recheck auth now.
 
             if not allow_rejected and entry.event.type == EventTypes.Redaction:
+                if not hasattr(entry.event, "redacts"):
+                    # A redacted redaction doesn't have a `redacts` key, in
+                    # which case lets just withhold the event.
+                    #
+                    # Note: Most of the time if the redactions has been
+                    # redacted we still have the un-redacted event in the DB
+                    # and so we'll still see the `redacts` key. However, this
+                    # isn't always true e.g. if we have censored the event.
+                    logger.debug(
+                        "Withholding redaction event %s as we don't have redacts key",
+                        event_id,
+                    )
+                    continue
+
                 redacted_event_id = entry.event.redacts
                 event_map = yield self._get_events_from_cache_or_db([redacted_event_id])
                 original_event_entry = event_map.get(redacted_event_id)

tests/storage/test_redaction.py

@@ -118,6 +118,8 @@ class RedactionTestCase(unittest.HomeserverTestCase):
 
         self.get_success(self.store.persist_event(event, context))
 
+        return event
+
     def test_redact(self):
         self.get_success(
             self.inject_room_member(self.room1, self.u_alice, Membership.JOIN)
@@ -361,3 +363,37 @@ class RedactionTestCase(unittest.HomeserverTestCase):
         )
 
         self.assert_dict({"content": {}}, json.loads(event_json))
+
+    def test_redact_redaction(self):
+        """Tests that we can redact a redaction and can fetch it again.
+        """
+
+        self.get_success(
+            self.inject_room_member(self.room1, self.u_alice, Membership.JOIN)
+        )
+
+        msg_event = self.get_success(self.inject_message(self.room1, self.u_alice, "t"))
+
+        first_redact_event = self.get_success(
+            self.inject_redaction(
+                self.room1, msg_event.event_id, self.u_alice, "Redacting message"
+            )
+        )
+
+        self.get_success(
+            self.inject_redaction(
+                self.room1,
+                first_redact_event.event_id,
+                self.u_alice,
+                "Redacting redaction",
+            )
+        )
+
+        # Now lets jump to the future where we have censored the redaction event
+        # in the DB.
+        self.reactor.advance(60 * 60 * 24 * 31)
+
+        # We just want to check that fetching the event doesn't raise an exception.
+        self.get_success(
+            self.store.get_event(first_redact_event.event_id, allow_none=True)
+        )