Allow retrieving the relations of a redacted event. (#12130)

This is allowed per MSC2675, although the original implementation did
not allow for it and would return an empty chunk / not bundle aggregations.

The main thing to improve is that the various caches get cleared properly
when an event is redacted, and that edits must not leak if the original
event is redacted (as that would presumably leak something similar to
the original event content).

tests/rest/client/test_relations.py

@@ -1475,12 +1475,13 @@ class RelationRedactionTestCase(BaseRelationsTestCase):
         self.assertEqual(relations, {})
 
     def test_redact_parent_annotation(self) -> None:
-        """Test that annotations of an event are redacted when the original event
+        """Test that annotations of an event are viewable when the original event
         is redacted.
         """
         # Add a relation
         channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", key="👍")
         self.assertEqual(200, channel.code, channel.json_body)
+        related_event_id = channel.json_body["event_id"]
 
         # The relations should exist.
         event_ids, relations = self._make_relation_requests()
@@ -1494,11 +1495,45 @@ class RelationRedactionTestCase(BaseRelationsTestCase):
         # Redact the original event.
         self._redact(self.parent_id)
 
-        # The relations are not returned.
+        # The relations are returned.
         event_ids, relations = self._make_relation_requests()
-        self.assertEqual(event_ids, [])
-        self.assertEqual(relations, {})
+        self.assertEquals(event_ids, [related_event_id])
+        self.assertEquals(
+            relations["m.annotation"],
+            {"chunk": [{"type": "m.reaction", "key": "👍", "count": 1}]},
+        )
 
         # There's nothing to aggregate.
         chunk = self._get_aggregations()
-        self.assertEqual(chunk, [])
+        self.assertEqual(chunk, [{"count": 1, "key": "👍", "type": "m.reaction"}])
+
+    @unittest.override_config({"experimental_features": {"msc3440_enabled": True}})
+    def test_redact_parent_thread(self) -> None:
+        """
+        Test that thread replies are still available when the root event is redacted.
+        """
+        channel = self._send_relation(
+            RelationTypes.THREAD,
+            EventTypes.Message,
+            content={"body": "reply 1", "msgtype": "m.text"},
+        )
+        self.assertEqual(200, channel.code, channel.json_body)
+        related_event_id = channel.json_body["event_id"]
+
+        # Redact one of the reactions.
+        self._redact(self.parent_id)
+
+        # The unredacted relation should still exist.
+        event_ids, relations = self._make_relation_requests()
+        self.assertEquals(len(event_ids), 1)
+        self.assertDictContainsSubset(
+            {
+                "count": 1,
+                "current_user_participated": True,
+            },
+            relations[RelationTypes.THREAD],
+        )
+        self.assertEqual(
+            relations[RelationTypes.THREAD]["latest_event"]["event_id"],
+            related_event_id,
+        )