Include bundled aggregations for the latest event in a thread. (#12273)

The `latest_event` field of the bundled aggregations for `m.thread` relations
did not include bundled aggregations itself. This resulted in clients needing to
immediately request the event from the server (and thus making it useless that
the latest event itself was serialized instead of just including an event ID).

tests/rest/client/test_relations.py

@@ -1029,7 +1029,106 @@ class BundledAggregationsTestCase(BaseRelationsTestCase):
                 bundled_aggregations.get("latest_event"),
             )
 
-        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 9)
+        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 10)
+
+    def test_thread_with_bundled_aggregations_for_latest(self) -> None:
+        """
+        Bundled aggregations should get applied to the latest thread event.
+        """
+        self._send_relation(RelationTypes.THREAD, "m.room.test")
+        channel = self._send_relation(RelationTypes.THREAD, "m.room.test")
+        thread_2 = channel.json_body["event_id"]
+
+        self._send_relation(
+            RelationTypes.ANNOTATION, "m.reaction", "a", parent_id=thread_2
+        )
+
+        def assert_thread(bundled_aggregations: JsonDict) -> None:
+            self.assertEqual(2, bundled_aggregations.get("count"))
+            self.assertTrue(bundled_aggregations.get("current_user_participated"))
+            # The latest thread event has some fields that don't matter.
+            self.assert_dict(
+                {
+                    "content": {
+                        "m.relates_to": {
+                            "event_id": self.parent_id,
+                            "rel_type": RelationTypes.THREAD,
+                        }
+                    },
+                    "event_id": thread_2,
+                    "sender": self.user_id,
+                    "type": "m.room.test",
+                },
+                bundled_aggregations.get("latest_event"),
+            )
+            # Check the unsigned field on the latest event.
+            self.assert_dict(
+                {
+                    "m.relations": {
+                        RelationTypes.ANNOTATION: {
+                            "chunk": [
+                                {"type": "m.reaction", "key": "a", "count": 1},
+                            ]
+                        },
+                    }
+                },
+                bundled_aggregations["latest_event"].get("unsigned"),
+            )
+
+        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 10)
+
+    def test_nested_thread(self) -> None:
+        """
+        Ensure that a nested thread gets ignored by bundled aggregations, as
+        those are forbidden.
+        """
+
+        # Start a thread.
+        channel = self._send_relation(RelationTypes.THREAD, "m.room.test")
+        reply_event_id = channel.json_body["event_id"]
+
+        # Disable the validation to pretend this came over federation, since it is
+        # not an event the Client-Server API will allow..
+        with patch(
+            "synapse.handlers.message.EventCreationHandler._validate_event_relation",
+            new=lambda self, event: make_awaitable(None),
+        ):
+            # Create a sub-thread off the thread, which is not allowed.
+            self._send_relation(
+                RelationTypes.THREAD, "m.room.test", parent_id=reply_event_id
+            )
+
+        # Fetch the thread root, to get the bundled aggregation for the thread.
+        relations_from_event = self._get_bundled_aggregations()
+
+        # Ensure that requesting the room messages also does not return the sub-thread.
+        channel = self.make_request(
+            "GET",
+            f"/rooms/{self.room}/messages?dir=b",
+            access_token=self.user_token,
+        )
+        self.assertEqual(200, channel.code, channel.json_body)
+        event = self._find_event_in_chunk(channel.json_body["chunk"])
+        relations_from_messages = event["unsigned"]["m.relations"]
+
+        # Check the bundled aggregations from each point.
+        for aggregations, desc in (
+            (relations_from_event, "/event"),
+            (relations_from_messages, "/messages"),
+        ):
+            # The latest event should have bundled aggregations.
+            self.assertIn(RelationTypes.THREAD, aggregations, desc)
+            thread_summary = aggregations[RelationTypes.THREAD]
+            self.assertIn("latest_event", thread_summary, desc)
+            self.assertEqual(
+                thread_summary["latest_event"]["event_id"], reply_event_id, desc
+            )
+
+            # The latest event should not have any bundled aggregations (since the
+            # only relation to it is another thread, which is invalid).
+            self.assertNotIn(
+                "m.relations", thread_summary["latest_event"]["unsigned"], desc
+            )
 
     def test_thread_edit_latest_event(self) -> None:
         """Test that editing the latest event in a thread works."""
@@ -1049,6 +1148,7 @@ class BundledAggregationsTestCase(BaseRelationsTestCase):
             content={"msgtype": "m.text", "body": "foo", "m.new_content": new_body},
             parent_id=threaded_event_id,
         )
+        edit_event_id = channel.json_body["event_id"]
 
         # Fetch the thread root, to get the bundled aggregation for the thread.
         relations_dict = self._get_bundled_aggregations()
@@ -1061,6 +1161,12 @@ class BundledAggregationsTestCase(BaseRelationsTestCase):
         self.assertIn("latest_event", thread_summary)
         latest_event_in_thread = thread_summary["latest_event"]
         self.assertEqual(latest_event_in_thread["content"]["body"], "I've been edited!")
+        # The latest event in the thread should have the edit appear under the
+        # bundled aggregations.
+        self.assertDictContainsSubset(
+            {"event_id": edit_event_id, "sender": "@alice:test"},
+            latest_event_in_thread["unsigned"]["m.relations"][RelationTypes.REPLACE],
+        )
 
     def test_aggregation_get_event_for_annotation(self) -> None:
         """Test that annotations do not get bundled aggregations included