Fix v12 rooms when using frozen dicts (#19235)

Fix #19233 

Synapse fails to handle events in v12 rooms when the server is run with
the `{use_frozen_dicts: True}` config.
This PR fixes the issue, and adds tests which cover room creation,
joining, and joining over federation, with both frozen and not frozen
config settings, by extending the existing `test_send_join` federation
tests.

This approach to testing was chosen as it is a simple way to get high
level integration style test coverage, without going through all our
existing tests and trying to retroactively add in coverage when using
frozen dicts.
This should provide an easy place for future room versions to extend the
suite of tests and reduce the chance of introducing subtle bugs like
this in the future.

### Pull Request Checklist

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* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
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- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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changelog.d/19235.bugfix

@@ -0,0 +1 @@
+Fix v12 rooms when running with `use_frozen_dicts: True`.

synapse/events/__init__.py

@@ -548,7 +548,7 @@ class FrozenEventV4(FrozenEventV3):
         assert create_event_id not in self._dict["auth_events"]
         if self.type == EventTypes.Create and self.get_state_key() == "":
             return self._dict["auth_events"]  # should be []
-        return self._dict["auth_events"] + [create_event_id]
+        return [*self._dict["auth_events"], create_event_id]
 
 
 def _event_type_from_format_version(

tests/federation/test_federation_server.py

@@ -30,6 +30,7 @@ from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import FederationError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersions
 from synapse.config.server import DEFAULT_ROOM_VERSION
+from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.events import EventBase, make_event_from_dict
 from synapse.federation.federation_base import event_from_pdu_json
 from synapse.http.types import QueryParams
@@ -356,19 +357,44 @@ class SendJoinFederationTests(unittest.FederatingHomeserverTestCase):
         self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
         return channel.json_body
 
-    def test_send_join(self) -> None:
+    def _test_send_join_common(self, room_version: str) -> None:
         """happy-path test of send_join"""
-        joining_user = "@misspiggy:" + self.OTHER_SERVER_NAME
+        creator_user_id = self.register_user(f"kermit_v{room_version}", "test")
-        join_result = self._make_join(joining_user)
+        tok = self.login(f"kermit_v{room_version}", "test")
+        room_id = self.helper.create_room_as(
+            room_creator=creator_user_id, tok=tok, room_version=room_version
+        )
 
+        # Second member joins
+        second_member_user_id = self.register_user(f"fozzie_v{room_version}", "bear")
+        tok2 = self.login(f"fozzie_v{room_version}", "bear")
+        self.helper.join(room_id, second_member_user_id, tok=tok2)
+
+        # Make join for remote user
+        joining_user = "@misspiggy:" + self.OTHER_SERVER_NAME
+        channel = self.make_signed_federation_request(
+            "GET",
+            f"/_matrix/federation/v1/make_join/{room_id}/{joining_user}?ver={room_version}",
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
+        join_result = channel.json_body
+
+        # Sign and send the join
         join_event_dict = join_result["event"]
         self.add_hashes_and_signatures_from_other_server(
             join_event_dict,
-            KNOWN_ROOM_VERSIONS[DEFAULT_ROOM_VERSION],
+            KNOWN_ROOM_VERSIONS[room_version],
         )
+        if room_version in ["1", "2"]:
+            add_hashes_and_signatures(
+                KNOWN_ROOM_VERSIONS[room_version],
+                join_event_dict,
+                signature_name=self.hs.hostname,
+                signing_key=self.hs.signing_key,
+            )
         channel = self.make_signed_federation_request(
             "PUT",
-            f"/_matrix/federation/v2/send_join/{self._room_id}/x",
+            f"/_matrix/federation/v2/send_join/{room_id}/x",
             content=join_event_dict,
         )
         self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
@@ -384,8 +410,8 @@ class SendJoinFederationTests(unittest.FederatingHomeserverTestCase):
                 ("m.room.power_levels", ""),
                 ("m.room.join_rules", ""),
                 ("m.room.history_visibility", ""),
-                ("m.room.member", "@kermit:test"),
+                ("m.room.member", f"@kermit_v{room_version}:test"),
-                ("m.room.member", "@fozzie:test"),
+                ("m.room.member", f"@fozzie_v{room_version}:test"),
                 # nb: *not* the joining user
             ],
         )
@@ -398,18 +424,28 @@ class SendJoinFederationTests(unittest.FederatingHomeserverTestCase):
             returned_auth_chain_events,
             [
                 ("m.room.create", ""),
-                ("m.room.member", "@kermit:test"),
+                ("m.room.member", f"@kermit_v{room_version}:test"),
                 ("m.room.power_levels", ""),
                 ("m.room.join_rules", ""),
             ],
         )
 
         # the room should show that the new user is a member
-        r = self.get_success(
+        r = self.get_success(self._storage_controllers.state.get_current_state(room_id))
-            self._storage_controllers.state.get_current_state(self._room_id)
-        )
         self.assertEqual(r[("m.room.member", joining_user)].membership, "join")
 
+    @parameterized.expand([(k,) for k in KNOWN_ROOM_VERSIONS.keys()])
+    @override_config({"use_frozen_dicts": True})
+    def test_send_join_with_frozen_dicts(self, room_version: str) -> None:
+        """Test send_join with USE_FROZEN_DICTS=True"""
+        self._test_send_join_common(room_version)
+
+    @parameterized.expand([(k,) for k in KNOWN_ROOM_VERSIONS.keys()])
+    @override_config({"use_frozen_dicts": False})
+    def test_send_join_without_frozen_dicts(self, room_version: str) -> None:
+        """Test send_join with USE_FROZEN_DICTS=False"""
+        self._test_send_join_common(room_version)
+
     def test_send_join_partial_state(self) -> None:
         """/send_join should return partial state, if requested"""
         joining_user = "@misspiggy:" + self.OTHER_SERVER_NAME