Restricted rooms (MSC3083) should not have their allow key redacted.

changelog.d/10489.feature

@@ -0,0 +1 @@
+Update support for [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083) to consider changes in the MSC around which servers can issue join events.

synapse/events/utils.py

@@ -109,6 +109,8 @@ def add_fields(*fields):
         add_fields("creator")
     elif event_type == EventTypes.JoinRules:
         add_fields("join_rule")
+        if room_version.msc3083_join_rules:
+            add_fields("allow")
     elif event_type == EventTypes.PowerLevels:
         add_fields(
             "users",

tests/events/test_utils.py

@@ -301,6 +301,49 @@ def test_redacts(self):
             room_version=RoomVersions.MSC2176,
         )
 
+    def test_join_rules(self):
+        """Join rules events have changed behavior starting with MSC3083."""
+        self.run_test(
+            {
+                "type": "m.room.join_rules",
+                "event_id": "$test:domain",
+                "content": {
+                    "join_rule": "invite",
+                    "allow": [],
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.join_rules",
+                "event_id": "$test:domain",
+                "content": {"join_rule": "invite"},
+                "signatures": {},
+                "unsigned": {},
+            },
+        )
+
+        # After MSC3083, alias events have no special behavior.
+        self.run_test(
+            {
+                "type": "m.room.join_rules",
+                "content": {
+                    "join_rule": "invite",
+                    "allow": [],
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.join_rules",
+                "content": {
+                    "join_rule": "invite",
+                    "allow": [],
+                },
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3083,
+        )
+
 
 class SerializeEventTestCase(unittest.TestCase):
     def serialize(self, ev, fields):