Read new field name from partial join response

synapse/federation/transport/client.py

@@ -910,6 +910,14 @@ def __init__(self, room_version: RoomVersion, v1_api: bool):
                     use_float="True",
                 )
             )
+            # The stable field name comes last, so it "wins" if the fields disagree
+            self._coros.append(
+                ijson.items_coro(
+                    _partial_state_parser(self._response),
+                    "members_omitted",
+                    use_float="True",
+                )
+            )
 
             self._coros.append(
                 ijson.items_coro(

tests/federation/transport/test_client.py

@@ -19,6 +19,7 @@
 
 from synapse.api.room_versions import RoomVersions
 from synapse.federation.transport.client import SendJoinParser
+from synapse.types import JsonDict
 from synapse.util import ExceptionBundle
 
 from tests.unittest import TestCase
@@ -71,19 +72,31 @@ def test_two_writes(self) -> None:
 
     def test_partial_state(self) -> None:
         """Check that the partial_state flag is correctly parsed"""
-        parser = SendJoinParser(RoomVersions.V1, False)
-        response = {
-            "org.matrix.msc3706.partial_state": True,
-        }
 
-        serialised_response = json.dumps(response).encode()
+        def parse(response: JsonDict) -> bool:
+            parser = SendJoinParser(RoomVersions.V1, False)
+            serialised_response = json.dumps(response).encode()
 
-        # Send data to the parser
-        parser.write(serialised_response)
+            # Send data to the parser
+            parser.write(serialised_response)
 
-        # Retrieve and check the parsed SendJoinResponse
-        parsed_response = parser.finish()
-        self.assertTrue(parsed_response.partial_state)
+            # Retrieve and check the parsed SendJoinResponse
+            parsed_response = parser.finish()
+            return parsed_response.partial_state
+
+        self.assertTrue(parse({"members_omitted": True}))
+        self.assertTrue(parse({"org.matrix.msc3706.partial_state": True}))
+
+        self.assertFalse(parse({"members_omitted": False}))
+        self.assertFalse(parse({"org.matrix.msc3706.partial_state": False}))
+
+        # If there's a conflict, the stable field wins.
+        self.assertTrue(
+            parse({"members_omitted": True, "org.matrix.msc3706.partial_state": False})
+        )
+        self.assertFalse(
+            parse({"members_omitted": False, "org.matrix.msc3706.partial_state": True})
+        )
 
     def test_servers_in_room(self) -> None:
         """Check that the servers_in_room field is correctly parsed"""