Use stable identifiers for faster joins (#14832)

* Use new query param when requesting a partial join

* Read new query param when serving partial join

* Provide new field names when serving partial joins

* Read new field names from partial join response

* Changelog

2 files changed, 58 insertions, 21 deletions

diff --git a/tests/federation/test_federation_server.py b/tests/federation/test_federation_server.py
index 177e5b5afc..27770304be 100644
--- a/tests/federation/test_federation_server.py
+++ b/tests/federation/test_federation_server.py
@@ -224,7 +224,7 @@ class SendJoinFederationTests(unittest.FederatingHomeserverTestCase):
         )
         channel = self.make_signed_federation_request(
             "PUT",
-            f"/_matrix/federation/v2/send_join/{self._room_id}/x?org.matrix.msc3706.partial_state=true",
+            f"/_matrix/federation/v2/send_join/{self._room_id}/x?omit_members=true",
             content=join_event_dict,
         )
         self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
diff --git a/tests/federation/transport/test_client.py b/tests/federation/transport/test_client.py
index b84c74fc0e..c90635e0a0 100644
--- a/tests/federation/transport/test_client.py
+++ b/tests/federation/transport/test_client.py
@@ -13,12 +13,14 @@
 # limitations under the License.
 
 import json
+from typing import List, Optional
 from unittest.mock import Mock
 
 import ijson.common
 
 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
@@ -71,33 +73,68 @@ class SendJoinParserTestCase(TestCase):
 
     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
 
-    def test_servers_in_room(self) -> None:
-        """Check that the servers_in_room field is correctly parsed"""
-        parser = SendJoinParser(RoomVersions.V1, False)
-        response = {"org.matrix.msc3706.servers_in_room": ["hs1", "hs2"]}
+        self.assertTrue(parse({"members_omitted": True}))
+        self.assertTrue(parse({"org.matrix.msc3706.partial_state": True}))
 
-        serialised_response = json.dumps(response).encode()
+        self.assertFalse(parse({"members_omitted": False}))
+        self.assertFalse(parse({"org.matrix.msc3706.partial_state": False}))
 
-        # Send data to the parser
-        parser.write(serialised_response)
+        # 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})
+        )
 
-        # Retrieve and check the parsed SendJoinResponse
-        parsed_response = parser.finish()
-        self.assertEqual(parsed_response.servers_in_room, ["hs1", "hs2"])
+    def test_servers_in_room(self) -> None:
+        """Check that the servers_in_room field is correctly parsed"""
+
+        def parse(response: JsonDict) -> Optional[List[str]]:
+            parser = SendJoinParser(RoomVersions.V1, False)
+            serialised_response = json.dumps(response).encode()
+
+            # Send data to the parser
+            parser.write(serialised_response)
+
+            # Retrieve and check the parsed SendJoinResponse
+            parsed_response = parser.finish()
+            return parsed_response.servers_in_room
+
+        self.assertEqual(
+            parse({"org.matrix.msc3706.servers_in_room": ["hs1", "hs2"]}),
+            ["hs1", "hs2"],
+        )
+        self.assertEqual(parse({"servers_in_room": ["example.com"]}), ["example.com"])
+
+        # If both are provided, the stable identifier should win
+        self.assertEqual(
+            parse(
+                {
+                    "org.matrix.msc3706.servers_in_room": ["old"],
+                    "servers_in_room": ["new"],
+                }
+            ),
+            ["new"],
+        )
+
+        # And lastly, we should be able to tell if neither field was present.
+        self.assertEqual(
+            parse({}),
+            None,
+        )
 
     def test_errors_closing_coroutines(self) -> None:
         """Check we close all coroutines, even if closing the first raises an Exception.