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, 28 insertions, 3 deletions

diff --git a/synapse/federation/transport/client.py b/synapse/federation/transport/client.py
index 77f1f39cac..c8471d4cf7 100644
--- a/synapse/federation/transport/client.py
+++ b/synapse/federation/transport/client.py
@@ -357,6 +357,7 @@ class TransportLayerClient:
         if self._faster_joins_enabled:
             # lazy-load state on join
             query_params["org.matrix.msc3706.partial_state"] = "true"
+            query_params["omit_members"] = "true"
 
         return await self.client.put_json(
             destination=destination,
@@ -909,6 +910,14 @@ class SendJoinParser(ByteParser[SendJoinResponse]):
                     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(
@@ -918,6 +927,15 @@ class SendJoinParser(ByteParser[SendJoinResponse]):
                 )
             )
 
+            # Again, stable field name comes last
+            self._coros.append(
+                ijson.items_coro(
+                    _servers_in_room_parser(self._response),
+                    "servers_in_room",
+                    use_float="True",
+                )
+            )
+
     def write(self, data: bytes) -> int:
         for c in self._coros:
             c.send(data)
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index 53e77b4bb6..c0a700905b 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -437,9 +437,16 @@ class FederationV2SendJoinServlet(BaseFederationServerServlet):
 
         partial_state = False
         if self._msc3706_enabled:
-            partial_state = parse_boolean_from_args(
-                query, "org.matrix.msc3706.partial_state", default=False
-            )
+            # The stable query parameter wins, if it disagrees with the unstable
+            # parameter for some reason.
+            stable_param = parse_boolean_from_args(query, "omit_members", default=None)
+            if stable_param is not None:
+                partial_state = stable_param
+            else:
+                partial_state = parse_boolean_from_args(
+                    query, "org.matrix.msc3706.partial_state", default=False
+                )
+
         result = await self.handler.on_send_join_request(
             origin, content, room_id, caller_supports_partial_state=partial_state
         )