Reinstate config option for reading old query parm

changelog.d/14839.feature

@@ -1 +1 @@
-Faster joins: always serve a partial join response to servers that request it.
+Faster joins: always serve a partial join response to servers that request it with the stable query param.

synapse/config/experimental.py

@@ -73,6 +73,13 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             "msc3202_transaction_extensions", False
         )
 
+        # MSC3706 (server-side support for partial state in /send_join responses)
+        # Synapse will always serve partial state responses to requests using the stable
+        # query parameter `omit_members`. If this flag is set, Synapse will also serve
+        # partial state responses to requests using the unstable query parameter
+        # `org.matrix.msc3706.partial_state`.
+        self.msc3706_enabled: bool = experimental.get("msc3706_enabled", False)
+
         # experimental support for faster joins over federation
         # (MSC2775, MSC3706, MSC3895)
         # requires a target server that can provide a partial join response (MSC3706)

synapse/federation/transport/server/federation.py

@@ -422,6 +422,7 @@ def __init__(
         server_name: str,
     ):
         super().__init__(hs, authenticator, ratelimiter, server_name)
+        self._read_msc3706_query_param = hs.config.experimental.msc3706_enabled
 
     async def on_PUT(
         self,
@@ -434,12 +435,13 @@ async def on_PUT(
         # TODO(paul): assert that event_id parsed from path actually
         #   match those given in content
 
+        partial_state = 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:
+        elif self._read_msc3706_query_param:
             partial_state = parse_boolean_from_args(
                 query, "org.matrix.msc3706.partial_state", default=False
             )