bugfix: allow gateways to permit authentication from v4 clients (#5862)

gateway/src/node/client_handling/websocket/connection_handler/fresh.rs

@@ -419,6 +419,13 @@ impl<R, S> FreshHandler<R, S> {
             return Ok(INITIAL_PROTOCOL_VERSION);
         };
 
+        // #####
+        // On backwards compat:
+        // Currently it is the case that gateways will understand all previous protocol versions
+        // and will downgrade accordingly, but this will now always be the case.
+        // For example, once we remove downgrade on legacy auth, anything below version 4 will be rejected
+        // #####
+
         // a v2 gateway will understand v1 requests, but v1 client will not understand v2 responses
         if client_protocol_version == 1 {
             return Ok(1);
@@ -434,6 +441,11 @@ impl<R, S> FreshHandler<R, S> {
             return Ok(3);
         }
 
+        // a v5 gateway will understand v4 requests (key-rotation)
+        if client_protocol_version == 4 {
+            return Ok(4);
+        }
+
         // we can't handle clients with higher protocol than ours
         // (perhaps we could try to negotiate downgrade on our end? sounds like a nice future improvement)
         if client_protocol_version <= CURRENT_PROTOCOL_VERSION {