Low Description
If clients connect to a cassandra cluster configured with rpc_server_type: sync with heterogeneous cql versions (2 and 3), the cql version used for execution on the server changes seemingly randomly.
It's due to the fact that CustomTThreadPoolServer.java does not set the remoteSocket anytime, or does not clear the cql version in the ThreadLocal clientState object.
When CassandraServer.java calls state() it gets the ThreadLocal object clientState, which has its cqlversion already changed by a previous socket that was using the same thread.
The easiest fix is probably to do a SocketSessionManagementService.instance.set when accepting a new client and SocketSessionManagementService.instance.remove when the client is closed, but if you really want to use the ThreadLocal clientState and not alloc/destroy a ClientState everytime, then you should clear this clientState on accept of a new client.
The problem can be reproduced with cqlsh -3 on one side and a client that does not set the cql version, expecting to get version 2 by default, but actually gettingv v2/v3 depending on which thread it connects to.
The problem does not happen with other rpc_server_types, nor with clients that set their cql version at connection.