IMPALA-7802 is fixed, the network connection of an idle session will be closed --idle_client_poll_time_s seconds latest after the session has been declared idle. This is a change of behavior from previous versions in which the client connection will hang around until the user explicitly closes the session and disconnects.
By default, --idle_client_poll_time_s is set to 30 seconds and the previous behavior can be restored by setting to 0. In addition, the session will only be closed if idle session timeout has been configured to be greater than 0 for the session either via the startup flag --idle_session_timeout or the query option IDLE_SESSION_TIMEOUT. If idle session timeout is not configured, a session cannot become idle by definition and therefore its connection cannot be closed until the client explicitly closes it.