As found in see NETVIRT-883 but certainly not causing CONTROLLER-1756 (because it's re. an open WriteOnlyTransaction and not TransactionChain), but should probably still be fixed, unless it's proven false positive.. in which case, if confirmed after analysis, this issue should be closed RESOLVED INVALID with an "explanation" why it may be perfectly logical to see non closed ReadOnlyTransaction created by DhcpExternalTunnelManager:

trace:transactions
TracingBroker found some not yet (or never..) closed transactions!

DataBroker : newWriteOnlyTransaction()
org.opendaylight.netvirt.dhcpservice.DhcpExternalTunnelManager.handleDesignatedDpnDown(DhcpExternalTunnelManager.java:340)
org.opendaylight.netvirt.dhcpservice.NodeListener.add(NodeListener.java:78)
org.opendaylight.netvirt.dhcpservice.NodeListener.add(NodeListener.java:29)
org.opendaylight.genius.datastoreutils.AsyncDataTreeChangeListenerBase$DataTreeChangeHandler.run(AsyncDataTreeChangeListenerBase.java:158)