KafkaAdminClient loses worker thread/enters zombie state when initial DNS lookup fails

Version: kafka-clients-2.1.0

I have some code that creates creates a KafkaAdminClient instance and then invokes listTopics(). I was seeing the following stacktrace in the logs, after which the KafkaAdminClient instance became unresponsive:

ERROR [kafka-admin-client-thread | adminclient-1] 2019-02-18 01:00:45,597 KafkaThread.java:51 - Uncaught exception in thread 'kafka-admin-client-thread | adminclient-1':
java.lang.IllegalStateException: No entry found for connection 0
    at org.apache.kafka.clients.ClusterConnectionStates.nodeState(ClusterConnectionStates.java:330)
    at org.apache.kafka.clients.ClusterConnectionStates.disconnected(ClusterConnectionStates.java:134)
    at org.apache.kafka.clients.NetworkClient.initiateConnect(NetworkClient.java:921)
    at org.apache.kafka.clients.NetworkClient.ready(NetworkClient.java:287)
    at org.apache.kafka.clients.admin.KafkaAdminClient$AdminClientRunnable.sendEligibleCalls(KafkaAdminClient.java:898)
    at org.apache.kafka.clients.admin.KafkaAdminClient$AdminClientRunnable.run(KafkaAdminClient.java:1113)
    at java.lang.Thread.run(Thread.java:748)

From looking at the code I was able to trace down a possible cause:

• NetworkClient.ready() invokes this.initiateConnect() as seen in the above stacktrace
• NetworkClient.initiateConnect() invokes ClusterConnectionStates.connecting(), which internally invokes ClientUtils.resolve() to to resolve the host when creating an entry for the connection.
• If this host lookup fails, a UnknownHostException can be thrown back to NetworkClient.initiateConnect() and the connection entry is not created in ClusterConnectionStates. This exception doesn't get logged so this is a guess on my part.
• NetworkClient.initiateConnect() catches the exception and attempts to call ClusterConnectionStates.disconnected(), which throws an IllegalStateException because no entry had yet been created due to the lookup failure.
• This IllegalStateException ends up killing the worker thread and KafkaAdminClient gets stuck, never returning from listTopics().