John Calcote created KAFKA-13686:
------------------------------------

             Summary: NullPointerException in Selector.pollSelectionKeys
                 Key: KAFKA-13686
                 URL: https://issues.apache.org/jira/browse/KAFKA-13686
             Project: Kafka
          Issue Type: Bug
          Components: clients
    Affects Versions: 0.11.0.1
         Environment: Linux/Centos 7.6, Java JVM - 1.8.0_181
            Reporter: John Calcote


Seeing repeated NPEs in Selector.pollSelectionKeys:

{noformat}
java.lang.NullPointerException: null
        at 
org.apache.kafka.common.network.Selector.pollSelectionKeys(Selector.java:353) 
~[kafka-clients-0.11.0.1.jar:?]
        at org.apache.kafka.common.network.Selector.poll(Selector.java:326) 
~[kafka-clients-0.11.0.1.jar:?]
        at org.apache.kafka.clients.NetworkClient.poll(NetworkClient.java:433) 
~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:232)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:208)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:184)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.internals.AbstractCoordinator.ensureCoordinatorReady(AbstractCoordinator.java:217)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.internals.AbstractCoordinator.ensureCoordinatorReady(AbstractCoordinator.java:203)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.internals.ConsumerCoordinator.poll(ConsumerCoordinator.java:286)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.KafkaConsumer.pollOnce(KafkaConsumer.java:1078)
 ~[kafka-clients-0.11.0.1.jar:?]
        at 
org.apache.kafka.clients.consumer.KafkaConsumer.poll(KafkaConsumer.java:1043) 
~[kafka-clients-0.11.0.1.jar:?]
{noformat}

{noformat}
    void pollSelectionKeys(Iterable<SelectionKey> selectionKeys,
                                   boolean isImmediatelyConnected,
                                   long currentTimeNanos) {
        Iterator<SelectionKey> iterator = selectionKeys.iterator();
        while (iterator.hasNext()) {
            SelectionKey key = iterator.next();
            iterator.remove();
            KafkaChannel channel = channel(key);
            long channelStartTimeNanos = recordTimePerConnection ? 
time.nanoseconds() : 0;

            // register all per-connection metrics at once
            sensors.maybeRegisterConnectionMetrics(channel.id());      <---- 
NPE HERE
            if (idleExpiryManager != null)
                idleExpiryManager.update(channel.id(), currentTimeNanos);

            try {
...
{noformat}

The only two possibilities are:
# sensors is null
# channel is null

sensors is not likely in my mind - it's more likely that the attachment on the 
key is null, but I don't have enough background in kafka client code to 
determine why this might be. 



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