KAFKA-6784: FindCoordinatorResponse cannot be cast to FetchResponse

[koqizhao]

fix bug: FindCoordinatorResponse cannot be cast to FetchResponse

[koqizhao]

JDK 8 and Scala 2.12 case fail:
kafka.api.ClientIdQuotaTest.testThrottledProducerConsumer
It's caused by another bug (opened as KAFKA-6787). No need fix it here.

java.lang.NullPointerException
at org.apache.kafka.common.metrics.stats.SampledStat.purgeObsoleteSamples(SampledStat.java:104)
at org.apache.kafka.common.metrics.stats.SampledStat.measure(SampledStat.java:74)
at org.apache.kafka.common.metrics.KafkaMetric.metricValue(KafkaMetric.java:68)
at kafka.api.QuotaTestClients$.metricValue(BaseQuotaTest.scala:163)
at kafka.api.QuotaTestClients.produceUntilThrottled(BaseQuotaTest.scala:193)
at kafka.api.BaseQuotaTest.testThrottledProducerConsumer(BaseQuotaTest.scala:84)

[hachikuji]

Thanks for the PR. I agree this would fix the problem, but the fact that the error is happening in the first place suggests that we have a bug somewhere else in the code. Do you have a way to reproduce the problem?

[koqizhao]

It's found when running test case. Cannot be reproduced stably. I will have a try sometime.

[hachikuji]

@koqizhao The test case itself may have a bug. Do you know which one causes it?

[koqizhao]

@hackikuji, test case is
kafka.api.ClientIdQuotaTest.testThrottledProducerConsumer

cannot be repro stably.