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https://issues.apache.org/jira/browse/KAFKA-20802?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Chia-Ping Tsai resolved KAFKA-20802.
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Fix Version/s: 4.4.0
Resolution: Fixed
> ClusterTestExtensions#afterEach throws NPE when a detected thread terminates
> before the leak report is built
> ------------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-20802
> URL: https://issues.apache.org/jira/browse/KAFKA-20802
> Project: Kafka
> Issue Type: Bug
> Reporter: Eric Chang
> Assignee: Eric Chang
> Priority: Major
> Fix For: 4.4.0
>
>
> {{ClusterTestExtensions#afterEach}} builds the thread-leak error message with
> {{t.getThreadGroup().getName()}}. {{Thread#getThreadGroup()}} returns null
> after a thread has terminated.
> If a thread returned by {{DetectThreadLeak#newThreads()}} terminates between
> the snapshot and message formatting, {{afterEach}} throws an NPE and fails
> the test even though the thread is no longer a leak.
> h3. CI evidence
> * Full job:
> https://github.com/apache/kafka/actions/runs/29228499012/job/86748625389
> * NPE in the JUnit test step:
> https://github.com/apache/kafka/actions/runs/29228499012/job/86748625389#step:6:136310
> * Flaky-test report:
> https://github.com/apache/kafka/actions/runs/29228499012/job/86748625389#step:10:1872
> * Affected test:
> {{MetadataQuorumCommandTest.testDescribeQuorumStatusSuccessful [5]}}
> {code}
> java.lang.NullPointerException: Cannot invoke
> "java.lang.ThreadGroup.getName()" because the return value of
> "java.lang.Thread.getThreadGroup()" is null
> at
> org.apache.kafka.common.test.junit.ClusterTestExtensions.lambda$afterEach$2(ClusterTestExtensions.java:188)
> {code}
> h3. Reproduction
> A deterministic regression test can reproduce the same NPE by making
> {{DetectThreadLeak#newThreads()}} return a thread that has already been
> joined, then invoking the real {{afterEach}} path. The proposed logic passes
> this regression test and continues to report live leaked threads.
> I also ran the actual affected test locally for 120 isolated cluster contexts
> using Java 17, 4 forks, and retries disabled. All 120 passed, so the natural
> timing window did not reproduce locally in that batch.
> h3. Expected behavior
> Threads that have already terminated should be ignored while constructing the
> leak report because they are not leaks. The reporting path must not throw an
> NPE.
> h3. Related issue
> KAFKA-17189 (https://issues.apache.org/jira/browse/KAFKA-17189) describes the
> underlying snapshot-to-report race and proposes adding {{Thread::isAlive}} to
> the leak-detector filter. This issue tracks the concrete
> {{ClusterTestExtensions#afterEach}} NPE observed in CI.
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