Edoardo Comar created KAFKA-20619:
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Summary: ClusterTests may report Thread leak detected: Cleaner-0,
Cleaner-1
Key: KAFKA-20619
URL: https://issues.apache.org/jira/browse/KAFKA-20619
Project: Kafka
Issue Type: Bug
Components: unit tests
Reporter: Edoardo Comar
Running in today's trunk (commit
89f3888c8718da91967badef560cf34ebe8e09ed)
A test like
./gradlew clean client:clients-integration-tests:test --tests
SocketServerMemoryPoolTest
I get consistently
testProduceRequestWithUnsupportedVersion [1] Type=Raft-Combined,
MetadataVersion=4.4-IV0,BrokerSecurityProtocol=PLAINTEXT,BrokerListenerName=ListenerName(EXTERNAL),ControllerSecurityProtocol=PLAINTEXT,ControllerListenerName=ListenerName(CONTROLLER)
FAILED
org.opentest4j.AssertionFailedError: Thread leak detected: Cleaner-0,
Cleaner-1 ==> expected: <true> but was: <false>
at
app//org.apache.kafka.common.test.junit.ClusterTestExtensions.afterEach(ClusterTestExtensions.java:184)
at [email protected]/java.util.Optional.ifPresent(Optional.java:178)
Which appears to be due to (Claude's report)
Root cause — Cleaner-0 / Cleaner-1 are JDK-internal daemon threads created
lazily by jdk.internal.ref.CleanerImpl (group: InnocuousThreadGroup). They're
not
application threads; they're spawned by JDK code for native resource cleanup
(e.g. direct ByteBuffer deallocation) and live in the InnocuousThreadGroup. The
ClusterTestExtensions thread-leak detector doesn't currently skip them, so it
flags them as leaks.
Why now — they're triggered the first time the broker happens to allocate
enough direct-buffer-backed objects whose cleanup uses
jdk.internal.ref.CleanerFactory. The testRequestWithUnsupportedVersion path
goes through processChannelException which logs a full stack trace via log4j2 —
log4j2's formatting / reflective stack walk can be what causes the JDK to
spin up these cleaners on first use. The corrupt-body test runs second, so by
beforeEach the cleaners already exist and aren't reported as "new".
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