KAFKA-18981: Fix flaky test QuorumControllerTest.testMinIsrUpdateWithElr

[Anshul-creator]

[KAFKA-18981] Deflake testMinIsrUpdateWithElr by maintaining broker heartbeats during waits

What

This PR removes nondeterminism in testMinIsrUpdateWithElr by ensuring broker sessions do not expire while the test is waiting/sleeping, through a lightweight background heartbeat pumper.

JIRA

KAFKA-18981

Reproduction of flakiness

The test is stable until you insert a sleep equal to the controller session timeout immediately after topic creation. With the original code:

1. In testMinIsrUpdateWithElr, add the line right after:

   CreateTopicsResponseData createTopicsResponseData = active.createTopics(
       ANONYMOUS_CONTEXT, createTopicsRequestData, Set.of("foo", "bar")).get();

   Insert:

   Thread.sleep(sessionTimeoutMillis);

2. Run the test loop a few times:

   pass=0; fail=0; for i in {1..5}; do \
    echo "=== Run $i ==="; \
    ./gradlew :metadata:test \
    --tests 'org.apache.kafka.controller.QuorumControllerTest' \
    -Pkafka.test.run.flaky=true \
    -PmaxParallelForks=1 -Dorg.gradle.workers.max=1 -Dtest.forkEvery=1 \
    --rerun-tasks --no-daemon --no-build-cache && pass=$((pass+1)) || fail=$((fail+1)); \
   done; echo "Summary: $pass pass / $fail fail"

Why this flakes

• The controller fences brokers that miss heartbeats for sessionTimeoutMillis.
• After initially unfencing all brokers, the test performs a sleep/wait before beginning the fencing phase.
• That pause allows the survivor broker (id 1) to lapse its session and get fenced transiently.
• Subsequent ISR/ELR assertions sometimes observe this unintended state, causing nondeterministic failures.

How (the fix)

Add a minimal background heartbeat pumper inside the test:

• Schedule rate: periodMs = max(50ms, sessionTimeoutMillis / 3) to guarantee multiple heartbeats per timeout window.
• Phase 1 (stabilize): heartbeat all brokers while creating topics and applying the initial broker-level config.
• Phase 2 (fencing): set a flag to heartbeat only brokersToKeepUnfenced right before waitForCondition that fences brokers 2 and 3.

This preserves the test’s intent (only broker 1 remains unfenced; brokers 2,3 fence) while eliminating timing races introduced by sleeps/awaits.

Verification of stability

With the heartbeat pumper in place and the flakiness sleep enabled, the loop below completed without failures:

pass=0; fail=0; for i in {1..50}; do \
 echo "=== Run $i ==="; \
 ./gradlew :metadata:test \
 --tests 'org.apache.kafka.controller.QuorumControllerTest' \
 -Pkafka.test.run.flaky=true \
 -PmaxParallelForks=1 -Dorg.gradle.workers.max=1 -Dtest.forkEvery=1 \
 --rerun-tasks --no-daemon --no-build-cache && pass=$((pass+1)) || fail=$((fail+1)); \
done; echo "Summary: $pass pass / $fail fail"

Note: I ran the loop 50 times (compared to the 5 times in reproducing the flakiness case) to increase trust in the fix

Scope

• Test-only change.
• No production behavior/APIs altered.
• Keeps existing assertions and ISR/ELR expectations intact.