[Test] Flaky ScalableTopicControllerAutoScaleTest.testRebucketCooldownSurvivesLeaderFailover #26552

Describe the bug

ScalableTopicControllerAutoScaleTest.testRebucketCooldownSurvivesLeaderFailover can assert before a coalesced auto-scale evaluation finishes.

Observed in CI - Unit - Brokers - Broker Group 2, on commit ba600bde951caa5697f7a8dce91b91586306bb5a:

java.lang.AssertionError: a hot-segment split must fire — the rollover is not a split expected [2] but found [1]
    at ScalableTopicControllerAutoScaleTest.testRebucketCooldownSurvivesLeaderFailover(ScalableTopicControllerAutoScaleTest.java:507)

The downloaded test report contains two failing attempts. In both, the logs show the expected split dispatched on the scheduler with trigger=coalesced, followed by creation of the two successor segments.

Timing issue

The test registers a consumer, reports hot-segment load, waits on evaluateAutoScaleForTest().get(), and immediately asserts the active segment count.

However, evaluateAndAct returns an already-completed future when another evaluation is in flight. It marks a pending evaluation, which is later scheduled asynchronously. Consequently, waiting on the test helper does not necessarily wait for the requested evaluation or its split.

This explains the timing-dependent assertion: the test can observe one active segment before the scheduler completes the split. The controller and test are unchanged by the PR where this occurred.

Suggested correction

Wait for the expected layout after triggering evaluation, or make the test helper await the coalesced evaluation. Preserve the assertions that rebucket cooldown survives failover and does not suppress a load-driven split.

Environment

• GitHub Actions, Linux, Corretto JDK 25
• Gradle 9.7.1
• Build scan

Evidence is from CI logs, the test-report artifact, and source inspection; no local reproduction run yet.