[fix][test] Fix flaky OffloadPrefixTest.testPositionOnEdgeOfLedger race with ledger rollover

[merlimat]

Motivation

OffloadPrefixTest.testPositionOnEdgeOfLedger is flaky. The test writes exactly 20 entries with maxEntriesPerLedger=10, which fills two ledgers. Once the second ledger is full, it is closed and a third (empty) ledger starts being created asynchronously on the managed-ledger executor. The test's next line asserts that only 2 ledgers exist — this races with the async rollover and the test intermittently fails with a third ledger already present.

Example failure

Gradle suite > Gradle test > org.apache.bookkeeper.mledger.impl.OffloadPrefixTest > testPositionOnEdgeOfLedger FAILED
    java.lang.AssertionError: expected [2] but found [3]
        at org.testng.Assert.fail(Assert.java:110)
        at org.testng.Assert.failNotEquals(Assert.java:1577)
        at org.testng.Assert.assertEqualsImpl(Assert.java:149)
        at org.testng.Assert.assertEquals(Assert.java:131)
        at org.testng.Assert.assertEquals(Assert.java:1418)
        at org.testng.Assert.assertEquals(Assert.java:1382)
        at org.testng.Assert.assertEquals(Assert.java:1428)
        at org.apache.bookkeeper.mledger.impl.OffloadPrefixTest.testPositionOnEdgeOfLedger(OffloadPrefixTest.java:237)

• Failure scan: https://scans.gradle.com/s/5uzx6ammf6lko/tests/task/:managed-ledger:test/details/org.apache.bookkeeper.mledger.impl.OffloadPrefixTest/testPositionOnEdgeOfLedger/2/output
• Failure scan: https://scans.gradle.com/s/lqkj2fztrznro/tests/task/:managed-ledger:test/details/org.apache.bookkeeper.mledger.impl.OffloadPrefixTest/testPositionOnEdgeOfLedger/2/output

Modifications

Replace the hard-coded assertEquals(size, 2) with an Awaitility.await() that waits for the 3-ledger steady state (l1 full, l2 full, l3 empty). The rest of the test (getLastConfirmedEntry returning the last entry of l2, then addEntry("entry-blah") being stored in l3, then the two offloadPrefix calls) already works correctly against this stable state, so no other changes are required.

Verifying this change

• Make sure that the change passes the CI checks.

This change is already covered by existing tests, such as OffloadPrefixTest.testPositionOnEdgeOfLedger.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment