Flaky test in TestCachePurging.testBackgroundPurges [LUCENE-9949]

While executing the `./gradlew check` command on an unrelated change, my tests crashed on this test case with the following error log:

>>>>>

org.apache.lucene.monitor.TestCachePurging > testBackgroundPurges FAILED
java.lang.AssertionError: expected:<-1> but was:<21196529334563>
at __randomizedtesting.SeedInfo.seed([4DDD0C97DB8E50CC:5D664A5C8BA9CD8C]:0)
at org.junit.Assert.fail(Assert.java:89)
at org.junit.Assert.failNotEquals(Assert.java:835)
at org.junit.Assert.assertEquals(Assert.java:647)
at org.junit.Assert.assertEquals(Assert.java:633)
at org.apache.lucene.monitor.TestCachePurging.testBackgroundPurges(TestCachePurging.java:142)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1754)
at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:942)
at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:978)
at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:992)
at org.apache.lucene.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:44)
at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
at org.apache.lucene.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:45)
at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
at org.junit.rules.RunRules.evaluate(RunRules.java:20)
at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:370)
at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:819)
at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:470)
at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:951)
at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:836)
at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:887)
at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:898)
at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
at org.apache.lucene.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:38)
at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
at org.apache.lucene.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
at org.apache.lucene.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:47)
at org.junit.rules.RunRules.evaluate(RunRules.java:20)
at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:370)
at com.carrotsearch.randomizedtesting.ThreadLeakControl.lambda$forkTimeoutingTask$0(ThreadLeakControl.java:826)
at java.base/java.lang.Thread.run(Thread.java:830)

org.apache.lucene.monitor.TestCachePurging > test suite's output saved to /home/ghw/study_ghw/lucene/lucene/monitor/build/test-results/test/outputs/OUTPUT-org.apache.lucene.monitor.TestCachePurging.txt, copied below:
> java.lang.AssertionError: expected:<-1> but was:<21196529334563>
> at __randomizedtesting.SeedInfo.seed([4DDD0C97DB8E50CC:5D664A5C8BA9CD8C]:0)
> at org.junit.Assert.fail(Assert.java:89)
> at org.junit.Assert.failNotEquals(Assert.java:835)
> at org.junit.Assert.assertEquals(Assert.java:647)
> at org.junit.Assert.assertEquals(Assert.java:633)
> at org.apache.lucene.monitor.TestCachePurging.testBackgroundPurges(TestCachePurging.java:142)
> at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
> at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
> at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
> at java.base/java.lang.reflect.Method.invoke(Method.java:567)
> at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1754)
> at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:942)
> at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:978)
> at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:992)
> at org.apache.lucene.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:44)
> at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
> at org.apache.lucene.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:45)
> at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
> at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
> at org.junit.rules.RunRules.evaluate(RunRules.java:20)
> at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
> at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:370)
> at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:819)
> at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:470)
> at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:951)
> at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:836)
> at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:887)
> at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:898)
> at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
> at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
> at org.apache.lucene.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:38)
> at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
> at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
> at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
> at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
> at org.apache.lucene.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
> at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
> at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
> at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
> at org.apache.lucene.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:47)
> at org.junit.rules.RunRules.evaluate(RunRules.java:20)
> at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
> at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:370)
> at com.carrotsearch.randomizedtesting.ThreadLeakControl.lambda$forkTimeoutingTask$0(ThreadLeakControl.java:826)
> at java.base/java.lang.Thread.run(Thread.java:830)
2> NOTE: reproduce with: gradlew test --tests TestCachePurging.testBackgroundPurges -Dtests.seed=4DDD0C97DB8E50CC -Dtests.slow=true -Dtests.badapples=true -Dtests.locale=shi-Tfng -Dtests.timezone=Antarctica/McMurdo -Dtests.asserts=true -Dtests.file.encoding=UTF-8
2> NOTE: test params are: codec=Asserting(Lucene90): \{field=TestBloomFilteredLucenePostings(BloomFilteringPostingsFormat(Lucene90)), _query_id=PostingsFormat(name=Direct)}, docValues:\{_mq=DocValuesFormat(name=Lucene90), _cache_id=DocValuesFormat(name=Lucene90), _query_id=DocValuesFormat(name=Asserting)}, maxPointsInLeafNode=1939, maxMBSortInHeap=5.0141647546104515, sim=Asserting(RandomSimilarity(queryNorm=false): {}), locale=shi-Tfng, timezone=Antarctica/McMurdo
2> NOTE: Linux 4.15.0-142-generic amd64/Oracle Corporation 13 (64-bit)/cpus=4,threads=1,free=146228880,total=270532608
2> NOTE: All tests run in this JVM: [TestSuffixingNGramTokenizer, TestTermsEnumTokenFilter, TestMultipassPresearcher, TestSpanExtractors, TestMatchAllPresearcher, TestParallelMatcher, TestQueryTermComparators, TestExplainingMatcher, TestRegexpQueryHandler, TestSimilarities, TestPresearcherMatchCollector, TestTermPresearcher, TestCachePurging]

:lucene:monitor:test (FAILURE): 171 test(s), 1 failure(s), 1 skipped

>>>>>

I tried replaying that test with the random seed with both `./gradlew check -Ptests.seed=4DDD0C97DB8E50CC:5D664A5C8BA9CD8C ` and `./gradlew test --tests TestCachePurging.testBackgroundPurges -Dtests.seed=4DDD0C97DB8E50CC -Dtests.slow=true -Dtests.badapples=true -Dtests.locale=shi-Tfng`, but both commands passed successfully.

This issue is to investigate why that flaky test failed.

---
Migrated from [LUCENE-9949](https://issues.apache.org/jira/browse/LUCENE-9949) by Gautam Worah (@gautamworah96), updated Jun 01 2021
Environment:
```
Ubuntu 18.04.5 LTS

Java 11
```

Start with TestCachePurging.java, especially testBackgroundPurges at line 142, and run the provided Gradle test command with the recorded seed to investigate the intermittent assertion. Done means identifying why the test can report a non--1 value and making the test reliable without masking failures, then verifying it with the focused test and the monitor test suite.

Written by the indexing model from the issue text.