[CI] test failure in o.o.search.aggregations.bucket.composite.CompositeAggregatorTests.testWithKeywordLongAndMissingBucket

[nknize]

Reproducible test failure related to #1942

REPRODUCE WITH: ./gradlew ':server:test' --tests "org.opensearch.search.aggregations.bucket.composite.CompositeAggregatorTests.testWithKeywordLongAndMissingBucket" -Dtests.seed=7AEBC6A11333343F -Dtests.security.manager=true -Dtests.jvm.argline="-XX:TieredStopAtLevel=1 -XX:ReservedCodeCacheSize=64m" -Dtests.locale=es-PY -Dtests.timezone=Pacific/Funafuti -Druntime.java=17

org.opensearch.search.aggregations.bucket.composite.CompositeAggregatorTests > testWithKeywordLongAndMissingBucket FAILED
    java.lang.AssertionError: expected:<3> but was:<4>
        at __randomizedtesting.SeedInfo.seed([7AEBC6A11333343F:9BD426C674ED5A70]: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.opensearch.search.aggregations.bucket.composite.CompositeAggregatorTests.lambda$testWithKeywordLongAndMissingBucket$93(CompositeAggregatorTests.java:1139)
        at org.opensearch.search.aggregations.bucket.composite.CompositeAggregatorTests.executeTestCase(CompositeAggregatorTests.java:2712)
        at org.opensearch.search.aggregations.bucket.composite.CompositeAggregatorTests.testSearchCase(CompositeAggregatorTests.java:2660)
        at org.opensearch.search.aggregations.bucket.composite.CompositeAggregatorTests.testWithKeywordLongAndMissingBucket(CompositeAggregatorTests.java:1128)

[nknize]

@penghuo can you have a look at this test failure?

[penghuo]

@penghuo can you have a look at this test failure?

Ack

[penghuo]

The root cause of the issue is InternalBucket does not use MissingOrders. In bucket reduce phase, null bucket's order is less than others.
The test cases not always fail because the bug in bucket reduce phase which required at least 2 shards.
I am preparing PR now, will publish soon.

[penghuo]

@nknize Could we close this issue? Failed test already been fixed by #2005.