Implement buildEmptyAggregations for MultiTermsAggregator (#7089)

CHANGELOG.md

@@ -150,6 +150,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Enforce 512 byte document ID limit in bulk updates ([#8039](https://github.com/opensearch-project/OpenSearch/pull/8039))
 - With only GlobalAggregation in request causes unnecessary wrapping with MultiCollector ([#8125](https://github.com/opensearch-project/OpenSearch/pull/8125))
 - Fix mapping char_filter when mapping a hashtag ([#7591](https://github.com/opensearch-project/OpenSearch/pull/7591))
+- Fix NPE in multiterms aggregations involving empty buckets ([#7318](https://github.com/opensearch-project/OpenSearch/pull/7318))
 
 ### Security
 

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/370_multi_terms.yml

@@ -712,3 +712,51 @@ setup:
   - match: { aggregations.m_terms.buckets.0.key: ["a", 1] }
   - match: { aggregations.m_terms.buckets.0.key_as_string: "a|1" }
   - match: { aggregations.m_terms.buckets.0.doc_count: 4 }
+
+---
+"aggregate over multi-terms test":
+  - skip:
+      version: "- 2.9.99"
+      reason: "multi_terms aggregation was introduced in 2.1.0, NPE bug checked by this test case will manifest in any version < 3.0"
+
+  - do:
+      bulk:
+        index: test_1
+        refresh: true
+        body:
+          - '{"index": {}}'
+          - '{"str": "a", "ip": "127.0.0.1", "date": "2022-03-23"}'
+          - '{"index": {}}'
+          - '{"str": "a", "ip": "127.0.0.1", "date": "2022-03-25"}'
+          - '{"index": {}}'
+          - '{"str": "b", "ip": "127.0.0.1", "date": "2022-03-23"}'
+          - '{"index": {}}'
+          - '{"str": "b", "ip": "127.0.0.1", "date": "2022-03-25"}'
+
+  - do:
+      search:
+        index: test_1
+        size: 0
+        body:
+          aggs:
+            histo:
+              date_histogram:
+                field: date
+                calendar_interval: day
+              aggs:
+                m_terms:
+                  multi_terms:
+                    terms:
+                      - field: str
+                      - field: ip
+
+  - match: { hits.total.value: 4 }
+  - length: { aggregations.histo.buckets: 3 }
+  - match: { aggregations.histo.buckets.0.key_as_string: "2022-03-23T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.0.m_terms.buckets.0.key: ["a", "127.0.0.1"] }
+  - match: { aggregations.histo.buckets.0.m_terms.buckets.1.key: ["b", "127.0.0.1"] }
+  - match: { aggregations.histo.buckets.1.key_as_string: "2022-03-24T00:00:00.000Z" }
+  - length: { aggregations.histo.buckets.1.m_terms.buckets: 0 }
+  - match: { aggregations.histo.buckets.2.key_as_string: "2022-03-25T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.2.m_terms.buckets.0.key: [ "a", "127.0.0.1" ] }
+  - match: { aggregations.histo.buckets.2.m_terms.buckets.1.key: [ "b", "127.0.0.1" ] }

server/src/main/java/org/opensearch/search/aggregations/bucket/terms/MultiTermsAggregator.java

@@ -196,7 +196,20 @@ InternalMultiTerms buildResult(long owningBucketOrd, long otherDocCount, Interna
 
     @Override
     public InternalAggregation buildEmptyAggregation() {
-        return null;
+        return new InternalMultiTerms(
+            name,
+            order,
+            order,
+            bucketCountThresholds.getRequiredSize(),
+            bucketCountThresholds.getMinDocCount(),
+            metadata(),
+            bucketCountThresholds.getShardSize(),
+            showTermDocCountError,
+            0,
+            0,
+            formats,
+            Collections.emptyList()
+        );
     }
 
     @Override

server/src/test/java/org/opensearch/search/aggregations/bucket/terms/MultiTermsAggregatorTests.java

@@ -28,36 +28,51 @@
 import org.opensearch.common.network.InetAddresses;
 import org.opensearch.common.settings.Settings;
 import org.opensearch.common.time.DateFormatter;
+import org.opensearch.common.util.BigArrays;
+import org.opensearch.common.util.MockPageCacheRecycler;
+import org.opensearch.index.IndexService;
+import org.opensearch.index.cache.IndexCache;
 import org.opensearch.index.mapper.BooleanFieldMapper;
 import org.opensearch.index.mapper.DateFieldMapper;
 import org.opensearch.index.mapper.GeoPointFieldMapper;
 import org.opensearch.index.mapper.IpFieldMapper;
 import org.opensearch.index.mapper.KeywordFieldMapper;
 import org.opensearch.index.mapper.MappedFieldType;
 import org.opensearch.index.mapper.NumberFieldMapper;
+import org.opensearch.index.query.QueryShardContext;
+import org.opensearch.index.shard.IndexShard;
+import org.opensearch.indices.breaker.NoneCircuitBreakerService;
 import org.opensearch.script.MockScriptEngine;
 import org.opensearch.script.Script;
 import org.opensearch.script.ScriptEngine;
 import org.opensearch.script.ScriptModule;
 import org.opensearch.script.ScriptService;
 import org.opensearch.script.ScriptType;
+import org.opensearch.search.DocValueFormat;
 import org.opensearch.search.aggregations.AggregationBuilder;
+import org.opensearch.search.aggregations.Aggregator;
+import org.opensearch.search.aggregations.AggregatorFactories;
 import org.opensearch.search.aggregations.AggregatorTestCase;
 import org.opensearch.search.aggregations.BucketOrder;
+import org.opensearch.search.aggregations.CardinalityUpperBound;
+import org.opensearch.search.aggregations.InternalAggregation;
 import org.opensearch.search.aggregations.metrics.InternalMax;
 import org.opensearch.search.aggregations.metrics.MaxAggregationBuilder;
 import org.opensearch.search.aggregations.support.CoreValuesSourceType;
 import org.opensearch.search.aggregations.support.MultiTermsValuesSourceConfig;
 import org.opensearch.search.aggregations.support.ValueType;
 import org.opensearch.search.aggregations.support.ValuesSourceType;
+import org.opensearch.search.internal.SearchContext;
 import org.opensearch.search.lookup.LeafDocLookup;
+import org.opensearch.test.TestSearchContext;
 
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.UUID;
 import java.util.function.Consumer;
 import java.util.function.Function;
 
@@ -68,8 +83,12 @@
 import static java.util.stream.Collectors.toList;
 import static org.hamcrest.Matchers.closeTo;
 import static org.hamcrest.Matchers.contains;
+import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasSize;
+import static org.hamcrest.Matchers.instanceOf;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 public class MultiTermsAggregatorTests extends AggregatorTestCase {
     private static final String FIELD_NAME = "field";
@@ -852,6 +871,56 @@ public void testIncludeExclude() throws IOException {
         );
     }
 
+    public void testEmptyAggregations() throws IOException {
+        QueryShardContext queryShardContext = mock(QueryShardContext.class);
+        IndexShard indexShard = mock(IndexShard.class);
+        BigArrays bigArrays = new BigArrays(new MockPageCacheRecycler(Settings.EMPTY), new NoneCircuitBreakerService(), "");
+        IndexService indexService = mock(IndexService.class);
+        when(indexService.getShardOrNull(0)).thenReturn(indexShard);
+        IndexCache cache = mock(IndexCache.class);
+        when(cache.bitsetFilterCache()).thenReturn(null);
+        when(indexService.cache()).thenReturn(cache);
+        SearchContext context = new TestSearchContext(bigArrays, indexService);
+        when(indexService.newQueryShardContext(0, null, () -> 0L, null)).thenReturn(queryShardContext);
+        AggregatorFactories factories = AggregatorFactories.EMPTY;
+        boolean showTermDocCountError = true;
+        MultiTermsAggregator.InternalValuesSource internalValuesSources = mock(MultiTermsAggregator.InternalValuesSource.class);
+        DocValueFormat format = mock(DocValueFormat.class);
+        BucketOrder order = mock(BucketOrder.class);
+        Aggregator.SubAggCollectionMode collectMode = Aggregator.SubAggCollectionMode.BREADTH_FIRST;
+        TermsAggregator.BucketCountThresholds bucketCountThresholds = mock(TermsAggregator.BucketCountThresholds.class);
+        Aggregator parent = mock(Aggregator.class);
+        CardinalityUpperBound cardinality = CardinalityUpperBound.ONE;
+        Map<String, Object> metadata = new HashMap<>();
+        String k1 = UUID.randomUUID().toString();
+        String v1 = UUID.randomUUID().toString();
+        metadata.put(k1, v1);
+
+        MultiTermsAggregator mAgg = new MultiTermsAggregator(
+            AGG_NAME,
+            factories,
+            showTermDocCountError,
+            List.of(internalValuesSources),
+            List.of(format),
+            order,
+            collectMode,
+            bucketCountThresholds,
+            context,
+            parent,
+            cardinality,
+            metadata
+        );
+        InternalAggregation emptyAgg = mAgg.buildEmptyAggregation();
+
+        MatcherAssert.assertThat(emptyAgg.getName(), equalTo(AGG_NAME));
+        MatcherAssert.assertThat(emptyAgg, instanceOf(InternalMultiTerms.class));
+
+        InternalMultiTerms mt = (InternalMultiTerms) emptyAgg;
+        MatcherAssert.assertThat(mt.getMetadata().keySet(), contains(k1));
+        MatcherAssert.assertThat(mt.getMetadata().get(k1), equalTo(v1));
+        MatcherAssert.assertThat(mt.getBuckets(), empty());
+    }
+
     private void testAggregation(
         Query query,
         List<MultiTermsValuesSourceConfig> terms,