aggs: Do not delegate a null scorer to LeafBucketCollectors

Closes #26611

core/src/main/java/org/elasticsearch/search/aggregations/AggregatorFactory.java

@@ -130,7 +130,11 @@ public void collect(int doc, long bucket) throws IOException {
                             aggregators.set(bucket, aggregator);
                         }
                         collector = aggregator.getLeafCollector(ctx);
-                        collector.setScorer(scorer);
+                        if (scorer != null) {
+                            // Passing a null scorer can cause unexpected NPE at a later time,
+                            // which can't not be directly linked to the fact that a null scorer has been supplied.
+                            collector.setScorer(scorer);
+                        }
                         collectors.set(bucket, collector);
                     }
                     collector.collect(doc, 0);

core/src/test/java/org/elasticsearch/search/aggregations/MultiBucketAggregatorWrapperTests.java

@@ -0,0 +1,93 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.search.aggregations;
+
+import org.apache.lucene.analysis.MockAnalyzer;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.memory.MemoryIndex;
+import org.apache.lucene.search.Scorer;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.util.BigArrays;
+import org.elasticsearch.common.util.MockBigArrays;
+import org.elasticsearch.indices.breaker.NoneCircuitBreakerService;
+import org.elasticsearch.search.internal.SearchContext;
+import org.elasticsearch.test.ESTestCase;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.reset;
+import static org.mockito.Mockito.same;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
+
+public class MultiBucketAggregatorWrapperTests extends ESTestCase {
+
+    public void testNoNullScorerIsDelegated() throws Exception {
+        LeafReaderContext leafReaderContext = MemoryIndex.fromDocument(Collections.emptyList(), new MockAnalyzer(random()))
+                .createSearcher().getIndexReader().leaves().get(0);
+        BigArrays bigArrays = new MockBigArrays(Settings.EMPTY, new NoneCircuitBreakerService());
+        SearchContext searchContext = mock(SearchContext.class);
+        when(searchContext.bigArrays()).thenReturn(bigArrays);
+
+        Aggregator aggregator = mock(Aggregator.class);
+        AggregatorFactory aggregatorFactory = new TestAggregatorFactory(searchContext, aggregator);
+        LeafBucketCollector wrappedCollector = mock(LeafBucketCollector.class);
+        when(aggregator.getLeafCollector(leafReaderContext)).thenReturn(wrappedCollector);
+        Aggregator wrapper = AggregatorFactory.asMultiBucketAggregator(aggregatorFactory, searchContext, null);
+
+        LeafBucketCollector collector = wrapper.getLeafCollector(leafReaderContext);
+
+        collector.collect(0, 0);
+        // setScorer should not be invoked as it has not been set
+        // Only collect should be invoked:
+        verify(wrappedCollector).collect(0, 0);
+        verifyNoMoreInteractions(wrappedCollector);
+
+        reset(wrappedCollector);
+        Scorer scorer = mock(Scorer.class);
+        collector.setScorer(scorer);
+        collector.collect(0, 1);
+        verify(wrappedCollector).setScorer(same(scorer));
+        verify(wrappedCollector).collect(0, 0);
+        verifyNoMoreInteractions(wrappedCollector);
+        wrapper.close();
+    }
+
+    static class TestAggregatorFactory extends AggregatorFactory {
+
+        private final Aggregator aggregator;
+
+        TestAggregatorFactory(SearchContext context, Aggregator aggregator) throws IOException {
+            super("_name", context, null, new AggregatorFactories.Builder(), Collections.emptyMap());
+            this.aggregator = aggregator;
+        }
+
+        @Override
+        protected Aggregator createInternal(Aggregator parent, boolean collectsFromSingleBucket, List list,
+                                            Map metaData) throws IOException {
+            return aggregator;
+        }
+    }
+
+}