[Backport 2.x] Fix double invocation of postCollection when MultiBucketCollector is present

CHANGELOG.md

@@ -49,6 +49,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Don't return negative scores from `multi_match` query with `cross_fields` type  ([#13829](https://github.com/opensearch-project/OpenSearch/pull/13829))
 - Pass parent filter to inner hit query ([#13903](https://github.com/opensearch-project/OpenSearch/pull/13903))
 - Fix NPE on restore searchable snapshot ([#13911](https://github.com/opensearch-project/OpenSearch/pull/13911))
+- Fix double invocation of postCollection when MultiBucketCollector is present ([#14015](https://github.com/opensearch-project/OpenSearch/pull/14015))
 
 ### Security
 

server/src/internalClusterTest/java/org/opensearch/search/aggregations/bucket/terms/StringTermsIT.java

@@ -42,10 +42,12 @@
 import org.opensearch.index.query.QueryBuilders;
 import org.opensearch.script.Script;
 import org.opensearch.script.ScriptType;
+import org.opensearch.search.aggregations.AggregationBuilders;
 import org.opensearch.search.aggregations.AggregationExecutionException;
 import org.opensearch.search.aggregations.Aggregator.SubAggCollectionMode;
 import org.opensearch.search.aggregations.BucketOrder;
 import org.opensearch.search.aggregations.bucket.filter.Filter;
+import org.opensearch.search.aggregations.bucket.filter.InternalFilters;
 import org.opensearch.search.aggregations.bucket.terms.Terms.Bucket;
 import org.opensearch.search.aggregations.metrics.Avg;
 import org.opensearch.search.aggregations.metrics.ExtendedStats;
@@ -999,6 +1001,72 @@ public void testOtherDocCount() {
         testOtherDocCount(SINGLE_VALUED_FIELD_NAME, MULTI_VALUED_FIELD_NAME);
     }
 
+    public void testDeferredSubAggs() {
+        // Tests subAgg doc count is the same with different collection modes and additional top level aggs
+        SearchResponse r1 = client().prepareSearch("idx")
+            .setSize(0)
+            .addAggregation(
+                terms("terms1").collectMode(SubAggCollectionMode.BREADTH_FIRST)
+                    .field("s_value")
+                    .size(2)
+                    .subAggregation(AggregationBuilders.filters("filter", QueryBuilders.boolQuery()))
+            )
+            .addAggregation(AggregationBuilders.min("min").field("constant"))
+            .get();
+
+        SearchResponse r2 = client().prepareSearch("idx")
+            .setSize(0)
+            .addAggregation(
+                terms("terms1").collectMode(SubAggCollectionMode.DEPTH_FIRST)
+                    .field("s_value")
+                    .size(2)
+                    .subAggregation(AggregationBuilders.filters("filter", QueryBuilders.boolQuery()))
+            )
+            .addAggregation(AggregationBuilders.min("min").field("constant"))
+            .get();
+
+        SearchResponse r3 = client().prepareSearch("idx")
+            .setSize(0)
+            .addAggregation(
+                terms("terms1").collectMode(SubAggCollectionMode.BREADTH_FIRST)
+                    .field("s_value")
+                    .size(2)
+                    .subAggregation(AggregationBuilders.filters("filter", QueryBuilders.boolQuery()))
+            )
+            .get();
+
+        SearchResponse r4 = client().prepareSearch("idx")
+            .setSize(0)
+            .addAggregation(
+                terms("terms1").collectMode(SubAggCollectionMode.DEPTH_FIRST)
+                    .field("s_value")
+                    .size(2)
+                    .subAggregation(AggregationBuilders.filters("filter", QueryBuilders.boolQuery()))
+            )
+            .get();
+
+        assertNotNull(r1.getAggregations().get("terms1"));
+        assertNotNull(r2.getAggregations().get("terms1"));
+        assertNotNull(r3.getAggregations().get("terms1"));
+        assertNotNull(r4.getAggregations().get("terms1"));
+
+        Terms terms = r1.getAggregations().get("terms1");
+        Bucket b1 = terms.getBucketByKey("val0");
+        InternalFilters f1 = b1.getAggregations().get("filter");
+        long docCount1 = f1.getBuckets().get(0).getDocCount();
+        Bucket b2 = terms.getBucketByKey("val1");
+        InternalFilters f2 = b2.getAggregations().get("filter");
+        long docCount2 = f1.getBuckets().get(0).getDocCount();
+
+        for (SearchResponse response : new SearchResponse[] { r2, r3, r4 }) {
+            terms = response.getAggregations().get("terms1");
+            f1 = terms.getBucketByKey(b1.getKeyAsString()).getAggregations().get("filter");
+            f2 = terms.getBucketByKey(b2.getKeyAsString()).getAggregations().get("filter");
+            assertEquals(docCount1, f1.getBuckets().get(0).getDocCount());
+            assertEquals(docCount2, f2.getBuckets().get(0).getDocCount());
+        }
+    }
+
     /**
      * Make sure that a request using a deterministic script or not using a script get cached.
      * Ensure requests using nondeterministic scripts do not get cached.

server/src/main/java/org/opensearch/search/aggregations/BucketCollectorProcessor.java

@@ -71,10 +71,10 @@ public void processPostCollection(Collector collectorTree) throws IOException {
                     collectors.offer(innerCollector);
                 }
             } else if (currentCollector instanceof BucketCollector) {
-                ((BucketCollector) currentCollector).postCollection();
-
                 // Perform build aggregation during post collection
                 if (currentCollector instanceof Aggregator) {
+                    // Do not perform postCollection for MultiBucketCollector as we are unwrapping that below
+                    ((BucketCollector) currentCollector).postCollection();
                     ((Aggregator) currentCollector).buildTopLevel();
                 } else if (currentCollector instanceof MultiBucketCollector) {
                     for (Collector innerCollector : ((MultiBucketCollector) currentCollector).getCollectors()) {

server/src/main/java/org/opensearch/search/aggregations/bucket/BestBucketsDeferringCollector.java

@@ -124,6 +124,7 @@ private void finishLeaf() {
         if (context != null) {
             assert docDeltasBuilder != null && bucketsBuilder != null;
             entries.add(new Entry(context, docDeltasBuilder.build(), bucketsBuilder.build()));
+            context = null;
         }
     }
 
@@ -161,6 +162,7 @@ public void preCollection() throws IOException {
 
     @Override
     public void postCollection() throws IOException {
+        assert searchContext.searcher().getLeafContexts().isEmpty() || finished != true;
         finishLeaf();
         finished = true;
     }