Remove empty results before merging (elastic#126770) (elastic#127031)

We addressed the empty top docs issue with elastic#126385 specifically for scenarios where
empty top docs don't go through the wire. Yet they may be serialized from data node
back to the coord node, in which case they will no longer be equal to Lucene#EMPTY_TOP_DOCS.

This commit expands the existing filtering of empty top docs to include also those that
did go through serialization.

Closes elastic#126742

Author: javanna

docs/changelog/126770.yaml

@@ -0,0 +1,6 @@
+pr: 126770
+summary: Remove empty results before merging
+area: Search
+type: bug
+issues:
+ - 126742

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -206,6 +206,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion AMAZON_BEDROCK_TASK_SETTINGS_8_19 = def(8_841_0_17);
     public static final TransportVersion SEMANTIC_TEXT_CHUNKING_CONFIG_8_19 = def(8_841_0_18);
     public static final TransportVersion BATCHED_QUERY_PHASE_VERSION_BACKPORT_8_X = def(8_841_0_19);
+    public static final TransportVersion SEARCH_INCREMENTAL_TOP_DOCS_NULL_BACKPORT_8_19 = def(8_841_0_20);
 
     /*
      * STOP! READ THIS FIRST! No, really,

server/src/main/java/org/elasticsearch/action/search/QueryPhaseResultConsumer.java

@@ -303,13 +303,19 @@ private static void consumePartialMergeResult(
         Collection<InternalAggregations> aggsList
     ) {
         if (topDocsList != null) {
-            topDocsList.add(partialResult.reducedTopDocs);
+            addTopDocsToList(partialResult, topDocsList);
         }
         if (aggsList != null) {
             addAggsToList(partialResult, aggsList);
         }
     }
 
+    private static void addTopDocsToList(MergeResult partialResult, List<TopDocs> topDocsList) {
+        if (partialResult.reducedTopDocs != null) {
+            topDocsList.add(partialResult.reducedTopDocs);
+        }
+    }
+
     private static void addAggsToList(MergeResult partialResult, Collection<InternalAggregations> aggsList) {
         var aggs = partialResult.reducedAggs;
         if (aggs != null) {
@@ -340,7 +346,7 @@ private MergeResult partialReduce(
         if (hasTopDocs) {
             topDocsList = new ArrayList<>(resultSetSize);
             if (lastMerge != null) {
-                topDocsList.add(lastMerge.reducedTopDocs);
+                addTopDocsToList(lastMerge, topDocsList);
             }
         } else {
             topDocsList = null;
@@ -358,7 +364,7 @@ private MergeResult partialReduce(
                 }
             }
             // we have to merge here in the same way we collect on a shard
-            newTopDocs = topDocsList == null ? Lucene.EMPTY_TOP_DOCS : mergeTopDocs(topDocsList, topNSize, 0);
+            newTopDocs = topDocsList == null ? null : mergeTopDocs(topDocsList, topNSize, 0);
             newAggs = hasAggs
                 ? aggregate(
                     toConsume.iterator(),
@@ -647,7 +653,7 @@ private static void releaseAggs(List<QuerySearchResult> toConsume) {
 
     record MergeResult(
         List<SearchShard> processedShards,
-        TopDocs reducedTopDocs,
+        @Nullable TopDocs reducedTopDocs,
         @Nullable InternalAggregations reducedAggs,
         long estimatedSize
     ) implements Writeable {

server/src/main/java/org/elasticsearch/action/search/SearchPhaseController.java

@@ -60,6 +60,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.concurrent.Executor;
 import java.util.function.BiFunction;
 import java.util.function.Consumer;
@@ -140,24 +141,26 @@ static SortedTopDocs sortDocs(
     }
 
     static TopDocs mergeTopDocs(Collection<TopDocs> results, int topN, int from) {
-        if (results.isEmpty()) {
+        List<TopDocs> topDocsList = results.stream().filter(Objects::nonNull).toList();
+        if (topDocsList.isEmpty()) {
             return null;
         }
-        final TopDocs topDocs = results.stream().findFirst().get();
-        final TopDocs mergedTopDocs;
+        final TopDocs topDocs = topDocsList.stream().findFirst().get();
         final int numShards = results.size();
         if (numShards == 1 && from == 0) { // only one shard and no pagination we can just return the topDocs as we got them.
             return topDocs;
-        } else if (topDocs instanceof TopFieldGroups firstTopDocs) {
+        }
+        final TopDocs mergedTopDocs;
+        if (topDocs instanceof TopFieldGroups firstTopDocs) {
             final Sort sort = new Sort(firstTopDocs.fields);
-            final TopFieldGroups[] shardTopDocs = results.stream().filter(td -> td != Lucene.EMPTY_TOP_DOCS).toArray(TopFieldGroups[]::new);
+            TopFieldGroups[] shardTopDocs = topDocsList.toArray(new TopFieldGroups[0]);
             mergedTopDocs = TopFieldGroups.merge(sort, from, topN, shardTopDocs, false);
         } else if (topDocs instanceof TopFieldDocs firstTopDocs) {
-            final Sort sort = checkSameSortTypes(results, firstTopDocs.fields);
-            final TopFieldDocs[] shardTopDocs = results.stream().filter((td -> td != Lucene.EMPTY_TOP_DOCS)).toArray(TopFieldDocs[]::new);
+            TopFieldDocs[] shardTopDocs = topDocsList.toArray(new TopFieldDocs[0]);
+            final Sort sort = checkSameSortTypes(topDocsList, firstTopDocs.fields);
             mergedTopDocs = TopDocs.merge(sort, from, topN, shardTopDocs);
         } else {
-            final TopDocs[] shardTopDocs = results.toArray(new TopDocs[numShards]);
+            final TopDocs[] shardTopDocs = topDocsList.toArray(new TopDocs[0]);
             mergedTopDocs = TopDocs.merge(from, topN, shardTopDocs);
         }
         return mergedTopDocs;

server/src/main/java/org/elasticsearch/action/search/SearchQueryThenFetchAsyncAction.java

@@ -27,7 +27,6 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
-import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.util.concurrent.CountDown;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.common.util.concurrent.ListenableFuture;
@@ -720,7 +719,7 @@ private static final class QueryPerNodeState {
 
         private static final QueryPhaseResultConsumer.MergeResult EMPTY_PARTIAL_MERGE_RESULT = new QueryPhaseResultConsumer.MergeResult(
             List.of(),
-            Lucene.EMPTY_TOP_DOCS,
+            null,
             null,
             0L
         );
@@ -780,10 +779,12 @@ void onShardDone() {
                 // also collect the set of indices that may be part of a subsequent fetch operation here so that we can release all other
                 // indices without a roundtrip to the coordinating node
                 final BitSet relevantShardIndices = new BitSet(searchRequest.shards.size());
-                for (ScoreDoc scoreDoc : mergeResult.reducedTopDocs().scoreDocs) {
-                    final int localIndex = scoreDoc.shardIndex;
-                    scoreDoc.shardIndex = searchRequest.shards.get(localIndex).shardIndex;
-                    relevantShardIndices.set(localIndex);
+                if (mergeResult.reducedTopDocs() != null) {
+                    for (ScoreDoc scoreDoc : mergeResult.reducedTopDocs().scoreDocs) {
+                        final int localIndex = scoreDoc.shardIndex;
+                        scoreDoc.shardIndex = searchRequest.shards.get(localIndex).shardIndex;
+                        relevantShardIndices.set(localIndex);
+                    }
                 }
                 final Object[] results = new Object[queryPhaseResultConsumer.getNumShards()];
                 for (int i = 0; i < results.length; i++) {

server/src/main/java/org/elasticsearch/common/lucene/Lucene.java

@@ -64,6 +64,7 @@
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.Version;
 import org.elasticsearch.ExceptionsHelper;
+import org.elasticsearch.TransportVersions;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -383,6 +384,14 @@ public static void writeTotalHits(StreamOutput out, TotalHits totalHits) throws
      * by shard for sorting purposes.
      */
     public static void writeTopDocsIncludingShardIndex(StreamOutput out, TopDocs topDocs) throws IOException {
+        if (topDocs == null) {
+            if (out.getTransportVersion().onOrAfter(TransportVersions.SEARCH_INCREMENTAL_TOP_DOCS_NULL_BACKPORT_8_19)) {
+                out.writeByte((byte) -1);
+                return;
+            } else {
+                topDocs = Lucene.EMPTY_TOP_DOCS;
+            }
+        }
         if (topDocs instanceof TopFieldGroups topFieldGroups) {
             out.writeByte((byte) 2);
             writeTotalHits(out, topDocs.totalHits);
@@ -423,7 +432,10 @@ public static void writeSortFieldArray(StreamOutput out, SortField[] sortFields)
      */
     public static TopDocs readTopDocsIncludingShardIndex(StreamInput in) throws IOException {
         byte type = in.readByte();
-        if (type == 0) {
+        if (type == -1) {
+            assert in.getTransportVersion().onOrAfter(TransportVersions.SEARCH_INCREMENTAL_TOP_DOCS_NULL_BACKPORT_8_19);
+            return null;
+        } else if (type == 0) {
             TotalHits totalHits = readTotalHits(in);
 
             final int scoreDocCount = in.readVInt();