Stop instantiating RankFeaturePhase unnecessarily (#115724)

We should not create the phase instance when we know we won't be doing any rank feature
execution up-front. An instance of these isn't free and entails creating an array of searched_shard_count size
which along is non-trivial. Also, this needlessly obscured the threading logic for fetch which has already led to a bug
before.

Author: original-brownbear

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

@@ -42,15 +42,17 @@ public class RankFeaturePhase extends SearchPhase {
     final SearchPhaseResults<SearchPhaseResult> rankPhaseResults;
     private final AggregatedDfs aggregatedDfs;
     private final SearchProgressListener progressListener;
-    private final Client client;
+    private final RankFeaturePhaseRankCoordinatorContext rankFeaturePhaseRankCoordinatorContext;
 
     RankFeaturePhase(
         SearchPhaseResults<SearchPhaseResult> queryPhaseResults,
         AggregatedDfs aggregatedDfs,
         SearchPhaseContext context,
-        Client client
+        RankFeaturePhaseRankCoordinatorContext rankFeaturePhaseRankCoordinatorContext
     ) {
         super("rank-feature");
+        assert rankFeaturePhaseRankCoordinatorContext != null;
+        this.rankFeaturePhaseRankCoordinatorContext = rankFeaturePhaseRankCoordinatorContext;
         if (context.getNumShards() != queryPhaseResults.getNumShards()) {
             throw new IllegalStateException(
                 "number of shards must match the length of the query results but doesn't:"
@@ -65,17 +67,10 @@ public class RankFeaturePhase extends SearchPhase {
         this.rankPhaseResults = new ArraySearchPhaseResults<>(context.getNumShards());
         context.addReleasable(rankPhaseResults);
         this.progressListener = context.getTask().getProgressListener();
-        this.client = client;
     }
 
     @Override
     public void run() {
-        RankFeaturePhaseRankCoordinatorContext rankFeaturePhaseRankCoordinatorContext = coordinatorContext(context.getRequest().source());
-        if (rankFeaturePhaseRankCoordinatorContext == null) {
-            moveToNextPhase(queryPhaseResults, null);
-            return;
-        }
-
         context.execute(new AbstractRunnable() {
             @Override
             protected void doRun() throws Exception {
@@ -122,7 +117,7 @@ void innerRun(RankFeaturePhaseRankCoordinatorContext rankFeaturePhaseRankCoordin
         }
     }
 
-    private RankFeaturePhaseRankCoordinatorContext coordinatorContext(SearchSourceBuilder source) {
+    static RankFeaturePhaseRankCoordinatorContext coordinatorContext(SearchSourceBuilder source, Client client) {
         return source == null || source.rankBuilder() == null
             ? null
             : source.rankBuilder().buildRankFeaturePhaseCoordinatorContext(source.size(), source.from(), client);
@@ -175,7 +170,6 @@ private void onPhaseDone(
         RankFeaturePhaseRankCoordinatorContext rankFeaturePhaseRankCoordinatorContext,
         SearchPhaseController.ReducedQueryPhase reducedQueryPhase
     ) {
-        assert rankFeaturePhaseRankCoordinatorContext != null;
         ThreadedActionListener<RankFeatureDoc[]> rankResultListener = new ThreadedActionListener<>(context, new ActionListener<>() {
             @Override
             public void onResponse(RankFeatureDoc[] docsWithUpdatedScores) {

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

@@ -105,7 +105,7 @@ protected SearchPhase getNextPhase(final SearchPhaseResults<DfsSearchResult> res
             aggregatedDfs,
             mergedKnnResults,
             queryPhaseResultConsumer,
-            (queryResults) -> new RankFeaturePhase(queryResults, aggregatedDfs, context, client),
+            (queryResults) -> SearchQueryThenFetchAsyncAction.nextPhase(client, context, queryResults, aggregatedDfs),
             context
         );
     }

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

@@ -18,6 +18,7 @@
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.search.SearchPhaseResult;
 import org.elasticsearch.search.SearchShardTarget;
+import org.elasticsearch.search.dfs.AggregatedDfs;
 import org.elasticsearch.search.internal.AliasFilter;
 import org.elasticsearch.search.internal.SearchContext;
 import org.elasticsearch.search.internal.ShardSearchRequest;
@@ -125,9 +126,22 @@ && getRequest().scroll() == null
         super.onShardResult(result, shardIt);
     }
 
+    static SearchPhase nextPhase(
+        Client client,
+        SearchPhaseContext context,
+        SearchPhaseResults<SearchPhaseResult> queryResults,
+        AggregatedDfs aggregatedDfs
+    ) {
+        var rankFeaturePhaseCoordCtx = RankFeaturePhase.coordinatorContext(context.getRequest().source(), client);
+        if (rankFeaturePhaseCoordCtx == null) {
+            return new FetchSearchPhase(queryResults, aggregatedDfs, context, null);
+        }
+        return new RankFeaturePhase(queryResults, aggregatedDfs, context, rankFeaturePhaseCoordCtx);
+    }
+
     @Override
     protected SearchPhase getNextPhase(final SearchPhaseResults<SearchPhaseResult> results, SearchPhaseContext context) {
-        return new RankFeaturePhase(results, null, this, client);
+        return nextPhase(client, this, results, null);
     }
 
     private ShardSearchRequest rewriteShardSearchRequest(ShardSearchRequest request) {

server/src/test/java/org/elasticsearch/action/search/RankFeaturePhaseTests.java

@@ -287,95 +287,6 @@ public void sendExecuteRankFeature(
         }
     }
 
-    public void testRankFeaturePhaseNoNeedForFetchingFieldData() {
-        AtomicBoolean phaseDone = new AtomicBoolean(false);
-        final ScoreDoc[][] finalResults = new ScoreDoc[1][1];
-
-        // build the appropriate RankBuilder; using a null rankFeaturePhaseRankShardContext
-        // and non-field based rankFeaturePhaseRankCoordinatorContext
-        RankBuilder rankBuilder = rankBuilder(
-            DEFAULT_RANK_WINDOW_SIZE,
-            defaultQueryPhaseRankShardContext(Collections.emptyList(), DEFAULT_RANK_WINDOW_SIZE),
-            negatingScoresQueryFeaturePhaseRankCoordinatorContext(DEFAULT_SIZE, DEFAULT_FROM, DEFAULT_RANK_WINDOW_SIZE),
-            null,
-            null
-        );
-        // create a SearchSource to attach to the request
-        SearchSourceBuilder searchSourceBuilder = searchSourceWithRankBuilder(rankBuilder);
-
-        SearchPhaseController controller = searchPhaseController();
-        SearchShardTarget shard1Target = new SearchShardTarget("node0", new ShardId("test", "na", 0), null);
-
-        MockSearchPhaseContext mockSearchPhaseContext = new MockSearchPhaseContext(1);
-        mockSearchPhaseContext.getRequest().source(searchSourceBuilder);
-        try (SearchPhaseResults<SearchPhaseResult> results = searchPhaseResults(controller, mockSearchPhaseContext)) {
-            // generate the QuerySearchResults that the RankFeaturePhase would have received from QueryPhase
-            // here we have 2 results, with doc ids 1 and 2
-            final ShardSearchContextId ctx = new ShardSearchContextId(UUIDs.base64UUID(), 123);
-            QuerySearchResult queryResult = new QuerySearchResult(ctx, shard1Target, null);
-
-            try {
-                queryResult.setShardIndex(shard1Target.getShardId().getId());
-                int totalHits = randomIntBetween(2, 100);
-                final ScoreDoc[] shard1Docs = new ScoreDoc[] { new ScoreDoc(1, 10.0F), new ScoreDoc(2, 9.0F) };
-                populateQuerySearchResult(queryResult, totalHits, shard1Docs);
-                results.consumeResult(queryResult, () -> {});
-                // do not make an actual http request, but rather generate the response
-                // as if we would have read it from the RankFeatureShardPhase
-                mockSearchPhaseContext.searchTransport = new SearchTransportService(null, null, null) {
-                    @Override
-                    public void sendExecuteRankFeature(
-                        Transport.Connection connection,
-                        final RankFeatureShardRequest request,
-                        SearchTask task,
-                        final ActionListener<RankFeatureResult> listener
-                    ) {
-                        // make sure to match the context id generated above, otherwise we throw
-                        if (request.contextId().getId() == 123 && Arrays.equals(request.getDocIds(), new int[] { 1, 2 })) {
-                            listener.onFailure(new UnsupportedOperationException("should not have reached here"));
-                        } else {
-                            listener.onFailure(new MockDirectoryWrapper.FakeIOException());
-                        }
-                    }
-                };
-            } finally {
-                queryResult.decRef();
-            }
-            // override the RankFeaturePhase to skip moving to next phase
-            RankFeaturePhase rankFeaturePhase = rankFeaturePhase(results, mockSearchPhaseContext, finalResults, phaseDone);
-            try {
-                rankFeaturePhase.run();
-                mockSearchPhaseContext.assertNoFailure();
-                assertTrue(mockSearchPhaseContext.failures.isEmpty());
-                assertTrue(phaseDone.get());
-
-                // in this case there was no additional "RankFeature" results on shards, so we shortcut directly to queryPhaseResults
-                SearchPhaseResults<SearchPhaseResult> rankPhaseResults = rankFeaturePhase.queryPhaseResults;
-                assertNotNull(rankPhaseResults.getAtomicArray());
-                assertEquals(1, rankPhaseResults.getAtomicArray().length());
-                assertEquals(1, rankPhaseResults.getSuccessfulResults().count());
-
-                SearchPhaseResult shardResult = rankPhaseResults.getAtomicArray().get(0);
-                assertTrue(shardResult instanceof QuerySearchResult);
-                QuerySearchResult rankResult = (QuerySearchResult) shardResult;
-                assertNull(rankResult.rankFeatureResult());
-                assertNotNull(rankResult.queryResult());
-
-                List<ExpectedRankFeatureDoc> expectedFinalResults = List.of(
-                    new ExpectedRankFeatureDoc(2, 1, -9.0F, null),
-                    new ExpectedRankFeatureDoc(1, 2, -10.0F, null)
-                );
-                assertFinalResults(finalResults[0], expectedFinalResults);
-            } finally {
-                rankFeaturePhase.rankPhaseResults.close();
-            }
-        } finally {
-            if (mockSearchPhaseContext.searchResponse.get() != null) {
-                mockSearchPhaseContext.searchResponse.get().decRef();
-            }
-        }
-    }
-
     public void testRankFeaturePhaseOneShardFails() {
         AtomicBoolean phaseDone = new AtomicBoolean(false);
         final ScoreDoc[][] finalResults = new ScoreDoc[1][1];
@@ -534,7 +445,12 @@ public void sendExecuteRankFeature(
                 queryResult.decRef();
             }
             // override the RankFeaturePhase to raise an exception
-            RankFeaturePhase rankFeaturePhase = new RankFeaturePhase(results, null, mockSearchPhaseContext, null) {
+            RankFeaturePhase rankFeaturePhase = new RankFeaturePhase(
+                results,
+                null,
+                mockSearchPhaseContext,
+                defaultRankFeaturePhaseRankCoordinatorContext(DEFAULT_SIZE, DEFAULT_FROM, DEFAULT_RANK_WINDOW_SIZE)
+            ) {
                 @Override
                 void innerRun(RankFeaturePhaseRankCoordinatorContext rankFeaturePhaseRankCoordinatorContext) {
                     throw new IllegalArgumentException("simulated failure");
@@ -890,36 +806,6 @@ public RankFeatureDoc[] rankAndPaginate(RankFeatureDoc[] rankFeatureDocs) {
         };
     }
 
-    private QueryPhaseRankCoordinatorContext negatingScoresQueryFeaturePhaseRankCoordinatorContext(int size, int from, int rankWindowSize) {
-        return new QueryPhaseRankCoordinatorContext(rankWindowSize) {
-            @Override
-            public ScoreDoc[] rankQueryPhaseResults(
-                List<QuerySearchResult> rankSearchResults,
-                SearchPhaseController.TopDocsStats topDocsStats
-            ) {
-                List<ScoreDoc> docScores = new ArrayList<>();
-                for (QuerySearchResult phaseResults : rankSearchResults) {
-                    docScores.addAll(Arrays.asList(phaseResults.topDocs().topDocs.scoreDocs));
-                }
-                ScoreDoc[] sortedDocs = docScores.toArray(new ScoreDoc[0]);
-                // negating scores
-                Arrays.stream(sortedDocs).forEach(doc -> doc.score *= -1);
-
-                Arrays.sort(sortedDocs, Comparator.comparing((ScoreDoc doc) -> doc.score).reversed());
-                sortedDocs = Arrays.stream(sortedDocs).limit(rankWindowSize).toArray(ScoreDoc[]::new);
-                RankFeatureDoc[] topResults = new RankFeatureDoc[Math.max(0, Math.min(size, sortedDocs.length - from))];
-                // perform pagination
-                for (int rank = 0; rank < topResults.length; ++rank) {
-                    ScoreDoc base = sortedDocs[from + rank];
-                    topResults[rank] = new RankFeatureDoc(base.doc, base.score, base.shardIndex);
-                    topResults[rank].rank = from + rank + 1;
-                }
-                topDocsStats.fetchHits = topResults.length;
-                return topResults;
-            }
-        };
-    }
-
     private RankFeaturePhaseRankShardContext defaultRankFeaturePhaseRankShardContext(String field) {
         return new RankFeaturePhaseRankShardContext(field) {
             @Override
@@ -1134,7 +1020,12 @@ private RankFeaturePhase rankFeaturePhase(
         AtomicBoolean phaseDone
     ) {
         // override the RankFeaturePhase to skip moving to next phase
-        return new RankFeaturePhase(results, null, mockSearchPhaseContext, null) {
+        return new RankFeaturePhase(
+            results,
+            null,
+            mockSearchPhaseContext,
+            RankFeaturePhase.coordinatorContext(mockSearchPhaseContext.getRequest().source(), null)
+        ) {
             @Override
             public void moveToNextPhase(
                 SearchPhaseResults<SearchPhaseResult> phaseResults,