Return an empty suggestion when suggest phase times out (#122575)

We recently fixed timeout handling in the suggest phase. A test failure on SearchTimeoutIT surfaced an issue with the current approach. In case partial results are allowed, it may happen that some shards time out while executing the suggest phase and some don't.

SearchPhaseController assumes that if one shard has suggest results, all of the other shards will have suggest results too. We could address that assertion and check is the search timed out, instead this commit changes timeout handling in the suggest phase to return an empty suggestion instead of null. This seems appropriate in terms of providing some results and makes the assertion about non null suggestions in SearchPhaseController happy.

Relates to #122357

Closes #122548

docs/changelog/122575.yaml

@@ -0,0 +1,6 @@
+pr: 122575
+summary: Return an empty suggestion when suggest phase times out
+area: Suggesters
+type: bug
+issues:
+ - 122548

muted-tests.yml

@@ -371,9 +371,6 @@ tests:
   issue: https://github.com/elastic/elasticsearch/issues/122378
 - class: org.elasticsearch.telemetry.apm.ApmAgentSettingsIT
   issue: https://github.com/elastic/elasticsearch/issues/122546
-- class: org.elasticsearch.search.SearchTimeoutIT
-  method: testSuggestTimeoutWithPartialResults
-  issue: https://github.com/elastic/elasticsearch/issues/122548
 - class: org.elasticsearch.xpack.inference.mapper.SemanticInferenceMetadataFieldsRecoveryTests
   method: testSnapshotRecovery {p0=false p1=false}
   issue: https://github.com/elastic/elasticsearch/issues/122549

server/src/internalClusterTest/java/org/elasticsearch/search/SearchTimeoutIT.java

@@ -484,8 +484,7 @@ public class SearchTimeoutIT extends ESIntegTestCase {
             CharsRefBuilder spare
         ) {
             contextIndexSearcher.throwTimeExceededException();
-            assert false;
-            return new TermSuggestion(name, suggestion.getSize(), SortBy.SCORE);
+            throw new AssertionError("should have thrown TimeExceededException");
         }
 
         @Override

server/src/main/java/org/elasticsearch/search/query/QueryPhase.java

@@ -126,15 +126,7 @@ public class QueryPhase {
 
     static void executeQuery(SearchContext searchContext) throws QueryPhaseExecutionException {
         if (searchContext.hasOnlySuggest()) {
-            try {
-                SuggestPhase.execute(searchContext);
-            } catch (ContextIndexSearcher.TimeExceededException timeExceededException) {
-                SearchTimeoutException.handleTimeout(
-                    searchContext.request().allowPartialSearchResults(),
-                    searchContext.shardTarget(),
-                    searchContext.queryResult()
-                );
-            }
+            SuggestPhase.execute(searchContext);
             searchContext.queryResult().topDocs(new TopDocsAndMaxScore(Lucene.EMPTY_TOP_DOCS, Float.NaN), new DocValueFormat[0]);
             return;
         }
@@ -150,18 +142,10 @@ public class QueryPhase {
 
         addCollectorsAndSearch(searchContext);
 
-        try {
-            RescorePhase.execute(searchContext);
-            SuggestPhase.execute(searchContext);
-            if (searchContext.getProfilers() != null) {
-                searchContext.queryResult().profileResults(searchContext.getProfilers().buildQueryPhaseResults());
-            }
-        } catch (ContextIndexSearcher.TimeExceededException timeExceededException) {
-            SearchTimeoutException.handleTimeout(
-                searchContext.request().allowPartialSearchResults(),
-                searchContext.shardTarget(),
-                searchContext.queryResult()
-            );
+        RescorePhase.execute(searchContext);
+        SuggestPhase.execute(searchContext);
+        if (searchContext.getProfilers() != null) {
+            searchContext.queryResult().profileResults(searchContext.getProfilers().buildQueryPhaseResults());
         }
     }
 

server/src/main/java/org/elasticsearch/search/rescore/RescorePhase.java

@@ -18,7 +18,9 @@ import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.common.lucene.search.TopDocsAndMaxScore;
 import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.lucene.grouping.TopFieldGroups;
+import org.elasticsearch.search.internal.ContextIndexSearcher;
 import org.elasticsearch.search.internal.SearchContext;
+import org.elasticsearch.search.query.SearchTimeoutException;
 import org.elasticsearch.search.sort.ShardDocSortField;
 import org.elasticsearch.search.sort.SortAndFormats;
 
@@ -84,6 +86,13 @@ public class RescorePhase {
                 .topDocs(new TopDocsAndMaxScore(topDocs, topDocs.scoreDocs[0].score), context.queryResult().sortValueFormats());
         } catch (IOException e) {
             throw new ElasticsearchException("Rescore Phase Failed", e);
+        } catch (ContextIndexSearcher.TimeExceededException timeExceededException) {
+            SearchTimeoutException.handleTimeout(
+                context.request().allowPartialSearchResults(),
+                context.shardTarget(),
+                context.queryResult()
+            );
+            // if the rescore phase times out and partial results are allowed, the returned top docs from this shard won't be rescored
         }
     }
 

server/src/main/java/org/elasticsearch/search/suggest/SuggestPhase.java

@@ -10,7 +10,9 @@ package org.elasticsearch.search.suggest;
 
 import org.apache.lucene.util.CharsRefBuilder;
 import org.elasticsearch.ElasticsearchException;
+import org.elasticsearch.search.internal.ContextIndexSearcher;
 import org.elasticsearch.search.internal.SearchContext;
+import org.elasticsearch.search.query.SearchTimeoutException;
 import org.elasticsearch.search.suggest.Suggest.Suggestion;
 import org.elasticsearch.search.suggest.Suggest.Suggestion.Entry;
 import org.elasticsearch.search.suggest.Suggest.Suggestion.Entry.Option;
@@ -40,12 +42,17 @@ public class SuggestPhase {
             for (Map.Entry<String, SuggestionSearchContext.SuggestionContext> entry : suggest.suggestions().entrySet()) {
                 SuggestionSearchContext.SuggestionContext suggestion = entry.getValue();
                 Suggester<SuggestionContext> suggester = suggestion.getSuggester();
-                Suggestion<? extends Entry<? extends Option>> result = suggester.execute(
-                    entry.getKey(),
-                    suggestion,
-                    context.searcher(),
-                    spare
-                );
+                Suggestion<? extends Entry<? extends Option>> result;
+                try {
+                    result = suggester.execute(entry.getKey(), suggestion, context.searcher(), spare);
+                } catch (ContextIndexSearcher.TimeExceededException timeExceededException) {
+                    SearchTimeoutException.handleTimeout(
+                        context.request().allowPartialSearchResults(),
+                        context.shardTarget(),
+                        context.queryResult()
+                    );
+                    result = suggester.emptySuggestion(entry.getKey(), suggestion, spare);
+                }
                 if (result != null) {
                     assert entry.getKey().equals(result.name);
                     suggestions.add(result);

server/src/test/java/org/elasticsearch/search/query/QueryPhaseTimeoutTests.java

@@ -297,7 +297,8 @@ public class QueryPhaseTimeoutTests extends IndexShardTestCase {
             assertTrue(context.hasOnlySuggest());
             QueryPhase.execute(context);
             assertTrue(context.queryResult().searchTimedOut());
-            assertNull(context.queryResult().suggest());
+            assertEquals(1, context.queryResult().suggest().size());
+            assertEquals(0, context.queryResult().suggest().getSuggestion("suggestion").getEntries().size());
             assertNotNull(context.queryResult().topDocs());
             assertEquals(0, context.queryResult().topDocs().topDocs.totalHits.value());
         }
@@ -311,7 +312,8 @@ public class QueryPhaseTimeoutTests extends IndexShardTestCase {
             QueryPhase.execute(context);
             assertThat(context.queryResult().topDocs().topDocs.totalHits.value(), Matchers.greaterThan(0L));
             assertTrue(context.queryResult().searchTimedOut());
-            assertNull(context.queryResult().suggest());
+            assertEquals(1, context.queryResult().suggest().size());
+            assertEquals(0, context.queryResult().suggest().getSuggestion("suggestion").getEntries().size());
         }
     }