Fix a bug in function_score queries where we use the wrong boost_mode. (#35148)

server/src/main/java/org/elasticsearch/common/lucene/search/function/FunctionScoreQuery.java

@@ -123,8 +123,7 @@ public class FunctionScoreQuery extends Query {
     final ScoreMode scoreMode;
     final float maxBoost;
     private final Float minScore;
-
+    private final CombineFunction combineFunction;
-    protected final CombineFunction combineFunction;
 
     /**
      * Creates a FunctionScoreQuery without function.
@@ -192,6 +191,10 @@ public class FunctionScoreQuery extends Query {
         return minScore;
     }
 
+    public CombineFunction getCombineFunction() {
+        return combineFunction;
+    }
+
     @Override
     public Query rewrite(IndexReader reader) throws IOException {
         Query rewritten = super.rewrite(reader);

server/src/main/java/org/elasticsearch/index/query/functionscore/FunctionScoreQueryBuilder.java

@@ -309,18 +309,14 @@ public class FunctionScoreQueryBuilder extends AbstractQueryBuilder<FunctionScor
             query = new MatchAllDocsQuery();
         }
 
+        CombineFunction boostMode = this.boostMode == null ? DEFAULT_BOOST_MODE : this.boostMode;
         // handle cases where only one score function and no filter was provided. In this case we create a FunctionScoreQuery.
         if (filterFunctions.length == 0) {
             return new FunctionScoreQuery(query, minScore, maxBoost);
         } else if (filterFunctions.length == 1 && filterFunctions[0] instanceof FunctionScoreQuery.FilterScoreFunction == false) {
-            CombineFunction combineFunction = this.boostMode;
+            return new FunctionScoreQuery(query, filterFunctions[0], boostMode, minScore, maxBoost);
-            if (combineFunction == null) {
-                combineFunction = filterFunctions[0].getDefaultScoreCombiner();
-            }
-            return new FunctionScoreQuery(query, filterFunctions[0], combineFunction, minScore, maxBoost);
         }
         // in all other cases we create a FunctionScoreQuery with filters
-        CombineFunction boostMode = this.boostMode == null ? DEFAULT_BOOST_MODE : this.boostMode;
         return new FunctionScoreQuery(query, scoreMode, filterFunctions, boostMode, minScore, maxBoost);
     }
 

server/src/test/java/org/elasticsearch/index/query/functionscore/FunctionScoreQueryBuilderTests.java

@@ -20,7 +20,6 @@
 package org.elasticsearch.index.query.functionscore;
 
 import com.fasterxml.jackson.core.JsonParseException;
-
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.Query;
@@ -672,6 +671,29 @@ public class FunctionScoreQueryBuilderTests extends AbstractQueryTestCase<Functi
         assertSame(rewrite.filterFunctionBuilders()[1].getFilter(), secondFunction);
     }
 
+    /**
+     * Please see https://github.com/elastic/elasticsearch/issues/35123 for context.
+     */
+    public void testSingleScriptFunction() throws IOException {
+        QueryBuilder queryBuilder = RandomQueryBuilder.createQuery(random());
+        ScoreFunctionBuilder functionBuilder = new ScriptScoreFunctionBuilder(
+            new Script(ScriptType.INLINE, MockScriptEngine.NAME, "1", Collections.emptyMap()));
+
+        FunctionScoreQueryBuilder builder = functionScoreQuery(queryBuilder, functionBuilder);
+        if (randomBoolean()) {
+            builder.boostMode(randomFrom(CombineFunction.values()));
+        }
+
+        Query query = builder.toQuery(createShardContext());
+        assertThat(query, instanceOf(FunctionScoreQuery.class));
+
+        CombineFunction expectedBoostMode = builder.boostMode() != null
+            ? builder.boostMode()
+            : FunctionScoreQueryBuilder.DEFAULT_BOOST_MODE;
+        CombineFunction actualBoostMode = ((FunctionScoreQuery) query).getCombineFunction();
+        assertEquals(expectedBoostMode, actualBoostMode);
+    }
+
     public void testQueryMalformedArrayNotSupported() throws IOException {
         String json =
             "{\n" +