RandomScoreFunction requires a defined field with disabled _seq_no

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

@@ -136,7 +136,7 @@ protected ScoreFunction doToFunction(SearchExecutionContext context) {
             // DocID-based random score generation
             return new RandomScoreFunction(hash(context.nowInMillis()), salt, null);
         } else {
-            final String fieldName = Objects.requireNonNullElse(field, SeqNoFieldMapper.NAME);
+            final String fieldName = field(context);
             if (context.isFieldMapped(fieldName) == false) {
                 if (context.hasMappings() == false) {
                     // no mappings: the index is empty anyway
@@ -155,6 +155,20 @@ protected ScoreFunction doToFunction(SearchExecutionContext context) {
         }
     }
 
+    private String field(SearchExecutionContext context) {
+        if (field != null) {
+            return field;
+        }
+        if (context.getIndexSettings().sequenceNumbersDisabled()) {
+            throw new IllegalArgumentException(
+                "random_score requires a [field] parameter when [index.disable_sequence_numbers] is [true] on index ["
+                    + context.index()
+                    + "]"
+            );
+        }
+        return SeqNoFieldMapper.NAME;
+    }
+
     private static int hash(long value) {
         return Long.hashCode(value);
     }

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

@@ -0,0 +1,72 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.index.query.functionscore;
+
+import org.elasticsearch.common.lucene.search.function.RandomScoreFunction;
+import org.elasticsearch.common.lucene.search.function.ScoreFunction;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.IndexSettings;
+import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.index.mapper.SeqNoFieldMapper;
+import org.elasticsearch.index.query.SearchExecutionContext;
+import org.elasticsearch.test.AbstractBuilderTestCase;
+
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.instanceOf;
+
+/**
+ * Tests for {@link RandomScoreFunctionBuilder} on an index with sequence numbers disabled.
+ * The default _seq_no behavior is covered by {@link FunctionScoreQueryBuilderTests}.
+ */
+public class RandomScoreFunctionBuilderTests extends AbstractBuilderTestCase {
+
+    @Override
+    protected Settings createTestIndexSettings() {
+        assumeTrue("Test requires disable_sequence_numbers feature flag", IndexSettings.DISABLE_SEQUENCE_NUMBERS_FEATURE_FLAG);
+        return Settings.builder()
+            .put("index.version.created", IndexVersion.current())
+            .put(IndexSettings.DISABLE_SEQUENCE_NUMBERS.getKey(), true)
+            .put(IndexSettings.SEQ_NO_INDEX_OPTIONS_SETTING.getKey(), SeqNoFieldMapper.SeqNoIndexOptions.DOC_VALUES_ONLY)
+            .build();
+    }
+
+    public void testRandomScoreWithoutFieldRequiresFieldWhenSeqNoDisabled() throws Exception {
+        assumeTrue("Test requires disable_sequence_numbers feature flag", IndexSettings.DISABLE_SEQUENCE_NUMBERS_FEATURE_FLAG);
+        RandomScoreFunctionBuilder builder = new RandomScoreFunctionBuilder();
+        builder.seed(42);
+        SearchExecutionContext context = createSearchExecutionContext();
+        assertTrue(context.getIndexSettings().sequenceNumbersDisabled());
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> builder.toFunction(context));
+        assertThat(e.getMessage(), containsString("random_score requires a [field] parameter"));
+        assertThat(e.getMessage(), containsString("index.disable_sequence_numbers"));
+    }
+
+    public void testRandomScoreWithExplicitFieldWhenSeqNoDisabled() throws Exception {
+        assumeTrue("Test requires disable_sequence_numbers feature flag", IndexSettings.DISABLE_SEQUENCE_NUMBERS_FEATURE_FLAG);
+        RandomScoreFunctionBuilder builder = new RandomScoreFunctionBuilder();
+        builder.seed(42);
+        builder.setField(KEYWORD_FIELD_NAME);
+        SearchExecutionContext context = createSearchExecutionContext();
+        assertTrue(context.getIndexSettings().sequenceNumbersDisabled());
+        ScoreFunction function = builder.toFunction(context);
+        assertNotNull(function);
+        assertThat(function, instanceOf(RandomScoreFunction.class));
+    }
+
+    public void testRandomScoreWithoutSeedFallsBackToDocIdWhenSeqNoDisabled() throws Exception {
+        assumeTrue("Test requires disable_sequence_numbers feature flag", IndexSettings.DISABLE_SEQUENCE_NUMBERS_FEATURE_FLAG);
+        RandomScoreFunctionBuilder builder = new RandomScoreFunctionBuilder();
+        SearchExecutionContext context = createSearchExecutionContext();
+        assertTrue(context.getIndexSettings().sequenceNumbersDisabled());
+        ScoreFunction function = builder.toFunction(context);
+        assertNotNull(function);
+        assertThat(function, instanceOf(RandomScoreFunction.class));
+    }
+}