Add test for int-to-long sort BWC conversion

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

@@ -625,7 +625,7 @@ public static void writeSortType(StreamOutput out, SortField.Type sortType) thro
      * Returns the generic version of the provided {@link SortField} that
      * can be used to merge documents coming from different shards.
      */
-    private static SortField rewriteMergeSortField(SortField sortField) {
+    public static SortField rewriteMergeSortField(SortField sortField) {
         if (sortField.getClass() == GEO_DISTANCE_SORT_TYPE_CLASS) {
             SortField newSortField = new SortField(sortField.getField(), SortField.Type.DOUBLE);
             newSortField.setMissingValue(sortField.getMissingValue());
@@ -642,6 +642,13 @@ private static SortField rewriteMergeSortField(SortField sortField) {
             );
             newSortField.setMissingValue(sortField.getMissingValue());
             return newSortField;
+        } else if (sortField.getComparatorSource() instanceof IndexFieldData.XFieldComparatorSource fcs) {
+            SortField newSortField = new SortField(sortField.getField(), fcs.reducedType(), sortField.getReverse());
+            Object missingValue = fcs.missingValue(sortField.getReverse());
+            if (missingValue != null) {
+                newSortField.setMissingValue(missingValue);
+            }
+            return newSortField;
         } else if (sortField.getClass() == ShardDocSortField.class) {
             return new SortField(sortField.getField(), SortField.Type.LONG, sortField.getReverse());
         } else {
@@ -655,15 +662,8 @@ static void writeSortField(StreamOutput out, SortField sortField) throws IOExcep
             throw new IllegalArgumentException("Cannot serialize SortField impl [" + sortField + "]");
         }
         out.writeOptionalString(sortField.getField());
-        if (sortField.getComparatorSource() != null) {
-            IndexFieldData.XFieldComparatorSource comparatorSource = (IndexFieldData.XFieldComparatorSource) sortField
-                .getComparatorSource();
-            writeSortType(out, comparatorSource.reducedType());
-            writeMissingValue(out, comparatorSource.missingValue(sortField.getReverse()));
-        } else {
-            writeSortType(out, sortField.getType());
-            writeMissingValue(out, sortField.getMissingValue());
-        }
+        writeSortType(out, sortField.getType());
+        writeMissingValue(out, sortField.getMissingValue());
         out.writeBoolean(sortField.getReverse());
     }
 

server/src/test/java/org/elasticsearch/search/sort/AbstractSortTestCase.java

@@ -183,14 +183,22 @@ public void testEqualsAndHashcode() {
     }
 
     protected final SearchExecutionContext createMockSearchExecutionContext() {
-        return createMockSearchExecutionContext(null);
+        return createMockSearchExecutionContext(null, IndexVersion.current());
+    }
+
+    protected final SearchExecutionContext createMockSearchExecutionContext(IndexVersion indexVersion) {
+        return createMockSearchExecutionContext(null, indexVersion);
     }
 
     protected final SearchExecutionContext createMockSearchExecutionContext(IndexSearcher searcher) {
+        return createMockSearchExecutionContext(searcher, IndexVersion.current());
+    }
+
+    protected final SearchExecutionContext createMockSearchExecutionContext(IndexSearcher searcher, IndexVersion indexVersion) {
         Index index = new Index(randomAlphaOfLengthBetween(1, 10), "_na_");
         IndexSettings idxSettings = IndexSettingsModule.newIndexSettings(
             index,
-            Settings.builder().put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current()).build()
+            Settings.builder().put(IndexMetadata.SETTING_VERSION_CREATED, indexVersion).build()
         );
         BitsetFilterCache bitsetFilterCache = new BitsetFilterCache(idxSettings, mock(BitsetFilterCache.Listener.class));
         BiFunction<MappedFieldType, FieldDataContext, IndexFieldData<?>> indexFieldDataLookup = (fieldType, fdc) -> {

server/src/test/java/org/elasticsearch/search/sort/FieldSortBuilderTests.java

@@ -31,6 +31,9 @@
 import org.apache.lucene.tests.index.RandomIndexWriter;
 import org.apache.lucene.tests.search.AssertingIndexSearcher;
 import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.common.lucene.Lucene;
+import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.fielddata.IndexFieldData.XFieldComparatorSource;
 import org.elasticsearch.index.fielddata.IndexFieldData.XFieldComparatorSource.Nested;
 import org.elasticsearch.index.mapper.DateFieldMapper;
@@ -48,6 +51,7 @@
 import org.elasticsearch.search.MultiValueMode;
 import org.elasticsearch.search.SearchSortValuesAndFormats;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
+import org.elasticsearch.test.index.IndexVersionUtils;
 import org.elasticsearch.xcontent.XContentParseException;
 import org.elasticsearch.xcontent.XContentParser;
 import org.elasticsearch.xcontent.json.JsonXContent;
@@ -657,4 +661,40 @@ public void testIsBottomSortShardDisjoint() throws Exception {
     protected FieldSortBuilder fromXContent(XContentParser parser, String fieldName) throws IOException {
         return FieldSortBuilder.fromXContent(parser, fieldName);
     }
+
+    public void testIntRewritesToLong() throws Exception {
+
+        assertIntegerSortRewrite(
+            IndexVersionUtils.randomPreviousCompatibleVersion(random(), IndexVersions.INDEX_INT_SORT_INT_TYPE_8_19),
+            SortField.Type.LONG
+        );
+        assertIntegerSortRewrite(
+            IndexVersionUtils.randomVersionBetween(
+                random(),
+                IndexVersions.INDEX_INT_SORT_INT_TYPE_8_19,
+                IndexVersionUtils.getPreviousVersion(IndexVersions.UPGRADE_TO_LUCENE_10_0_0)
+            ),
+            SortField.Type.INT
+        );
+        assertIntegerSortRewrite(
+            IndexVersionUtils.randomVersionBetween(
+                random(),
+                IndexVersions.UPGRADE_TO_LUCENE_10_0_0,
+                IndexVersionUtils.getPreviousVersion(IndexVersions.INDEX_INT_SORT_INT_TYPE)
+            ),
+            SortField.Type.LONG
+        );
+        assertIntegerSortRewrite(
+            IndexVersionUtils.randomVersionBetween(random(), IndexVersions.INDEX_INT_SORT_INT_TYPE, IndexVersion.current()),
+            SortField.Type.INT
+        );
+        assertIntegerSortRewrite(IndexVersion.current(), SortField.Type.INT);
+    }
+
+    private void assertIntegerSortRewrite(IndexVersion version, SortField.Type expectedType) throws IOException {
+        FieldSortBuilder builder = new FieldSortBuilder("custom-integer");
+        SortFieldAndFormat sff = builder.build(createMockSearchExecutionContext(version));
+        SortField sf = Lucene.rewriteMergeSortField(sff.field());
+        assertThat(sf.getType(), equalTo(expectedType));
+    }
 }