[Backport 2.x] Support retrieving doc values of unsigned long field

CHANGELOG.md

@@ -25,6 +25,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Revert changes to upload remote state manifest using minimum codec version([#16403](https://github.com/opensearch-project/OpenSearch/pull/16403))
 - Ensure index templates are not applied to system indices ([#16418](https://github.com/opensearch-project/OpenSearch/pull/16418))
 - Remove resource usages object from search response headers ([#16532](https://github.com/opensearch-project/OpenSearch/pull/16532))
+- Support retrieving doc values of unsigned long field ([#16543](https://github.com/opensearch-project/OpenSearch/pull/16543))
 
 ### Security
 

server/src/main/java/org/opensearch/index/fielddata/plain/SortedNumericIndexFieldData.java

@@ -41,6 +41,7 @@
 import org.apache.lucene.sandbox.document.HalfFloatPoint;
 import org.apache.lucene.util.Accountable;
 import org.apache.lucene.util.NumericUtils;
+import org.opensearch.common.Numbers;
 import org.opensearch.common.time.DateUtils;
 import org.opensearch.core.indices.breaker.CircuitBreakerService;
 import org.opensearch.index.fielddata.FieldData;
@@ -573,6 +574,28 @@ public final SortedBinaryDocValues getBytesValues() {
             return FieldData.toUnsignedString(getLongValues());
         }
 
+        @Override
+        public DocValueFetcher.Leaf getLeafValueFetcher(DocValueFormat format) {
+            SortedNumericDocValues values = getLongValues();
+            return new DocValueFetcher.Leaf() {
+                @Override
+                public boolean advanceExact(int docId) throws IOException {
+                    return values.advanceExact(docId);
+                }
+
+                @Override
+                public int docValueCount() {
+                    return values.docValueCount();
+                }
+
+                @Override
+                public Object nextValue() throws IOException {
+                    final BigInteger value = Numbers.toUnsignedBigInteger(values.nextValue());
+                    return format.format(value);
+                }
+            };
+        }
+
         @Override
         public long ramBytesUsed() {
             return 0L;

server/src/test/java/org/opensearch/index/mapper/NumberFieldTypeTests.java

@@ -73,6 +73,7 @@
 import org.opensearch.index.mapper.NumberFieldMapper.NumberFieldType;
 import org.opensearch.index.mapper.NumberFieldMapper.NumberType;
 import org.opensearch.index.query.QueryShardContext;
+import org.opensearch.search.DocValueFormat;
 import org.opensearch.search.MultiValueMode;
 import org.opensearch.search.query.BitmapDocValuesQuery;
 import org.junit.Before;
@@ -981,4 +982,28 @@ public void testBitmapQuery() throws IOException {
         NumberFieldType finalFt = ft;
         assertThrows(IllegalArgumentException.class, () -> finalFt.bitmapQuery(bitmap));
     }
+
+    public void testFetchUnsignedLongDocValues() throws IOException {
+        Directory dir = newDirectory();
+        IndexWriter w = new IndexWriter(dir, new IndexWriterConfig(null));
+        Document doc = new Document();
+        final BigInteger expectedValue = randomUnsignedLong();
+        doc.add(new SortedNumericDocValuesField("ul", expectedValue.longValue()));
+        w.addDocument(doc);
+        try (DirectoryReader reader = DirectoryReader.open(w)) {
+            final NumberFieldType ft = new NumberFieldType("ul", NumberType.UNSIGNED_LONG);
+            IndexNumericFieldData fielddata = (IndexNumericFieldData) ft.fielddataBuilder(
+                "index",
+                () -> { throw new UnsupportedOperationException(); }
+            ).build(null, null);
+            assertEquals(IndexNumericFieldData.NumericType.UNSIGNED_LONG, fielddata.getNumericType());
+            DocValueFetcher.Leaf fetcher = fielddata.load(reader.leaves().get(0)).getLeafValueFetcher(DocValueFormat.UNSIGNED_LONG);
+            assertTrue(fetcher.advanceExact(0));
+            assertEquals(1, fetcher.docValueCount());
+            final Object value = fetcher.nextValue();
+            assertTrue(value instanceof BigInteger);
+            assertEquals(expectedValue, value);
+        }
+        IOUtils.close(w, dir);
+    }
 }