[8.18] fix stale data in synthetic source for string stored field (#123105)

docs/changelog/123105.yaml

@@ -0,0 +1,6 @@
+pr: 123105
+summary: fix stale data in synthetic source for string stored field
+area: Mapping
+type: bug
+issues:
+  - 123110

server/src/main/java/org/elasticsearch/index/mapper/StringStoredFieldFieldLoader.java

@@ -54,7 +54,7 @@ public final void write(XContentBuilder b) throws IOException {
             case 1:
                 b.field(simpleName);
                 write(b, values.get(0));
-                return;
+                break;
             default:
                 b.startArray(simpleName);
                 for (Object value : values) {

server/src/test/java/org/elasticsearch/index/mapper/StringStoredFieldFieldLoaderTests.java

@@ -0,0 +1,52 @@
+/*
+ * 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.mapper;
+
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xcontent.XContentType;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+public class StringStoredFieldFieldLoaderTests extends ESTestCase {
+
+    public void testLoadStoredFieldAndReset() throws IOException {
+        var sfl = new StringStoredFieldFieldLoader("foo", "foo") {
+            @Override
+            protected void write(XContentBuilder b, Object value) throws IOException {
+                b.value((String) value);
+            }
+        };
+
+        var storedFieldLoaders = sfl.storedFieldLoaders().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
+        storedFieldLoaders.get("foo").load(List.of("one"));
+
+        var result = XContentBuilder.builder(XContentType.JSON.xContent());
+        result.startObject();
+        sfl.write(result);
+        result.endObject();
+
+        assertEquals("""
+            {"foo":"one"}""", Strings.toString(result));
+
+        var empty = XContentBuilder.builder(XContentType.JSON.xContent());
+        empty.startObject();
+        // reset() should have been called after previous write
+        sfl.write(empty);
+        empty.endObject();
+
+        assertEquals("{}", Strings.toString(empty));
+    }
+
+}