Fix _id field fetch issue (#94528)

When queries through the "fields" API, the metadata _id field 
is not returned consistently due to frequently clearing an internal 
data cache. This commit fixes this by making copies of those values 
in the value fetcher so clearing the cache doesn't affect displaying 
the results in the search hits.

Closes #94515

Author: howardhuanghua

docs/changelog/94528.yaml

@@ -0,0 +1,5 @@
+pr: 94528
+summary: Fix _id field fetch issue.
+area: Search
+type: bug
+issues: [94515]

server/src/internalClusterTest/java/org/elasticsearch/search/query/SearchQueryIT.java

@@ -2079,4 +2079,21 @@ public void testIssueFuzzyInsideSpanMulti() {
         SearchResponse response = client().prepareSearch("test").setQuery(query).get();
         assertHitCount(response, 1);
     }
+
+    public void testFetchIdFieldQuery() {
+        createIndex("test");
+        int docCount = randomIntBetween(10, 50);
+        for (int i = 0; i < docCount; i++) {
+            client().prepareIndex("test").setSource("field", "foobarbaz").get();
+        }
+        ensureGreen();
+        refresh();
+
+        SearchResponse response = client().prepareSearch("test").addFetchField("_id").setSize(docCount).get();
+        SearchHit[] hits = response.getHits().getHits();
+        assertEquals(docCount, hits.length);
+        for (SearchHit hit : hits) {
+            assertNotNull(hit.getFields().get("_id").getValue());
+        }
+    }
 }

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

@@ -38,7 +38,12 @@ public void setNextReader(LeafReaderContext context) {
     @Override
     public List<Object> fetchValues(Source source, int doc, List<Object> ignoredValues) throws IOException {
         leafSearchLookup.setDocument(doc);
-        return leafSearchLookup.fields().get(fieldname).getValues();
+        List<Object> values = leafSearchLookup.fields().get(fieldname).getValues();
+        if (values == null) {
+            return values;
+        } else {
+            return List.copyOf(values);
+        }
     }
 
 }