Avoid nested docs in painless execute api

docs/changelog/127991.yaml

@@ -0,0 +1,6 @@
+pr: 127991
+summary: Avoid nested docs in painless execute api
+area: Infra/Scripting
+type: bug
+issues:
+ - 41004

modules/lang-painless/src/main/java/org/elasticsearch/painless/action/PainlessExecuteAction.java

@@ -831,7 +831,8 @@ private static Response prepareRamIndex(
                     // This is a problem especially for indices that have no mappings, as no fields will be accessible, neither through doc
                     // nor _source (if there are no mappings there are no metadata fields).
                     ParsedDocument parsedDocument = documentMapper.parse(sourceToParse);
-                    indexWriter.addDocuments(parsedDocument.docs());
+                    // only index the root doc since nested docs are not supported in painless anyways
+                    indexWriter.addDocuments(List.of(parsedDocument.rootDoc()));
                     try (IndexReader indexReader = DirectoryReader.open(indexWriter)) {
                         final IndexSearcher searcher = new IndexSearcher(indexReader);
                         searcher.setQueryCache(null);

modules/lang-painless/src/test/java/org/elasticsearch/painless/action/PainlessExecuteApiTests.java

@@ -70,6 +70,18 @@ public void testDefaults() throws IOException {
         assertThat(e.getCause().getMessage(), equalTo("cannot resolve symbol [doc]"));
     }
 
+    public void testNestedDocs() throws IOException {
+        ScriptService scriptService = getInstanceFromNode(ScriptService.class);
+        IndexService indexService = createIndex("index", Settings.EMPTY, "doc", "rank", "type=long", "nested", "type=nested");
+
+        Request.ContextSetup contextSetup = new Request.ContextSetup("index", new BytesArray("""
+            {"rank": 4.0, "nested": [{"text": "foo"}, {"text": "bar"}]}"""), new MatchAllQueryBuilder());
+        contextSetup.setXContentType(XContentType.JSON);
+        Request request = new Request(new Script(ScriptType.INLINE, "painless", "doc['rank'].value", Map.of()), "score", contextSetup);
+        Response response = innerShardOperation(request, scriptService, indexService);
+        assertThat(response.getResult(), equalTo(4.0D));
+    }
+
     public void testFilterExecutionContext() throws IOException {
         ScriptService scriptService = getInstanceFromNode(ScriptService.class);
         IndexService indexService = createIndex("index", Settings.EMPTY, "doc", "field", "type=long");