Avoid nested docs in painless execute api (#127991)

Painless does not support accessing nested docs (except through
_source). Yet the painless execute api indexes any nested docs that are
found when parsing the sample document. This commit changes the ram
indexing to only index the root document, ignoring any nested docs.

fixes #41004

Author: rjernst

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");