Ignore script fields when size is 0 (elastic#31917)

This change adds a check so that when parsing the search source, script fields are 
ignored when the requested search result size is 0. This helps with e.g. clients like 
Kibana that sends a list of script fields that they may need for convenience, but they
don't require any hits. Before this change, user sometimes ran into confusing behaviour, 
e.g. the script compilation limit to breaking although no hits were requested.

Closes elastic#31824

Author: Parth-Vader

server/src/main/java/org/elasticsearch/search/SearchService.java

@@ -806,7 +806,7 @@ private void parseSource(DefaultSearchContext context, SearchSourceBuilder sourc
                 throw new SearchContextException(context, "failed to create SearchContextHighlighter", e);
             }
         }
-        if (source.scriptFields() != null) {
+        if (source.scriptFields() != null && source.size() != 0) {
             int maxAllowedScriptFields = context.mapperService().getIndexSettings().getMaxScriptFields();
             if (source.scriptFields().size() > maxAllowedScriptFields) {
                 throw new IllegalArgumentException(

server/src/test/java/org/elasticsearch/search/SearchServiceTests.java

@@ -345,6 +345,23 @@ searchSourceBuilder, new String[0], false, new AliasFilter(null, Strings.EMPTY_A
         }
     }
 
+    public void testIgnoreScriptfieldIfSizeZero() throws IOException {
+        createIndex("index");
+        final SearchService service = getInstanceFromNode(SearchService.class);
+        final IndicesService indicesService = getInstanceFromNode(IndicesService.class);
+        final IndexService indexService = indicesService.indexServiceSafe(resolveIndex("index"));
+        final IndexShard indexShard = indexService.getShard(0);
+
+        SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
+        searchSourceBuilder.scriptField("field" + 0,
+                new Script(ScriptType.INLINE, MockScriptEngine.NAME, CustomScriptPlugin.DUMMY_SCRIPT, Collections.emptyMap()));
+        searchSourceBuilder.size(0);
+        try (SearchContext context = service.createContext(new ShardSearchLocalRequest(indexShard.shardId(), 1, SearchType.DEFAULT,
+                searchSourceBuilder, new String[0], false, new AliasFilter(null, Strings.EMPTY_ARRAY), 1.0f, true, null, null))) {
+                assertEquals(0, context.scriptFields().fields().size());
+        }
+    }
+
     public static class FailOnRewriteQueryPlugin extends Plugin implements SearchPlugin {
         @Override
         public List<QuerySpec<?>> getQueries() {