Update by Query is modified to accept short `script` parameter. (#26841)

Update by Query is modified to accept short `script` parameter.

Closes issue #24898

modules/reindex/src/main/java/org/elasticsearch/index/reindex/RestUpdateByQueryAction.java

@@ -67,7 +67,7 @@ public class RestUpdateByQueryAction extends AbstractBulkByQueryRestHandler<Upda
 
         Map<String, Consumer<Object>> consumers = new HashMap<>();
         consumers.put("conflicts", o -> internal.setConflicts((String) o));
-        consumers.put("script", o -> internal.setScript(parseScript((Map<String, Object>)o)));
+        consumers.put("script", o -> internal.setScript(parseScript(o)));
 
         parseInternalRequest(internal, request, consumers);
 
@@ -76,12 +76,18 @@ public class RestUpdateByQueryAction extends AbstractBulkByQueryRestHandler<Upda
     }
 
     @SuppressWarnings("unchecked")
-    private static Script parseScript(Map<String, Object> config) {
+    private static Script parseScript(Object config) {
+        assert config != null : "Script should not be null";
+
+        if (config instanceof String) {
+            return new Script((String) config);
+        } else if (config instanceof Map) {
+            Map<String,Object> configMap = (Map<String, Object>) config;
             String script = null;
             ScriptType type = null;
             String lang = DEFAULT_SCRIPT_LANG;
             Map<String, Object> params = Collections.emptyMap();
-        for (Iterator<Map.Entry<String, Object>> itr = config.entrySet().iterator(); itr.hasNext();) {
+            for (Iterator<Map.Entry<String, Object>> itr = configMap.entrySet().iterator(); itr.hasNext();) {
                 Map.Entry<String, Object> entry = itr.next();
                 String parameterName = entry.getKey();
                 Object parameterValue = entry.getValue();
@@ -120,5 +126,8 @@ public class RestUpdateByQueryAction extends AbstractBulkByQueryRestHandler<Upda
             assert type != null : "if script is not null, type should definitely not be null";
 
             return new Script(type, lang, script, params);
+        } else {
+            throw new IllegalArgumentException("Script value should be a String or a Map");
+        }
     }
 }

qa/smoke-test-reindex-with-all-modules/src/test/resources/rest-api-spec/test/update_by_query/10_script.yml

@@ -29,6 +29,34 @@
               user: notkimchy
   - match: { hits.total: 1 }
 
+---
+"Update document using short `script` form":
+  - do:
+      index:
+        index:  twitter
+        type:   tweet
+        id:     1
+        body:   { "user": "kimchy" }
+  - do:
+        indices.refresh: {}
+
+  - do:
+      update_by_query:
+        index:   twitter
+        refresh: true
+        body:    { "script": "ctx._source.user = \"not\" + ctx._source.user" }
+  - match: {updated: 1}
+  - match: {noops: 0}
+
+  - do:
+      search:
+        index: twitter
+        body:
+          query:
+            match:
+              user: notkimchy
+  - match: { hits.total: 1 }
+
 ---
 "Noop one doc":
   - do: