Handle Null in FetchSourceContext#fetchSource (#36839)

This change ensures that when null is passed as the value to one of
UpdateRequest#fetchSource @nullable parameters, it is not wrapped in a
String array. That would in turn cause a NPE when attempting to serialize
FetchSourceContext as its constructor checks explicitly for Null and not for
arrays of Null objects.

In master, the problematic behavior was corrected as part of #29293

server/src/main/java/org/elasticsearch/action/update/UpdateRequest.java

@@ -410,7 +410,9 @@ public UpdateRequest fields(String... fields) {
      */
     public UpdateRequest fetchSource(@Nullable String include, @Nullable String exclude) {
         FetchSourceContext context = this.fetchSourceContext == null ? FetchSourceContext.FETCH_SOURCE : this.fetchSourceContext;
-        this.fetchSourceContext = new FetchSourceContext(context.fetchSource(), new String[] {include}, new String[]{exclude});
+        String[] includes = include == null ? Strings.EMPTY_ARRAY : new String[]{include};
+        String[] excludes = exclude == null ? Strings.EMPTY_ARRAY : new String[]{exclude};
+        this.fetchSourceContext = new FetchSourceContext(context.fetchSource(), includes, excludes);
         return this;
     }
 

server/src/test/java/org/elasticsearch/update/UpdateIT.java

@@ -379,6 +379,34 @@ public void testUpdate() throws Exception {
         assertThat(updateResponse.getGetResult().sourceAsMap().size(), equalTo(1));
         assertThat(updateResponse.getGetResult().sourceAsMap().get("field1"), equalTo(2));
 
+        // check updates with null excludes
+        client().prepareIndex("test", "type1", "1").setSource("field1", 1, "field2", 2).execute().actionGet();
+        updateResponse = client().prepareUpdate(indexOrAlias(), "type1", "1")
+            .setScript(new Script(ScriptType.INLINE, UPDATE_SCRIPTS, FIELD_INC_SCRIPT, Collections.singletonMap("field", "field1")))
+            .setFetchSource("field1", null)
+            .get();
+        assertThat(updateResponse.getIndex(), equalTo("test"));
+        assertThat(updateResponse.getGetResult(), notNullValue());
+        assertThat(updateResponse.getGetResult().getIndex(), equalTo("test"));
+        assertThat(updateResponse.getGetResult().sourceRef(), notNullValue());
+        assertThat(updateResponse.getGetResult().field("field1"), nullValue());
+        assertThat(updateResponse.getGetResult().sourceAsMap().size(), equalTo(1));
+        assertThat(updateResponse.getGetResult().sourceAsMap().get("field1"), equalTo(2));
+
+        // check updates with null includes
+        client().prepareIndex("test", "type1", "1").setSource("field1", 1, "field2", 2).execute().actionGet();
+        updateResponse = client().prepareUpdate(indexOrAlias(), "type1", "1")
+            .setScript(new Script(ScriptType.INLINE, UPDATE_SCRIPTS, FIELD_INC_SCRIPT, Collections.singletonMap("field", "field1")))
+            .setFetchSource(null, "field1")
+            .get();
+        assertThat(updateResponse.getIndex(), equalTo("test"));
+        assertThat(updateResponse.getGetResult(), notNullValue());
+        assertThat(updateResponse.getGetResult().getIndex(), equalTo("test"));
+        assertThat(updateResponse.getGetResult().sourceRef(), notNullValue());
+        assertThat(updateResponse.getGetResult().field("field2"), nullValue());
+        assertThat(updateResponse.getGetResult().sourceAsMap().size(), equalTo(1));
+        assertThat(updateResponse.getGetResult().sourceAsMap().get("field2"), equalTo(2));
+
         // check updates without script
         // add new field
         client().prepareIndex("test", "type1", "1").setSource("field", 1).execute().actionGet();