Add tests for some failure conditions

server/src/main/java/org/elasticsearch/common/xcontent/ChunkedToXContentBuilder.java

@@ -175,7 +175,7 @@ public <T> ChunkedToXContentBuilder appendXContentFields(Map<String, ? extends T
         return forEach(map.entrySet().iterator(), (e, b) -> b.field(e.getKey()).appendXContent(e.getValue()));
     }
 
-    private ChunkedToXContentBuilder appendXContent(ToXContent xContent) {
+    ChunkedToXContentBuilder appendXContent(ToXContent xContent) {
         if (xContent != ToXContent.EMPTY) {
             addChunk(xContent);
         }

server/src/test/java/org/elasticsearch/common/xcontent/ChunkedToXContentBuilderTest.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.common.xcontent;
+
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xcontent.ToXContent;
+import org.elasticsearch.xcontent.XContentType;
+
+import java.io.IOException;
+import java.util.Map;
+
+import static org.hamcrest.Matchers.sameInstance;
+
+public class ChunkedToXContentBuilderTest extends ESTestCase {
+
+    private static ChunkedToXContentBuilder builder() {
+        return new ChunkedToXContentBuilder(ToXContent.EMPTY_PARAMS);
+    }
+
+    public void testCannotAddAfterRead() {
+        var builder = builder();
+        builder.hasNext();
+        expectThrows(AssertionError.class, builder::startObject);
+    }
+
+    public void testParamsPassedToNestedXContent() throws IOException {
+        ToXContent.Params params = new ToXContent.MapParams(Map.of());
+        var builder = new ChunkedToXContentBuilder(params);
+        builder.appendXContent((b, p) -> {assertThat(p, sameInstance(params)); return b;});
+        read(params, builder);
+    }
+
+    public void testOutOfOrderElementsThrows() {
+        expectThrows(IllegalStateException.class, () -> builder().endObject());
+        expectThrows(IllegalStateException.class, () -> builder().endArray());
+        expectThrows(IllegalStateException.class, () -> builder().startObject().endArray());
+        expectThrows(IllegalStateException.class, () -> builder().startArray().endObject());
+
+        expectThrows(IllegalStateException.class, builder().startObject()::hasNext);
+        expectThrows(IllegalStateException.class, builder().startArray()::hasNext);
+        expectThrows(IllegalStateException.class, builder().startObject().startObject().endObject()::hasNext);
+        expectThrows(IllegalStateException.class, builder().startArray().endArray().startArray()::hasNext);
+    }
+
+    private static void read(ChunkedToXContentBuilder builder) throws IOException {
+        XContentHelper.toXContent(p -> builder, XContentType.JSON, false);
+    }
+
+    private static void read(ToXContent.Params params, ChunkedToXContentBuilder builder) throws IOException {
+        XContentHelper.toXContent(p -> builder, XContentType.JSON, params, false);
+    }
+}