CompressedXContent to honour ToXContent#isFragment (#69574)

CompressedXContent can be constructed providing a ToXContent as argument, which gets then serialized by calling its toXContent method. Depending on whether it is a whole object or a fragment, a start and end object should or should not be written. Currently, we only use CompressedXContent to serialize ToXContentFragments hence we never caught this issue, but it is something that we can easily fix before we stumble upon it.

Author: javanna

server/src/main/java/org/elasticsearch/common/compress/CompressedXContent.java

@@ -64,9 +64,13 @@ public CompressedXContent(ToXContent xcontent, XContentType type, ToXContent.Par
         CRC32 crc32 = new CRC32();
         OutputStream checkedStream = new CheckedOutputStream(compressedStream, crc32);
         try (XContentBuilder builder = XContentFactory.contentBuilder(type, checkedStream)) {
-            builder.startObject();
+            if (xcontent.isFragment()) {
+                builder.startObject();
+            }
             xcontent.toXContent(builder, params);
-            builder.endObject();
+            if (xcontent.isFragment()) {
+                builder.endObject();
+            }
         }
         this.bytes = BytesReference.toBytes(bStream.bytes());
         this.crc32 = (int) crc32.getValue();

server/src/test/java/org/elasticsearch/common/compress/DeflateCompressedXContentTests.java

@@ -11,6 +11,10 @@
 import org.apache.lucene.util.TestUtil;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.ToXContentFragment;
+import org.elasticsearch.common.xcontent.ToXContentObject;
+import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.test.ESTestCase;
 import org.junit.Assert;
 
@@ -87,4 +91,19 @@ public void testHashCode() throws IOException {
         assertFalse(new CompressedXContent("{\"a\":\"b\"}").hashCode() == new CompressedXContent("{\"a\":\"c\"}").hashCode());
     }
 
+    public void testToXContentObject() throws IOException {
+        ToXContentObject toXContentObject = (builder, params) -> {
+            builder.startObject();
+            builder.endObject();
+            return builder;
+        };
+        CompressedXContent compressedXContent = new CompressedXContent(toXContentObject, XContentType.JSON, ToXContent.EMPTY_PARAMS);
+        assertEquals("{}", compressedXContent.string());
+    }
+
+    public void testToXContentFragment() throws IOException {
+        ToXContentFragment toXContentFragment = (builder, params) -> builder.field("field", "value");
+        CompressedXContent compressedXContent = new CompressedXContent(toXContentFragment, XContentType.JSON, ToXContent.EMPTY_PARAMS);
+        assertEquals("{\"field\":\"value\"}", compressedXContent.string());
+    }
 }