Make sure HEAD / has 0 Content-Length (#21077)

Before this commit `curl -XHEAD localhost:9200?pretty` would return
`Content-Length: 1` and a body which is fairly upsetting to standards
compliant tools. Now it'll return `Content-Length: 0` with an empty
body like every other `HEAD` request.

Relates to #21075

core/src/main/java/org/elasticsearch/rest/action/RestMainAction.java

@@ -23,6 +23,7 @@ import org.elasticsearch.action.main.MainAction;
 import org.elasticsearch.action.main.MainRequest;
 import org.elasticsearch.action.main.MainResponse;
 import org.elasticsearch.client.node.NodeClient;
+import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -60,7 +61,7 @@ public class RestMainAction extends BaseRestHandler {
     static BytesRestResponse convertMainResponse(MainResponse response, RestRequest request, XContentBuilder builder) throws IOException {
         RestStatus status = response.isAvailable() ? RestStatus.OK : RestStatus.SERVICE_UNAVAILABLE;
         if (request.method() == RestRequest.Method.HEAD) {
-            return new BytesRestResponse(status, builder);
+            return new BytesRestResponse(status, BytesRestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY);
         }
 
         // Default to pretty printing, but allow ?pretty=false to disable

distribution/integ-test-zip/src/test/java/org/elasticsearch/test/rest/HeadBodyIsEmptyIT.java

@@ -0,0 +1,72 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.test.rest;
+
+import org.apache.http.entity.StringEntity;
+import org.elasticsearch.client.Response;
+
+import java.io.IOException;
+import java.io.UnsupportedEncodingException;
+import java.util.Map;
+
+import static java.util.Collections.emptyMap;
+import static java.util.Collections.singletonMap;
+
+/**
+ * Tests that HTTP HEAD requests don't respond with a body.
+ */
+public class HeadBodyIsEmptyIT extends ESRestTestCase {
+    public void testHeadRoot() throws IOException {
+        headTestCase("/", emptyMap());
+        headTestCase("/", singletonMap("pretty", ""));
+        headTestCase("/", singletonMap("pretty", "true"));
+    }
+
+    private void createTestDoc() throws UnsupportedEncodingException, IOException {
+        client().performRequest("PUT", "test/test/1", emptyMap(), new StringEntity("{\"test\": \"test\"}"));
+    }
+
+    public void testDocumentExists() throws IOException {
+        createTestDoc();
+        headTestCase("test/test/1", emptyMap());
+        headTestCase("test/test/1", singletonMap("pretty", "true"));
+    }
+
+    public void testIndexExists() throws IOException {
+        createTestDoc();
+        headTestCase("test", emptyMap());
+        headTestCase("test", singletonMap("pretty", "true"));
+    }
+
+    public void testTypeExists() throws IOException {
+        createTestDoc();
+        headTestCase("test/test", emptyMap());
+        headTestCase("test/test", singletonMap("pretty", "true"));
+    }
+
+    private void headTestCase(String url, Map<String, String> params) throws IOException {
+        Response response = client().performRequest("HEAD", url, params);
+        assertEquals(200, response.getStatusLine().getStatusCode());
+        /* Check that the content-length header is always 0. This isn't what we should be doing in the long run but it is what we expect
+         * that we are *actually* doing. */
+        assertEquals("We expect HEAD requests to have 0 Content-Length but " + url + " didn't", "0", response.getHeader("Content-Length"));
+        assertNull("HEAD requests shouldn't have a response body but " + url + " did", response.getEntity());
+    }
+}