Fix alias HEAD requests

Alias HEAD requests incorrectly return a content-length header of
0. This commit addresses this by removing the special handling for alias
HEAD requests, and just relying on the general mechanism that exists for
handling HEAD requests in the REST layer.

Relates elastic#23094

Author: jasontedor

core/src/main/java/org/elasticsearch/action/ActionModule.java

@@ -234,7 +234,6 @@
 import org.elasticsearch.rest.action.admin.cluster.RestRestoreSnapshotAction;
 import org.elasticsearch.rest.action.admin.cluster.RestSnapshotsStatusAction;
 import org.elasticsearch.rest.action.admin.cluster.RestVerifyRepositoryAction;
-import org.elasticsearch.rest.action.admin.indices.RestAliasesExistAction;
 import org.elasticsearch.rest.action.admin.indices.RestAnalyzeAction;
 import org.elasticsearch.rest.action.admin.indices.RestClearIndicesCacheAction;
 import org.elasticsearch.rest.action.admin.indices.RestCloseIndexAction;
@@ -535,7 +534,6 @@ public void initRestHandlers(Supplier<DiscoveryNodes> nodesInCluster) {
         registerHandler.accept(new RestIndicesSegmentsAction(settings, restController));
         registerHandler.accept(new RestIndicesShardStoresAction(settings, restController));
         registerHandler.accept(new RestGetAliasesAction(settings, restController));
-        registerHandler.accept(new RestAliasesExistAction(settings, restController));
         registerHandler.accept(new RestIndexDeleteAliasesAction(settings, restController));
         registerHandler.accept(new RestIndexPutAliasAction(settings, restController));
         registerHandler.accept(new RestIndicesAliasesAction(settings, restController));

core/src/main/java/org/elasticsearch/rest/action/admin/indices/RestAliasesExistAction.java

@@ -44,6 +44,7 @@
 import java.util.stream.Collectors;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
+import static org.elasticsearch.rest.RestRequest.Method.HEAD;
 import static org.elasticsearch.rest.RestStatus.OK;
 
 /**
@@ -54,7 +55,9 @@ public class RestGetAliasesAction extends BaseRestHandler {
     public RestGetAliasesAction(final Settings settings, final RestController controller) {
         super(settings);
         controller.registerHandler(GET, "/_alias/{name}", this);
+        controller.registerHandler(HEAD, "/_alias/{name}", this);
         controller.registerHandler(GET, "/{index}/_alias/{name}", this);
+        controller.registerHandler(HEAD, "/{index}/_alias/{name}", this);
     }
 
     @Override

core/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetAliasesAction.java

@@ -21,6 +21,7 @@
 
 import org.apache.http.entity.StringEntity;
 import org.elasticsearch.client.Response;
+import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.test.rest.ESRestTestCase;
 import org.hamcrest.Matcher;
 
@@ -29,6 +30,7 @@
 
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonMap;
+import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 
@@ -62,10 +64,39 @@ public void testTypeExists() throws IOException {
         headTestCase("test/test", singletonMap("pretty", "true"), equalTo(0));
     }
 
+    public void testAliasExists() throws IOException {
+        createTestDoc();
+        try (XContentBuilder builder = jsonBuilder()) {
+            builder.startObject();
+            {
+                builder.startArray("actions");
+                {
+                    builder.startObject();
+                    {
+                        builder.startObject("add");
+                        {
+                            builder.field("index", "test");
+                            builder.field("alias", "test_alias");
+                        }
+                        builder.endObject();
+                    }
+                    builder.endObject();
+                }
+                builder.endArray();
+            }
+            builder.endObject();
+
+            client().performRequest("POST", "_aliases", emptyMap(), new StringEntity(builder.string()));
+            headTestCase("/_alias/test_alias", emptyMap(), greaterThan(0));
+            headTestCase("/test/_alias/test_alias", emptyMap(), greaterThan(0));
+        }
+    }
+
     private void headTestCase(String url, Map<String, String> params, Matcher<Integer> matcher) throws IOException {
         Response response = client().performRequest("HEAD", url, params);
         assertEquals(200, response.getStatusLine().getStatusCode());
         assertThat(Integer.valueOf(response.getHeader("Content-Length")), matcher);
         assertNull("HEAD requests shouldn't have a response body but " + url + " did", response.getEntity());
     }
+
 }