Improve handling of empty response (#125562)

Today `ActionResponse$Empty` implements `ToXContentObject`, but yields
no bytes of content when serialized which creates an invalid JSON
response. This commit removes the bogus interface and adjusts the
affected REST APIs to send a `text/plain` response instead.

docs/changelog/125562.yaml

@@ -0,0 +1,6 @@
+pr: 125562
+summary: Improve handling of empty response
+area: Infra/REST API
+type: bug
+issues:
+ - 57639

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/cluster.desired_balance/10_basic.yml

@@ -180,6 +180,11 @@ setup:
   - requires:
       cluster_features: ["gte_v8.8.0"]
       reason: "reset API added in in 8.8.0"
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: DELETE
+          path: /_internal/desired_balance
+          capabilities: [ plain_text_empty_response ]
 
   - do:
       _internal.delete_desired_balance: { }

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/cluster.desired_nodes/10_basic.yml

@@ -3,6 +3,11 @@ setup:
   - requires:
       cluster_features: ["gte_v8.13.0"]
       reason: "API added in in 8.1.0 but modified in 8.13 (node_version field removed)"
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: DELETE
+          path: /_internal/desired_nodes
+          capabilities: [ plain_text_empty_response ]
 ---
 teardown:
   - do:

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/cluster.desired_nodes/11_old_format.yml

@@ -6,6 +6,11 @@ setup:
   - requires:
       cluster_features: ["gte_v8.3.0"]
       reason: "API added in in 8.1.0 but modified in 8.3"
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: DELETE
+          path: /_internal/desired_nodes
+          capabilities: [ plain_text_empty_response ]
 ---
 teardown:
   - do:

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/cluster.desired_nodes/20_dry_run.yml

@@ -3,6 +3,11 @@ setup:
   - requires:
       cluster_features: ["gte_v8.4.0"]
       reason: "Support for the dry run option was added in in 8.4.0"
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: DELETE
+          path: /_internal/desired_nodes
+          capabilities: [ plain_text_empty_response ]
 ---
 teardown:
   - do:

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/cluster.voting_config_exclusions/10_basic.yml

@@ -1,3 +1,16 @@
+setup:
+  - requires:
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: POST
+          path: /_cluster/voting_config_exclusions
+          capabilities: [ plain_text_empty_response ]
+        - method: DELETE
+          path: /_cluster/voting_config_exclusions
+          capabilities: [ plain_text_empty_response ]
+      reason: needs these capabilities
+
+---
 teardown:
   - do:
       cluster.delete_voting_config_exclusions: {}

server/src/internalClusterTest/java/org/elasticsearch/cluster/coordination/VotingConfigurationIT.java

@@ -9,22 +9,24 @@
 package org.elasticsearch.cluster.coordination;
 
 import org.elasticsearch.ElasticsearchException;
-import org.elasticsearch.action.admin.cluster.configuration.AddVotingConfigExclusionsRequest;
-import org.elasticsearch.action.admin.cluster.configuration.TransportAddVotingConfigExclusionsAction;
+import org.elasticsearch.client.Request;
+import org.elasticsearch.client.Response;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.Priority;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.test.transport.MockTransportService;
 import org.elasticsearch.transport.TransportService;
 
+import java.io.IOException;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.Set;
-import java.util.concurrent.ExecutionException;
 
+import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasItem;
 import static org.hamcrest.Matchers.hasSize;
@@ -38,18 +40,39 @@ public class VotingConfigurationIT extends ESIntegTestCase {
         return Collections.singletonList(MockTransportService.TestPlugin.class);
     }
 
-    public void testAbdicateAfterVotingConfigExclusionAdded() throws ExecutionException, InterruptedException {
+    @Override
+    protected boolean addMockHttpTransport() {
+        return false; // enable HTTP
+    }
+
+    public void testAbdicateAfterVotingConfigExclusionAdded() throws IOException {
         internalCluster().setBootstrapMasterNodeIndex(0);
         internalCluster().startNodes(2);
         final String originalMaster = internalCluster().getMasterName();
+        final var restClient = getRestClient();
 
         logger.info("--> excluding master node {}", originalMaster);
-        client().execute(
-            TransportAddVotingConfigExclusionsAction.TYPE,
-            new AddVotingConfigExclusionsRequest(TEST_REQUEST_TIMEOUT, originalMaster)
-        ).get();
+        final var excludeRequest = new Request("POST", "/_cluster/voting_config_exclusions");
+        excludeRequest.addParameter("node_names", originalMaster);
+        assertEmptyResponse(restClient.performRequest(excludeRequest));
+
         clusterAdmin().prepareHealth(TEST_REQUEST_TIMEOUT).setWaitForEvents(Priority.LANGUID).get();
         assertNotEquals(originalMaster, internalCluster().getMasterName());
+
+        final var clearRequest = new Request("DELETE", "/_cluster/voting_config_exclusions");
+        clearRequest.addParameter("wait_for_removal", "false");
+        assertEmptyResponse(restClient.performRequest(clearRequest));
+
+        assertThat(
+            internalCluster().getInstance(ClusterService.class).state().metadata().coordinationMetadata().getVotingConfigExclusions(),
+            empty()
+        );
+    }
+
+    private void assertEmptyResponse(Response response) throws IOException {
+        assertEquals("text/plain; charset=UTF-8", response.getHeader("content-type"));
+        assertEquals(0, response.getEntity().getContentLength());
+        assertEquals(0, response.getEntity().getContent().readAllBytes().length);
     }
 
     public void testElectsNodeNotInVotingConfiguration() throws Exception {

server/src/main/java/org/elasticsearch/action/ActionResponse.java

@@ -10,9 +10,10 @@
 package org.elasticsearch.action;
 
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.rest.action.EmptyResponseListener;
 import org.elasticsearch.transport.TransportResponse;
-import org.elasticsearch.xcontent.ToXContentObject;
-import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xcontent.ToXContent;
+import org.elasticsearch.xcontent.XContent;
 
 /**
  * Base class for responses to action requests.
@@ -21,20 +22,23 @@ public abstract class ActionResponse extends TransportResponse {
 
     public ActionResponse() {}
 
-    public static final class Empty extends ActionResponse implements ToXContentObject {
+    /**
+     * A response with no payload. This is deliberately not an implementation of {@link ToXContent} or similar because an empty response
+     * has no valid {@link XContent} representation. Use {@link EmptyResponseListener} to convert this to a valid (plain-text) REST
+     * response instead.
+     */
+    public static final class Empty extends ActionResponse {
+
+        private Empty() { /* singleton */ }
+
         public static final ActionResponse.Empty INSTANCE = new ActionResponse.Empty();
 
         @Override
         public String toString() {
-            return "EmptyActionResponse{}";
+            return "ActionResponse.Empty{}";
         }
 
         @Override
         public void writeTo(StreamOutput out) {}
-
-        @Override
-        public XContentBuilder toXContent(final XContentBuilder builder, final Params params) {
-            return builder;
-        }
     }
 }

server/src/main/java/org/elasticsearch/rest/action/EmptyResponseListener.java

@@ -0,0 +1,38 @@
+/*
+ * 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", the "GNU Affero General Public License v3.0 only", 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", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.rest.action;
+
+import org.elasticsearch.action.ActionResponse;
+import org.elasticsearch.common.bytes.BytesArray;
+import org.elasticsearch.rest.RestChannel;
+import org.elasticsearch.rest.RestResponse;
+import org.elasticsearch.rest.RestStatus;
+
+/**
+ * A listener which converts a successful {@link ActionResponse.Empty} action response into a {@code 200 OK} REST response with empty body.
+ */
+public final class EmptyResponseListener extends RestResponseListener<ActionResponse.Empty> {
+    public EmptyResponseListener(RestChannel channel) {
+        super(channel);
+    }
+
+    @Override
+    public RestResponse buildResponse(ActionResponse.Empty ignored) throws Exception {
+        // Content-type header is not required for an empty body but some clients may expect it; the empty body is a valid text/plain entity
+        // so we use that here.
+        return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY);
+    }
+
+    /**
+     * Capability name for APIs that previously would return an invalid zero-byte {@code application/json} response so that the YAML test
+     * runner can avoid those APIs.
+     */
+    public static final String PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME = "plain_text_empty_response";
+}

server/src/main/java/org/elasticsearch/rest/action/RestBuilderListener.java

@@ -27,6 +27,7 @@ public abstract class RestBuilderListener<Response> extends RestResponseListener
         try (XContentBuilder builder = channel.newBuilder()) {
             final RestResponse restResponse = buildResponse(response, builder);
             assert assertBuilderClosed(builder);
+            assert restResponse.content() != null && restResponse.content().length() > 0 : "Use EmptyResponseListener for empty responses";
             return restResponse;
         }
     }

server/src/main/java/org/elasticsearch/rest/action/admin/cluster/RestAddVotingConfigExclusionAction.java

@@ -16,13 +16,15 @@ import org.elasticsearch.common.Strings;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
-import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.rest.action.EmptyResponseListener;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Set;
 
 import static org.elasticsearch.rest.RestRequest.Method.POST;
 import static org.elasticsearch.rest.RestUtils.getMasterNodeTimeout;
+import static org.elasticsearch.rest.action.EmptyResponseListener.PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME;
 
 public class RestAddVotingConfigExclusionAction extends BaseRestHandler {
     private static final TimeValue DEFAULT_TIMEOUT = TimeValue.timeValueSeconds(30L);
@@ -37,6 +39,11 @@ public class RestAddVotingConfigExclusionAction extends BaseRestHandler {
         return List.of(new Route(POST, "/_cluster/voting_config_exclusions"));
     }
 
+    @Override
+    public Set<String> supportedCapabilities() {
+        return Set.of(PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME);
+    }
+
     @Override
     public boolean canTripCircuitBreaker() {
         return false;
@@ -48,7 +55,7 @@ public class RestAddVotingConfigExclusionAction extends BaseRestHandler {
         return channel -> client.execute(
             TransportAddVotingConfigExclusionsAction.TYPE,
             votingConfigExclusionsRequest,
-            new RestToXContentListener<>(channel)
+            new EmptyResponseListener(channel)
         );
     }
 

server/src/main/java/org/elasticsearch/rest/action/admin/cluster/RestClearVotingConfigExclusionsAction.java

@@ -14,13 +14,15 @@ import org.elasticsearch.action.admin.cluster.configuration.TransportClearVoting
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
-import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.rest.action.EmptyResponseListener;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Set;
 
 import static org.elasticsearch.rest.RestRequest.Method.DELETE;
 import static org.elasticsearch.rest.RestUtils.getMasterNodeTimeout;
+import static org.elasticsearch.rest.action.EmptyResponseListener.PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME;
 
 public class RestClearVotingConfigExclusionsAction extends BaseRestHandler {
 
@@ -39,10 +41,15 @@ public class RestClearVotingConfigExclusionsAction extends BaseRestHandler {
         return "clear_voting_config_exclusions_action";
     }
 
+    @Override
+    public Set<String> supportedCapabilities() {
+        return Set.of(PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME);
+    }
+
     @Override
     protected RestChannelConsumer prepareRequest(final RestRequest request, final NodeClient client) throws IOException {
         final var req = resolveVotingConfigExclusionsRequest(request);
-        return channel -> client.execute(TransportClearVotingConfigExclusionsAction.TYPE, req, new RestToXContentListener<>(channel));
+        return channel -> client.execute(TransportClearVotingConfigExclusionsAction.TYPE, req, new EmptyResponseListener(channel));
     }
 
     static ClearVotingConfigExclusionsRequest resolveVotingConfigExclusionsRequest(final RestRequest request) {

server/src/main/java/org/elasticsearch/rest/action/admin/cluster/RestDeleteDesiredBalanceAction.java

@@ -17,10 +17,13 @@ import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.RestUtils;
 import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
-import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.rest.action.EmptyResponseListener;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Set;
+
+import static org.elasticsearch.rest.action.EmptyResponseListener.PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME;
 
 @ServerlessScope(Scope.INTERNAL)
 public class RestDeleteDesiredBalanceAction extends BaseRestHandler {
@@ -35,9 +38,14 @@ public class RestDeleteDesiredBalanceAction extends BaseRestHandler {
         return List.of(new Route(RestRequest.Method.DELETE, "_internal/desired_balance"));
     }
 
+    @Override
+    public Set<String> supportedCapabilities() {
+        return Set.of(PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME);
+    }
+
     @Override
     protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) throws IOException {
         final var req = new DesiredBalanceRequest(RestUtils.getMasterNodeTimeout(request));
-        return channel -> client.execute(TransportDeleteDesiredBalanceAction.TYPE, req, new RestToXContentListener<>(channel));
+        return channel -> client.execute(TransportDeleteDesiredBalanceAction.TYPE, req, new EmptyResponseListener(channel));
     }
 }

server/src/main/java/org/elasticsearch/rest/action/admin/cluster/RestDeleteDesiredNodesAction.java

@@ -14,13 +14,15 @@ import org.elasticsearch.action.support.master.AcknowledgedRequest;
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
-import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.rest.action.EmptyResponseListener;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.Set;
 
 import static org.elasticsearch.rest.RestUtils.getAckTimeout;
 import static org.elasticsearch.rest.RestUtils.getMasterNodeTimeout;
+import static org.elasticsearch.rest.action.EmptyResponseListener.PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME;
 
 public class RestDeleteDesiredNodesAction extends BaseRestHandler {
     @Override
@@ -33,13 +35,18 @@ public class RestDeleteDesiredNodesAction extends BaseRestHandler {
         return List.of(new Route(RestRequest.Method.DELETE, "_internal/desired_nodes"));
     }
 
+    @Override
+    public Set<String> supportedCapabilities() {
+        return Set.of(PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME);
+    }
+
     @Override
     protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) throws IOException {
         final var deleteDesiredNodesRequest = new AcknowledgedRequest.Plain(getMasterNodeTimeout(request), getAckTimeout(request));
         return restChannel -> client.execute(
             TransportDeleteDesiredNodesAction.TYPE,
             deleteDesiredNodesRequest,
-            new RestToXContentListener<>(restChannel)
+            new EmptyResponseListener(restChannel)
         );
     }
 }

server/src/test/java/org/elasticsearch/rest/action/RestBuilderListenerTests.java

@@ -23,16 +23,19 @@ import java.util.concurrent.atomic.AtomicReference;
 
 public class RestBuilderListenerTests extends ESTestCase {
 
+    // bypass the check that XContent responses are never empty - we're ignoring the builder and sending a text/plain response anyway
+    private static final BytesArray NONEMPTY_BODY = new BytesArray(new byte[] { '\n' });
+
     public void testXContentBuilderClosedInBuildResponse() throws Exception {
         AtomicReference<XContentBuilder> builderAtomicReference = new AtomicReference<>();
-        RestBuilderListener<TransportResponse.Empty> builderListener = new RestBuilderListener<Empty>(
+        RestBuilderListener<TransportResponse.Empty> builderListener = new RestBuilderListener<>(
             new FakeRestChannel(new FakeRestRequest(), randomBoolean(), 1)
         ) {
             @Override
-            public RestResponse buildResponse(Empty empty, XContentBuilder builder) throws Exception {
+            public RestResponse buildResponse(Empty empty, XContentBuilder builder) {
                 builderAtomicReference.set(builder);
                 builder.close();
-                return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY);
+                return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, NONEMPTY_BODY);
             }
         };
 
@@ -43,13 +46,13 @@ public class RestBuilderListenerTests extends ESTestCase {
 
     public void testXContentBuilderNotClosedInBuildResponseAssertionsDisabled() throws Exception {
         AtomicReference<XContentBuilder> builderAtomicReference = new AtomicReference<>();
-        RestBuilderListener<TransportResponse.Empty> builderListener = new RestBuilderListener<Empty>(
+        RestBuilderListener<TransportResponse.Empty> builderListener = new RestBuilderListener<>(
             new FakeRestChannel(new FakeRestRequest(), randomBoolean(), 1)
         ) {
             @Override
-            public RestResponse buildResponse(Empty empty, XContentBuilder builder) throws Exception {
+            public RestResponse buildResponse(Empty empty, XContentBuilder builder) {
                 builderAtomicReference.set(builder);
-                return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY);
+                return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, NONEMPTY_BODY);
             }
 
             @Override
@@ -64,15 +67,15 @@ public class RestBuilderListenerTests extends ESTestCase {
         assertTrue(builderAtomicReference.get().generator().isClosed());
     }
 
-    public void testXContentBuilderNotClosedInBuildResponseAssertionsEnabled() throws Exception {
+    public void testXContentBuilderNotClosedInBuildResponseAssertionsEnabled() {
         assumeTrue("tests are not being run with assertions", RestBuilderListener.class.desiredAssertionStatus());
 
-        RestBuilderListener<TransportResponse.Empty> builderListener = new RestBuilderListener<Empty>(
+        RestBuilderListener<TransportResponse.Empty> builderListener = new RestBuilderListener<>(
             new FakeRestChannel(new FakeRestRequest(), randomBoolean(), 1)
         ) {
             @Override
-            public RestResponse buildResponse(Empty empty, XContentBuilder builder) throws Exception {
-                return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY);
+            public RestResponse buildResponse(Empty empty, XContentBuilder builder) {
+                return new RestResponse(RestStatus.OK, RestResponse.TEXT_CONTENT_TYPE, NONEMPTY_BODY);
             }
         };
 

server/src/test/java/org/elasticsearch/search/vectors/QueryVectorBuilderTests.java

@@ -64,6 +64,6 @@ public class QueryVectorBuilderTests extends AbstractQueryVectorBuilderTestCase<
 
     @Override
     protected ActionResponse createResponse(float[] array, TestQueryVectorBuilderPlugin.TestQueryVectorBuilder builder) {
-        return new ActionResponse.Empty();
+        return ActionResponse.Empty.INSTANCE;
     }
 }

test/yaml-rest-runner/src/main/java/org/elasticsearch/test/rest/yaml/ClientYamlTestResponse.java

@@ -50,6 +50,9 @@ public class ClientYamlTestResponse {
                 byte[] bytes = EntityUtils.toByteArray(response.getEntity());
                 // skip parsing if we got text back (e.g. if we called _cat apis)
                 if (bodyContentType != null) {
+                    if (bytes.length == 0) {
+                        throw new IllegalArgumentException("Empty body is invalid for content-type [" + contentType + "]");
+                    }
                     this.parsedResponse = ObjectPath.createFromXContent(bodyContentType.xContent(), new BytesArray(bytes));
                 }
                 this.body = bytes;

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/logging/DeprecationCacheResetAction.java

@@ -67,7 +67,7 @@ public class DeprecationCacheResetAction extends ActionType<DeprecationCacheRese
         }
     }
 
-    public static class Response extends BaseNodesResponse<NodeResponse> implements Writeable, ToXContentObject {
+    public static class Response extends BaseNodesResponse<NodeResponse> implements Writeable {
         public Response(ClusterName clusterName, List<NodeResponse> nodes, List<FailedNodeException> failures) {
             super(clusterName, nodes, failures);
         }
@@ -82,11 +82,6 @@ public class DeprecationCacheResetAction extends ActionType<DeprecationCacheRese
             TransportAction.localOnly();
         }
 
-        @Override
-        public XContentBuilder toXContent(XContentBuilder builder, Params params) {
-            return builder;
-        }
-
         @Override
         public boolean equals(Object o) {
             if (this == o) return true;

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/logging/RestDeprecationCacheResetAction.java

@@ -7,15 +7,17 @@
 
 package org.elasticsearch.xpack.deprecation.logging;
 
+import org.elasticsearch.action.ActionResponse;
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
-import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.rest.action.EmptyResponseListener;
 
-import java.io.IOException;
 import java.util.List;
+import java.util.Set;
 
 import static org.elasticsearch.rest.RestRequest.Method.DELETE;
+import static org.elasticsearch.rest.action.EmptyResponseListener.PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME;
 
 public class RestDeprecationCacheResetAction extends BaseRestHandler {
 
@@ -30,8 +32,17 @@ public class RestDeprecationCacheResetAction extends BaseRestHandler {
     }
 
     @Override
-    public RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) throws IOException {
+    public Set<String> supportedCapabilities() {
+        return Set.of(PLAIN_TEXT_EMPTY_RESPONSE_CAPABILITY_NAME);
+    }
+
+    @Override
+    public RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) {
         DeprecationCacheResetAction.Request resetRequest = new DeprecationCacheResetAction.Request();
-        return channel -> client.execute(DeprecationCacheResetAction.INSTANCE, resetRequest, new RestToXContentListener<>(channel));
+        return channel -> client.execute(
+            DeprecationCacheResetAction.INSTANCE,
+            resetRequest,
+            new EmptyResponseListener(channel).map(ignored -> ActionResponse.Empty.INSTANCE)
+        );
     }
 }

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/rest/inference/RestUpdateTrainedModelDeploymentActionTests.java

@@ -12,15 +12,19 @@ import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.test.rest.FakeRestRequest;
 import org.elasticsearch.test.rest.RestActionTestCase;
+import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentType;
 import org.elasticsearch.xpack.core.ml.action.CreateTrainedModelAssignmentAction;
 import org.elasticsearch.xpack.core.ml.action.UpdateTrainedModelDeploymentAction;
 
+import java.io.IOException;
 import java.util.HashMap;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
+import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestCase {
     public void testNumberOfAllocationInParam() {
@@ -33,7 +37,7 @@ public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestC
             assertEquals(request.getNumberOfAllocations().intValue(), 5);
 
             executeCalled.set(true);
-            return mock(CreateTrainedModelAssignmentAction.Response.class);
+            return newMockResponse();
         }));
         var params = new HashMap<String, String>();
         params.put("number_of_allocations", "5");
@@ -56,7 +60,7 @@ public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestC
             assertEquals(request.getNumberOfAllocations().intValue(), 6);
 
             executeCalled.set(true);
-            return mock(CreateTrainedModelAssignmentAction.Response.class);
+            return newMockResponse();
         }));
 
         final String content = """
@@ -69,4 +73,16 @@ public class RestUpdateTrainedModelDeploymentActionTests extends RestActionTestC
         dispatchRequest(inferenceRequest);
         assertThat(executeCalled.get(), equalTo(true));
     }
+
+    private static CreateTrainedModelAssignmentAction.Response newMockResponse() {
+        final var response = mock(CreateTrainedModelAssignmentAction.Response.class);
+        try {
+            when(response.toXContent(any(), any())).thenAnswer(
+                invocation -> asInstanceOf(XContentBuilder.class, invocation.getArgument(0)).startObject().endObject()
+            );
+        } catch (IOException e) {
+            fail(e);
+        }
+        return response;
+    }
 }

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/rest/action/profile/RestSuggestProfilesActionTests.java

@@ -7,6 +7,7 @@
 
 package org.elasticsearch.xpack.security.rest.action.profile;
 
+import org.apache.lucene.search.TotalHits;
 import org.elasticsearch.ElasticsearchSecurityException;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.settings.Settings;
@@ -33,7 +34,6 @@ import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.nullValue;
-import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -54,7 +54,7 @@ public class RestSuggestProfilesActionTests extends RestActionTestCase {
         verifyingClient.setExecuteLocallyVerifier(((actionType, actionRequest) -> {
             assertThat(actionRequest, instanceOf(SuggestProfilesRequest.class));
             requestHolder.set((SuggestProfilesRequest) actionRequest);
-            return mock(SuggestProfilesResponse.class);
+            return new SuggestProfilesResponse(new SuggestProfilesResponse.ProfileHit[0], 0, new TotalHits(0, TotalHits.Relation.EQUAL_TO));
         }));
     }
 

x-pack/qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/130_operator_privileges.yml

@@ -2,9 +2,13 @@
 "Test operator privileges will work in the mixed cluster":
 
   - requires:
-      test_runner_features: headers
+      test_runner_features: [headers, capabilities]
       cluster_features: ["gte_v7.11.0"]
       reason: "operator privileges are available since 7.11"
+      capabilities:
+        - method: DELETE
+          path: /_cluster/voting_config_exclusions
+          capabilities: [ plain_text_empty_response ]
 
   # The default user ("test_user") is an operator, so this works
   - do: