Add ANONYMOUS SASL mechanism

Fixes #1405

src/main/java/com/rabbitmq/client/DefaultSaslConfig.java

@@ -15,6 +15,7 @@
 
 package com.rabbitmq.client;
 
+import com.rabbitmq.client.impl.AnonymousMechanism;
 import com.rabbitmq.client.impl.ExternalMechanism;
 import com.rabbitmq.client.impl.PlainMechanism;
 
@@ -30,6 +31,7 @@ public class DefaultSaslConfig implements SaslConfig {
 
     public static final DefaultSaslConfig PLAIN = new DefaultSaslConfig("PLAIN");
     public static final DefaultSaslConfig EXTERNAL = new DefaultSaslConfig("EXTERNAL");
+    public static final DefaultSaslConfig ANONYMOUS = new DefaultSaslConfig("ANONYMOUS");
 
     /**
      * Create a DefaultSaslConfig with an explicit mechanism to use.
@@ -50,6 +52,8 @@ public SaslMechanism getSaslMechanism(String[] serverMechanisms) {
             }
             else if (mechanism.equals("EXTERNAL")) {
                 return new ExternalMechanism();
+            } else if (mechanism.equals("ANONYMOUS")) {
+                return new AnonymousMechanism();
             }
         }
         return null;

src/main/java/com/rabbitmq/client/impl/AnonymousMechanism.java

@@ -0,0 +1,34 @@
+// Copyright (c) 2007-2023 Broadcom. All Rights Reserved. The term "Broadcom" refers to Broadcom Inc. and/or its subsidiaries.
+//
+// This software, the RabbitMQ Java client library, is triple-licensed under the
+// Mozilla Public License 2.0 ("MPL"), the GNU General Public License version 2
+// ("GPL") and the Apache License version 2 ("ASL"). For the MPL, please see
+// LICENSE-MPL-RabbitMQ. For the GPL, please see LICENSE-GPL2.  For the ASL,
+// please see LICENSE-APACHE2.
+//
+// This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND,
+// either express or implied. See the LICENSE file for specific language governing
+// rights and limitations of this software.
+//
+// If you have any questions regarding licensing, please contact us at
+// [email protected].
+
+package com.rabbitmq.client.impl;
+
+import com.rabbitmq.client.LongString;
+import com.rabbitmq.client.SaslMechanism;
+
+/**
+ * The ANONYMOUS auth mechanism
+ */
+public class AnonymousMechanism implements SaslMechanism {
+    @Override
+    public String getName() {
+        return "ANONYMOUS";
+    }
+
+    @Override
+    public LongString handleChallenge(LongString challenge, String username, String password) {
+        return LongStringHelper.asLongString("");
+    }
+}

src/test/java/com/rabbitmq/client/test/TestUtils.java

@@ -485,7 +485,8 @@ private BrokerVersionAtLeast310Condition() {
 
     public enum BrokerVersion {
         RABBITMQ_3_8("3.8.0"),
-        RABBITMQ_3_10("3.10.0");
+        RABBITMQ_3_10("3.10.0"),
+        RABBITMQ_4_0("4.0.0");
 
         final String value;
 

src/test/java/com/rabbitmq/client/test/functional/SaslMechanisms.java

@@ -22,16 +22,10 @@
 import java.util.Map;
 import java.util.concurrent.TimeoutException;
 
+import com.rabbitmq.client.*;
 import com.rabbitmq.client.test.TestUtils;
 import org.junit.jupiter.api.Test;
 
-import com.rabbitmq.client.AuthenticationFailureException;
-import com.rabbitmq.client.Connection;
-import com.rabbitmq.client.ConnectionFactory;
-import com.rabbitmq.client.LongString;
-import com.rabbitmq.client.PossibleAuthenticationFailureException;
-import com.rabbitmq.client.SaslConfig;
-import com.rabbitmq.client.SaslMechanism;
 import com.rabbitmq.client.impl.AMQConnection;
 import com.rabbitmq.client.impl.LongStringHelper;
 import com.rabbitmq.client.test.BrokerTestCase;
@@ -105,6 +99,15 @@ public SaslMechanism getSaslMechanism(String[] mechanisms) {
         connectionCloseAuthFailure(connectionFactory.getUsername(), "incorrect-password");
     }
 
+    @Test
+    @TestUtils.BrokerVersionAtLeast(TestUtils.BrokerVersion.RABBITMQ_4_0)
+    public void anonymousShouldSucceed() throws Exception {
+        ConnectionFactory factory = TestUtils.connectionFactory();
+        factory.setSaslConfig(DefaultSaslConfig.ANONYMOUS);
+        Connection connection = factory.newConnection();
+        connection.close();
+    }
+
     public void connectionCloseAuthFailure(String username, String password) throws IOException, TimeoutException {
         String failDetail =  "for username " + username + " and password " + password;
         try {