Log warning when receiving basic.cancel for unknown consumer

Fixes #525

(cherry picked from commit be13273)

Author: acogoluegnes

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

@@ -402,6 +402,8 @@ private void releaseChannel() {
                                                                                       consumerTag,
                                                                                       "handleCancel");
                     }
+                } else {
+                    LOGGER.warn("Could not cancel consumer with unknown tag {}", consumerTag);
                 }
                 return true;
             } else {

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

@@ -0,0 +1,51 @@
+// Copyright (c) 2019 Pivotal Software, Inc.  All rights reserved.
+//
+// This software, the RabbitMQ Java client library, is triple-licensed under the
+// Mozilla Public License 1.1 ("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.test;
+
+import com.rabbitmq.client.Method;
+import com.rabbitmq.client.impl.*;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+import org.mockito.Mockito;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+public class ChannelNTest {
+
+    ConsumerWorkService consumerWorkService;
+    ExecutorService executorService;
+
+    @Before public void init() {
+        executorService = Executors.newSingleThreadExecutor();
+        consumerWorkService = new ConsumerWorkService(executorService, null, 1000, 1000);
+    }
+
+    @After public void tearDown() {
+        consumerWorkService.shutdown();
+        executorService.shutdownNow();
+    }
+
+    @Test
+    public void cancelUnknownConsumerDoesNotThrowException() throws Exception {
+        AMQConnection connection = Mockito.mock(AMQConnection.class);
+        ChannelN channel = new ChannelN(connection, 1, consumerWorkService);
+        Method method = new AMQImpl.Basic.Cancel.Builder().consumerTag("does-not-exist").build();
+        channel.processAsync(new AMQCommand(method));
+    }
+
+}

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

@@ -70,7 +70,8 @@
     GeneratedClassesTest.class,
     RpcTopologyRecordingTest.class,
     ConnectionTest.class,
-    TlsUtilsTest.class
+    TlsUtilsTest.class,
+    ChannelNTest.class
 })
 public class ClientTests {