Fix：PulsarKafkaProducer is not thread safe (apache#4745)

fix apache#4707

pulsar-client-kafka-compat/pulsar-client-kafka/src/main/java/org/apache/kafka/clients/producer/PulsarKafkaProducer.java

@@ -283,7 +283,9 @@ public void close(Duration duration) {
     private org.apache.pulsar.client.api.Producer<byte[]> createNewProducer(String topic) {
         try {
             // Add the partitions info for the new topic
-            cluster = cluster.withPartitions(readPartitionsInfo(topic));
+            synchronized (this){
+                cluster = cluster.withPartitions(readPartitionsInfo(topic));
+            }
             List<org.apache.pulsar.client.api.ProducerInterceptor> wrappedInterceptors = interceptors.stream()
                     .map(interceptor -> new KafkaProducerInterceptorWrapper(interceptor, keySchema, valueSchema, topic))
                     .collect(Collectors.toList());

tests/pulsar-kafka-compat-client-test/src/test/java/org/apache/pulsar/tests/integration/compat/kafka/PulsarKafkaProducerThreadSafeTest.java

@@ -0,0 +1,61 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF 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.apache.pulsar.tests.integration.compat.kafka;
+
+import org.apache.kafka.clients.producer.KafkaProducer;
+import org.apache.kafka.clients.producer.Producer;
+import org.apache.kafka.clients.producer.ProducerRecord;
+import org.apache.kafka.clients.producer.PulsarKafkaProducer;
+import org.apache.kafka.common.serialization.IntegerSerializer;
+import org.apache.kafka.common.serialization.StringSerializer;
+import org.apache.pulsar.tests.integration.suites.PulsarStandaloneTestSuite;
+import org.testng.annotations.BeforeTest;
+import org.testng.annotations.Test;
+import java.util.Properties;
+/**
+ * A test that tests if {@link PulsarKafkaProducer} is thread safe.
+ */
+public class PulsarKafkaProducerThreadSafeTest extends PulsarStandaloneTestSuite {
+    private Producer producer;
+
+    private static String getPlainTextServiceUrl() {
+        return container.getPlainTextServiceUrl();
+    }
+
+    @BeforeTest
+    private void setup() {
+        Properties producerProperties = new Properties();
+        producerProperties.put("bootstrap.servers", getPlainTextServiceUrl());
+        producerProperties.put("key.serializer", IntegerSerializer.class.getName());
+        producerProperties.put("value.serializer", StringSerializer.class.getName());
+        producer = new KafkaProducer<>(producerProperties);
+    }
+
+    /**
+     * This test run 10 times in threadPool witch size is 5.
+     * Different threads have same producer and different topics witch is based on thread time.
+     * This test will be failed when producer failed to send if PulsarKafkaProducer is not thread safe.
+     */
+    @Test(threadPoolSize = 5, invocationCount = 10)
+    public void testPulsarKafkaProducerThreadSafe() {
+        String topic1 = "persistent://public/default/topic-" + System.currentTimeMillis();
+        ProducerRecord<String, String> record = new ProducerRecord<>(topic1, "Hello");
+        producer.send(record);
+    }
+}