synchronized ProducerIdAndEpoch producerIdAndEpoch(TopicPartition topicPartition) {
        return txnPartitionMap.getOrCreate(topicPartition).producerIdAndEpoch();
    }
Returns the current producer id/epoch of the given TopicPartition.
clients/src/main/java/org/apache/kafka/clients/producer/internals/TransactionManager.java