Note : See #2144 for auth workaround that is required, and independent of this issue.

  Publisher publisher = Publisher.defaultBuilder(topicName).build();
    PubsubMessage pubsubMessage =
        PubsubMessage.newBuilder().setData(ByteString.copyFromUtf8(message)).build();
    String messageId = publisher.publish(pubsubMessage).get();

hangs if run directly from a request.

This works only if executed using an ExecutorService. Users should n't be required to provide an executor to publish messages within a request.