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After reading http://wiki.ros.org/roscpp/Internals and a few source files, I think I got what is going on.

The behavior depends on whether no-copy publishing (via shared_ptr messages) is used or not. In no-copy intraprocess publishing, the published message is directly put on the subscriber's callback queue. In intraprocess serialized pubsub connection (when you publish just a reference or a copy of the message), the callback queue of the subscriber is not directly updated from the publisher. It instead goes into the publisher output queue and waits for PollManager to move the callback from the publisher output queue to the subscriber callback queue.

As referenced later in the source code analysis, the PollSet gets signaled right after a serialized message is put in the publisher output queue. The signaling is done via a pipe, which can be buffered in kernel. So I think this is what brings asynchronicity in the code.

Relevant source lines:

• Publisher::publish() calls TopicManager::publish()
• TopicManager::publish() calls Publication::publish()
• Publication::publish() has two options based on the type of the published message
  • if the published message is the no-copy shared pointer, IntraProcessSubscriberLink::enqueueMessage() is called directly
  • if the published message is not no-copy, it is pushed to the publisher output queue
    • right after Publication::publish() finishes, TopicManager signals the PollSet
• The publication queue is asynchronously processed by the PollManager:
  • Registration of processPublishQueues() callback to PollManager
  • TopicManager::processPublishQueues() calss Publication::processPublishQueue()
  • Publication::processPublishQueue() calls enqueueMessage()
  • Publication::enqueueMessage() calls IntraProcessSubscriberLink::enqueueMessage()
• From now on both paths of no-copy and serialized messages meet again
• IntraProcessSubscriberLink::enqueueMessage() calls IntraProcessPublisherLink::handleMessage()
• IntraProcessPublisherLink::handleMessage() calls Subscription::handleMessage()
• Subscription::handleMessage() adds a callback to the callback queues of all its registered callbacks