As per http://qpid.2158936.n2.nabble.com/Deadlock-in-pn-messenger-send-when-using-more-than-2-publishers-td7641239.html, I think I have found an issue with qpid proton when running in point to point mode. If running a single recv thread and 3 concurrent messenger links are set up with it, it seems to cause a deadlock in the third pn_messenger_send. All subsequent attempts to send will also hang (i.e. the proton example send.c application).
We found this behaviour in our own code for OpenMAMA, but I think we have a valid recreation in native qpid proton code here too - see https://github.com/OpenMAMA/OpenMAMA/files/200901/om-issue-153.zip (attached as part of where it was discovered - https://github.com/OpenMAMA/OpenMAMA/issues/153).
If you compile and run that code on latest yum versions for Fedora 23 / qpid proton, you'll get:
Then it hangs - you never get "Sent from third publisher".
The application sets up one messenger to run on its own recv thread, then on the main thread, it fires up 3 distinct messengers and attempts to send a single message from each messenger, and the third one hangs.