libports is not (at least, not for now) a generalization / abstraction of Mach ports to the functionality the Hurd needs, that is, it is not meant to provide an interface independently of the underlying microkernel.
When a message is recieved, the thread acting as receiver checks if any other thread is also waiting for requests. If there is none, a new thread is spawned. Thus, the current thread continues processing the message while the newly created thread starts listening for new ones. (open issue hurd: multithreading.)
Also, there are configurable timeouts for translators who want to go away
when they are not used. (open issue hurd: there used to be bugs
in this area,
id:"email@example.com", but it may be
fixed as of
hurd/hurd.git commit 9b5429e834cde56f73b8ff605e36afc7d9bb6e1b.)