Past exam of the computer science course of the University of Oxford / 2015 / Concurrent programming / 1 / b / Solution

A cyclic Communicating Scala Objects component has a communication loop followed by a close-down phase. repeat { ... } catches the library's normal Stopped family of exceptions, including closure, and leaves the loop; ordinary while code instead needs the explicit handler used above. In either case, cleanup closes every owned input port with closeIn and every owned output port with closeOut, preferably in finally. Reclosing a one-to-one port is harmless. A parallel composition completes only when every component has completed; one component returning does not automatically cancel all the others.

The producer closes left on return. The strict buffer observes this, drains its FIFO queue into a cooperating consumer, and closes right; the consumer's next read detects closure and enters its own cleanup. Conversely, the consumer closes right on return; the strict buffer then closes left, releasing a blocked producer write or causing its next write to stop. Thus,Eventual termination assumes that surviving components reach their next communication or closure check, and that the consumer keeps accepting a finite outstanding backlog after producer termination. A component stuck forever in unrelated computation cannot be stopped by closing a port alone.