barrier is not a suitable primitive for a WaitGroup. The ideea is simple. You accept sockets in a loop and handle the connections in parallel threads.At one point you want to stop this loop and the main thread must wait for all the active threads to finish before exiting, otherwise some clients may receive "connection reset by peer". With a WaitGroup, every starting thread increments a counter, and every finishing thread decrements it;when the counter is zero -> all the threads finished.The main thread calls WaitGroup.Wait and it remains blocked until all the worker threads finish the jobs. I guess you could simulate it with core.sync.condition
At the program level, this is built in (D has daemon and non-daemon threads like Java). Or you could use ThreadGroup in core.thread. It would be pretty trivial to do this at the user level with messaging as well.