Swift test harness: a blocking call on the main thread defeats a DispatchQueue.main.asyncAfter whole-run timeout

A common pattern in standalone Swift/AppKit check binaries is DispatchQueue.main.asyncAfter(deadline: .now()+N) { exit(1) } as a run-wide timeout. If a check under test blocks the main thread (for example, opening a FIFO without O_NONBLOCK), that timer never fires and the run hangs forever, so a regression shows up as a hang instead of a red result. Fix: run potentially-blocking checks on a separate Thread with a semaphore wait (and unblock a FIFO reader by opening the write end with O_NONBLOCK), or arm the timeout on a background DispatchSourceTimer that calls exit. Also: perl -e "alarm N; exec script.sh" only kills the script process, and children it started keep running as orphans. Use a process-group kill if they must die.