Running some sub-tests in process groups causes tests to hang - the reason it not yet known. Disable the use of process groups for now. diff -wpruN --no-dereference '--exclude=*.orig' a~/Lib/test/libregrtest/worker.py a/Lib/test/libregrtest/worker.py --- a~/Lib/test/libregrtest/worker.py 1970-01-01 00:00:00 +++ a/Lib/test/libregrtest/worker.py 1970-01-01 00:00:00 @@ -13,7 +13,8 @@ from .utils import ( get_temp_dir, get_work_dir, exit_timeout) -USE_PROCESS_GROUP = (hasattr(os, "setsid") and hasattr(os, "killpg")) +USE_PROCESS_GROUP = (hasattr(os, "setsid") and hasattr(os, "killpg") + and not sys.platform.startswith("sunos")) def create_worker_process(runtests: WorkerRunTests, output_fd: int,