Class _ThreadedChildWatcher

Lib/asyncio/unix_events.py:893–951

Threaded child watcher implementation. The watcher uses a thread per process for waiting for the process finish. It doesn't require subscription on POSIX signal but a thread creation is not free. The watcher has O(1) complexity, its performance doesn't depend on amount of

        callback(pid, returncode, *args)

class _ThreadedChildWatcher:
    """Threaded child watcher implementation.

    The watcher uses a thread per process
    for waiting for the process finish.

    It doesn't require subscription on POSIX signal
    but a thread creation is not free.

    The watcher has O(1) complexity, its performance doesn't depend
    on amount of spawn processes.
    """

    def __init__(self):
        self._pid_counter = itertools.count(0)
        self._threads = {}

    def __del__(self, _warn=warnings.warn):
        threads = [thread for thread in list(self._threads.values())
                   if thread.is_alive()]
        if threads:
            _warn(f"{self.__class__} has registered but not finished child processes",
                  ResourceWarning,
                  source=self)

    def add_child_handler(self, pid, callback, *args):
        loop = events.get_running_loop()
        thread = threading.Thread(target=self._do_waitpid,
                                  name=f"asyncio-waitpid-{next(self._pid_counter)}",
                                  args=(loop, pid, callback, args),
                                  daemon=True)
        self._threads[pid] = thread
        thread.start()

    def _do_waitpid(self, loop, expected_pid, callback, args):
        assert expected_pid > 0

        try:
            pid, status = os.waitpid(expected_pid, 0)
        except ChildProcessError:
            # The child process is already reaped
            # (may happen if waitpid() is called elsewhere).
            pid = expected_pid
            returncode = 255
            logger.warning(
                "Unknown child process pid %d, will report returncode 255",
                pid)
        else:
            returncode = waitstatus_to_exitcode(status)
            if loop.get_debug():
                logger.debug('process %s exited with returncode %s',
                             expected_pid, returncode)

        if loop.is_closed():
            logger.warning("Loop %r that handles pid %r is closed", loop, pid)
        else:
            loop.call_soon_threadsafe(callback, pid, returncode, *args)