human
Complete this python program:
def fork(pkg, function, dirty=False):
assistant
#Here is the Python program you requested.

    """Fork a child process to do part of a spack build.
    Args:
    pkg (PackageBase): package whose environemnt we should set up the
    forked process for.
    function (callable): argless function to run in the child
    process.
    dirty (bool): If True, do NOT clean the environment before
    building.
    Usage::
    def child_fun():
    # do stuff
    build_env.fork(pkg, child_fun)
    Forked processes are run with the build environment set up by
    spack.build_environment.  This allows package authors to have full
    control over the environment, etc. without affecting other builds
    that might be executed in the same spack call.
    If something goes wrong, the child process is expected to print the
    error and the parent process will exit with error as well. If things
    go well, the child exits and the parent carries on.
    """
    def child_execution(child_connection, input_stream):
        try:
            setup_package(pkg, dirty=dirty)
            function(input_stream)
            child_connection.send(None)
        except StopIteration as e:
            tty.msg(e.message)
            child_connection.send(None)
        except:
            (exc_type, exc, tb) = sys.exc_info()
            tb_string = traceback.format_exc()
            package_context = get_package_context(tb)
            build_log = None
            if hasattr(pkg, 'log_path'):
                build_log = pkg.log_path
            msg = ('%s:    %s' % (str(exc_type.__name__), str(exc)))
            ce = ChildError(msg, tb_string, build_log, package_context)
            child_connection.send(ce)
        finally:
            child_connection.close()
    (parent_connection, child_connection) = multiprocessing.Pipe()
    try:
        try:
            input_stream = lang.duplicate_stream(sys.stdin)
        except OSError as e:
            if (e.errno == errno.EINVAL):
                tty.debug('Using    devnull    as    input_stream')
                input_stream = open(os.devnull)
        p = multiprocessing.Process(target=child_execution, args=(child_connection, input_stream))
        p.start()
    finally:
        input_stream.close()
    child_exc = parent_connection.recv()
    p.join()
    if (child_exc is not None):
        raise child_exc