Copy of apple/swift/utils/update_checkout Python script's concurrent clone function

run_parallel

def run_parallel(fn, pool_args, n_processes=0):
    """Function used to run a given closure in parallel.

    NOTE: This function was originally located in the shell module of
    swift_build_support and should eventually be replaced with a better
    parallel implementation.
    """

    if n_processes == 0:
        n_processes = cpu_count() * 2

    lk = Lock()
    print("Running ``%s`` with up to %d processes." %
          (fn.__name__, n_processes))
    pool = Pool(processes=n_processes, initializer=child_init, initargs=(lk,))
    results = pool.map_async(func=fn, iterable=pool_args).get(999999)
    pool.close()
    pool.join()
    return results