def build_walker(concurrency):
    """This will return a function suitable for passing to
    :class:`stacker.plan.Plan` for walking the graph.

    If concurrency is 1 (no parallelism) this will return a simple topological
    walker that doesn't use any multithreading.

    If concurrency is 0, this will return a walker that will walk the graph as
    fast as the graph topology allows.

    If concurrency is greater than 1, it will return a walker that will only
    execute a maximum of concurrency steps at any given time.

    Returns:
        func: returns a function to walk a :class:`stacker.dag.DAG`.
    """
    if concurrency == 1:
        return walk

    semaphore = UnlimitedSemaphore()
    if concurrency > 1:
        semaphore = threading.Semaphore(concurrency)

    return ThreadedWalker(semaphore).walk