util.py

# from http://codereview.stackexchange.com/questions/42802/a-non-blocking-lock-decorator-in-python
def non_blocking_lock(fn):
    """Decorator. Prevents the function from being called multiple times simultaneously.

    If thread A is executing the function and thread B attempts to call the
    function, thread B will immediately receive a return value of None instead.

    """
    locks = defaultdict(Lock)

    @wraps(fn)
    def locker(*args, **kwargs):
        # args[0] is the clustername
        lock = locks[args[0]]
        if lock.acquire(False):
            try:
                return fn(*args, **kwargs)
            finally:
                lock.release()

    return locker