Thread-safe way to prevent decorator from being nested

[Peter Otten]

Michael wrote:

> I'm writing a decorator that I never want to be nested. Following from the
> answer on my StackOverflow question
> (http://stackoverflow.com/a/16905779/106244), I've adapted it to the
> following.
> 
> Can anyone spot any issues with this? It'll be run in a multi-threaded
> environment serving Django requests and also be a part of Celery tasks.

I'm not sure I understand what you are trying to do, but this

>             if not within_special_wrapper():
>                 with flag():

looks suspiciously like race condition.

>    thread_safe_globals = threading.local()

I'm not an expert in the area, but I think you need a lock, something like

class NestingError(Exception):
    pass

nest_lock = threading.Lock()

def my_special_wrapper(f):
    @wraps(f)
    def internal(*args, **kwargs):
        if nest_lock.acquire(False): # non-blocking
            try:
                f(*args, **kwargs)
            finally:
                nest_lock.release()
        else:
            raise NestingError
    return internal