Add custom SharedCounter / Queue

This will not fix anything related to Django 2.0 but will allow MAC
users to run Django-Q allowing tests to be run locally.

django_q/queues.py

@@ -0,0 +1,69 @@
+"""
+The code is derived from https://github.com/althonos/pronto/commit/3384010dfb4fc7c66a219f59276adef3288a886b
+"""
+
+import multiprocessing
+import multiprocessing.queues
+
+
+class SharedCounter(object):
+    """ A synchronized shared counter.
+
+    The locking done by multiprocessing.Value ensures that only a single
+    process or thread may read or write the in-memory ctypes object. However,
+    in order to do n += 1, Python performs a read followed by a write, so a
+    second process may read the old value before the new one is written by
+    the first process. The solution is to use a multiprocessing.Lock to
+    guarantee the atomicity of the modifications to Value.
+
+    This class comes almost entirely from Eli Bendersky's blog:
+    http://eli.thegreenplace.net/2012/01/04/shared-counter-with-pythons-multiprocessing/
+    """
+
+    def __init__(self, n=0):
+        self.count = multiprocessing.Value('i', n)
+
+    def increment(self, n=1):
+        """ Increment the counter by n (default = 1) """
+        with self.count.get_lock():
+            self.count.value += n
+
+    @property
+    def value(self):
+        """ Return the value of the counter """
+        return self.count.value
+
+
+class Queue(multiprocessing.queues.Queue):
+    """ A portable implementation of multiprocessing.Queue.
+
+    Because of multithreading / multiprocessing semantics, Queue.qsize() may
+    raise the NotImplementedError exception on Unix platforms like Mac OS X
+    where sem_getvalue() is not implemented. This subclass addresses this
+    problem by using a synchronized shared counter (initialized to zero) and
+    increasing / decreasing its value every time the put() and get() methods
+    are called, respectively. This not only prevents NotImplementedError from
+    being raised, but also allows us to implement a reliable version of both
+    qsize() and empty().
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(Queue, self).__init__(*args, ctx=multiprocessing.get_context(), **kwargs)
+        self.size = SharedCounter(0)
+
+    def put(self, *args, **kwargs):
+        super(Queue, self).put(*args, **kwargs)
+        self.size.increment(1)
+
+    def get(self, *args, **kwargs):
+        x = super(Queue, self).get(*args, **kwargs)
+        self.size.increment(-1)
+        return x
+
+    def qsize(self):
+        """ Reliable implementation of multiprocessing.Queue.qsize() """
+        return self.size.value
+
+    def empty(self):
+        """ Reliable implementation of multiprocessing.Queue.empty() """
+        return not self.qsize() > 0