INT-78: Add a helper class for concurrency

ns1/helpers.py

@@ -0,0 +1,14 @@
+from threading import Lock
+
+
+class SingletonMixin(object):
+    """double-locked thread safe singleton"""
+    _instance = None
+    _lock = Lock()
+
+    def __new__(cls, *args, **kwargs):
+        if cls._instance is None:
+            with cls._lock:
+                if cls._instance is None:
+                    cls._instance = object.__new__(cls, *args, **kwargs)
+        return cls._instance

tests/unit/test_helpers.py

@@ -0,0 +1,67 @@
+import multiprocessing
+import pytest
+import threading
+
+from uuid import uuid4
+
+import ns1.helpers
+
+try:  # Python 3.3 +
+    import queue
+except ImportError:
+    import Queue as queue
+
+
+JOIN_TIMEOUT = 2
+
+
+class A(ns1.helpers.SingletonMixin):
+    def __init__(self):
+        self.uuid = uuid4()
+
+
+class B(ns1.helpers.SingletonMixin):
+    def __init__(self):
+        self.uuid = uuid4()
+
+
+def test_singleton_mixin():
+    """
+    it should be the same instance per mixed-in class
+    it should not be the same instance across subclassers
+    """
+    a, a2 = A(), A()
+    b, b2 = B(), B()
+
+    assert a is a2
+    assert b is b2
+
+    assert a is not b
+
+
+@pytest.mark.parametrize('queue_class,process,repetitions', [
+    (multiprocessing.Queue, multiprocessing.Process, 64),
+    (queue.Queue, threading.Thread, 64),
+])
+def test_singleton_mixin_with_concurrency(queue_class, process, repetitions):
+    """
+    it should hold up under multiple processes or threads
+    """
+    def inner(queue):
+        a = A()
+        b = A()
+        queue.put((a.uuid, b.uuid))
+
+    test_queue = queue_class()
+    processes = []
+    for _ in range(repetitions):
+        p = process(target=inner, args=(test_queue,))
+        p.start()
+        processes.append(p)
+
+    seen_uuids = set()
+    while len(seen_uuids) < repetitions:
+        a, b = test_queue.get(timeout=JOIN_TIMEOUT)
+        assert a is b
+        assert a not in seen_uuids
+        seen_uuids.add(a)