item allocator should return same value for same key

When ItemAllocator.allocate called with same key again,
it is returning different value.
To fix this, we check if a value for key already exists in
self.allocations, and return that value if already exists.

Closes-bug: #1533216
Change-Id: I1f6191b07d33a1f542de18a942cefaf7ecb6c143
(cherry picked from commit 334dae0190)

neutron/agent/l3/item_allocator.py

@@ -65,6 +65,9 @@ class ItemAllocator(object):
         allocations to free the pool.  This final desperate step will not
         happen often in practice.
         """
+        if key in self.allocations:
+            return self.allocations[key]
+
         if key in self.remembered:
             self.allocations[key] = self.remembered.pop(key)
             return self.allocations[key]

neutron/tests/unit/agent/l3/test_item_allocator.py

@@ -62,6 +62,23 @@ class TestItemAllocator(base.BaseTestCase):
         self.assertTrue(test_object not in a.pool)
         self.assertTrue(write.called)
 
+    def test_allocate_repeated_call_with_same_key(self):
+        test_pool = set([TestObject(33000), TestObject(33001),
+                         TestObject(33002), TestObject(33003),
+                         TestObject(33004), TestObject(33005)])
+        a = ia.ItemAllocator('/file', TestObject, test_pool)
+        with mock.patch.object(ia.ItemAllocator, '_write'):
+            test_object = a.allocate('test')
+            test_object1 = a.allocate('test')
+            test_object2 = a.allocate('test')
+            test_object3 = a.allocate('test1')
+
+        # same value for same key on repeated calls
+        self.assertEqual(test_object, test_object1)
+        self.assertEqual(test_object1, test_object2)
+        # values for different keys should be diffent
+        self.assertNotEqual(test_object, test_object3)
+
     def test_allocate_from_file(self):
         test_pool = set([TestObject(33000), TestObject(33001)])
         with mock.patch.object(ia.ItemAllocator, '_read') as read: