Allow creation of cache classes associated with a context

This will allow heat.object implementations to create a single object
per context to manage its particular context caching needs.

Change-Id: I9b626efee45164617a73b790bfad4808172d2c12
Related-Bug: #1578854

heat/common/context.py

@@ -111,6 +111,17 @@ class RequestContext(context.RequestContext):
         else:
             self.is_admin = is_admin
 
+        # context scoped cache dict where the key is a class of the type of
+        # object being cached and the value is the cache implementation class
+        self._object_cache = {}
+
+    def cache(self, cache_cls):
+        cache = self._object_cache.get(cache_cls)
+        if not cache:
+            cache = cache_cls()
+            self._object_cache[cache_cls] = cache
+        return cache
+
     @property
     def session(self):
         if self._session is None:

heat/tests/test_common_context.py

@@ -206,6 +206,29 @@ class TestRequestContext(common.HeatTestCase):
                                            auth_url='http://abc/v3',
                                            trust_id=None)
 
+    def test_cache(self):
+        ctx = context.RequestContext.from_dict(self.ctx)
+
+        class Class1(object):
+            pass
+
+        class Class2(object):
+            pass
+
+        self.assertEqual(0, len(ctx._object_cache))
+
+        cache1 = ctx.cache(Class1)
+        self.assertIsInstance(cache1, Class1)
+        self.assertEqual(1, len(ctx._object_cache))
+
+        cache1a = ctx.cache(Class1)
+        self.assertEqual(cache1, cache1a)
+        self.assertEqual(1, len(ctx._object_cache))
+
+        cache2 = ctx.cache(Class2)
+        self.assertIsInstance(cache2, Class2)
+        self.assertEqual(2, len(ctx._object_cache))
+
 
 class RequestContextMiddlewareTest(common.HeatTestCase):