caching: Add test to validate shared caching across threads

This patch set simply adds a unit test to sanity check that
rendered documents cache shares the same cached documents across
different threads. This is to ensure that each thread doesn't
use different document sets for rendering.

Change-Id: If473f92888bf67550bf0e002d2299c9bc6e4dca9

deckhand/tests/unit/engine/test_cache.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from threading import Thread
+
 import testtools
 
 from deckhand.engine import cache
@@ -55,3 +57,36 @@ class RenderedDocumentsCacheTest(test_base.DeckhandTestCase):
         # The cache won't be hit this time - expect AttributeError.
         with testtools.ExpectedException(AttributeError):
             cache.lookup_by_revision_id(1, None)
+
+    def test_lookup_by_revision_id_cache_multiple_threads(self):
+        """Validate that cache works across multiple threads: each thread
+        should use the same set of rendered documents.
+        """
+        document_factory = factories.DocumentFactory(1, [1])
+        documents1 = document_factory.gen_test({})
+        documents2 = document_factory.gen_test({})
+        # Sanity-check that the document sets differ.
+        self.assertNotEqual(documents1, documents2)
+
+        rendered_documents_by_thread = []
+
+        def threaded_function(documents):
+            # Validate that caching the ref returns expected payload.
+            rendered_documents = cache.lookup_by_revision_id(1, documents)
+            rendered_documents_by_thread.append(rendered_documents)
+            return rendered_documents
+
+        thread1 = Thread(target=threaded_function,
+                         kwargs={'documents': documents1})
+        thread2 = Thread(target=threaded_function,
+                         kwargs={'documents': documents2})
+        thread1.start()
+        thread2.start()
+        thread1.join()
+        thread2.join()
+
+        # Validate that 2nd thread uses 1st thread's document set which proves
+        # caching working across threads.
+        self.assertEqual(2, len(rendered_documents_by_thread))
+        self.assertEqual(rendered_documents_by_thread[0],
+                         rendered_documents_by_thread[1])