Can not delete images if db deadlock occurs

Glance api returns 500 Internal Server Error, if db deadlock occurs
in glance-registry for some reason while deleting an image.

Added retry on image_destroy when db deadlock occures which will again
try to delete the image from database.

Closes-bug: 1378215
Change-Id: Ifad403e363daf368e846b5b6838432a7bedbe81a
(cherry picked from commit dae0fa8f10)

glance/db/sqlalchemy/api.py

@@ -135,6 +135,8 @@ def image_update(context, image_id, values, purge_props=False,
                          from_state=from_state)
 
 
+@retry(retry_on_exception=_retry_on_deadlock, wait_fixed=500,
+       stop_max_attempt_number=50)
 def image_destroy(context, image_id):
     """Destroy the image or raise if it does not exist."""
     session = get_session()

glance/tests/unit/test_db.py

@@ -717,3 +717,13 @@ class RetryOnDeadlockTestCase(test_utils.BaseTestCase):
                 api._image_update(None, {}, 'fake-id')
             except TestException:
                 self.assertEqual(sess.call_count, 3)
+
+        # Test retry on image destroy if db deadlock occurs
+        self.attempts = 3
+        with mock.patch.object(api, 'get_session') as sess:
+            sess.side_effect = _mock_get_session()
+
+            try:
+                api.image_destroy(None, 'fake-id')
+            except TestException:
+                self.assertEqual(sess.call_count, 3)