Logging a policy index when container PUT request conflicts

This is a continuing work of patch afdbf73. This patch enables
proxy-server to log a policy index when container PUT request
conflicts with existing container's policy index.

Change-Id: I6d40044c510632a0f61b817a9af2f6c13a721d39
Implements: blueprint logging-policy-number

swift/container/server.py

@@ -308,7 +308,9 @@ class ContainerController(BaseStorageServer):
         elif requested_policy_index is not None:
             # validate requested policy with existing container
             if requested_policy_index != broker.storage_policy_index:
-                raise HTTPConflict(request=req)
+                raise HTTPConflict(request=req,
+                                   headers={'x-backend-storage-policy-index':
+                                            broker.storage_policy_index})
         broker.update_put_timestamp(timestamp)
         if broker.is_deleted():
             raise HTTPConflict(request=req)

test/unit/container/test_server.py

@@ -476,6 +476,9 @@ class TestContainerController(unittest.TestCase):
             })
             resp = req.get_response(self.controller)
             self.assertEquals(resp.status_int, 409)
+            self.assertEquals(
+                resp.headers.get('X-Backend-Storage-Policy-Index'),
+                str(policy.idx))
 
         # and make sure there is no change!
         req = Request.blank('/sda1/p/a/c')