pickle_async_update should create tmp_dir

While creating a probe test for the expirer daemon, I found
the following error scenario:

1. Introduce a new object server. Initially it doesn't have a tmp_dir.
2. Have the object-replicator replicate some objects, one of them
    with an expiration (X-Delete-At).
3. Send a DELETE request for the expired object.

While beginning to process the DELETE request, the fresh
object server still doesn't have a tmp_dir created.
Since the object has an old expiration value, the object server
will first call "delete_at_update", before creating a tombstone.
delete_at_update then must create an async_pending,
which will lead to an IO error, since tmp_dir doesn't exist.

As said, I have witnessed this in practice in the probe test I wrote
at https://review.openstack.org/#/c/326903/.

This patch changes pickle_async_update behavior to create
tmp_dir, in case it doesn't exist.

Change-Id: I88b0e5f75a2a28d6880694ff327ac2763c816d24

swift/obj/diskfile.py

@@ -1115,12 +1115,14 @@ class BaseDiskFileManager(object):
         """
         device_path = self.construct_dev_path(device)
         async_dir = os.path.join(device_path, get_async_dir(policy))
+        tmp_dir = os.path.join(device_path, get_tmp_dir(policy))
+        mkdirs(tmp_dir)
         ohash = hash_path(account, container, obj)
         write_pickle(
             data,
             os.path.join(async_dir, ohash[-3:], ohash + '-' +
                          Timestamp(timestamp).internal),
-            os.path.join(device_path, get_tmp_dir(policy)))
+            tmp_dir)
         self.logger.increment('async_pendings')
 
     def get_diskfile(self, device, partition, account, container, obj,

test/unit/obj/test_diskfile.py

@@ -246,10 +246,6 @@ class TestDiskFileModuleMethods(unittest.TestCase):
             self.assertFalse(os.path.isdir(tmp_path))
             pickle_args = (self.existing_device, 'a', 'c', 'o',
                            'data', 0.0, policy)
-            # async updates don't create their tmpdir on their own
-            self.assertRaises(OSError, self.df_mgr.pickle_async_update,
-                              *pickle_args)
-            os.makedirs(tmp_path)
             # now create a async update
             self.df_mgr.pickle_async_update(*pickle_args)
             # check tempdir