relinking from next part power partition fails to makedir during PUT

[Alistair Coles]

This exception was seen during a partition power increase:

object-server: err Relinking <path> to <path> failed: [Errno 17] File exists: '<path>':
 Traceback (most recent call last):
   File "/opt/ss/lib/python2.7/site-packages/swift/obj/diskfile.py", line 1845, in _finalize_put
     relink_paths(target_path, new_target_path)
   File "/opt/ss/lib/python2.7/site-packages/swift/obj/diskfile.py", line 462, in relink_paths
     os.makedirs(new_target_dir)
   File "/opt/ss/lib/python2.7/os.py", line 157, in makedirs
     mkdir(name, mode)
 OSError: [Errno 17] File exists: '<path>'

It would appear that there were two concurrent PUTs to the same path during the relinking phase of a partition power increase. Both attempt to create the object dir in the new (next partition power) dir but one fails when the dir already exists: the 'look before you leap' check before os.makedirs in diskfile.relink_paths was not sufficient when the requests were concurrent.

If the relinker has already processed (relinked) the old partition, then the failure to create the link from the next part power dir can result in object data that is not relinked to the next part power.

[Alistair Coles (alistair-coles)]

https://review.opendev.org/c/openstack/swift/+/778474