Extending a volume fail after snapshoting the vm when volume driver is vmware.vmdk.VMwareVcVmdkDriver

[wind]

I use vmware.vmdk.VMwareVcVmdkDriver as volume driver, when I try to extend a volume, I fail and the
volume stataus is "error_extending".

The /var/log/cinder/volume.log shows:
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager [req-8607343c-e3b8-4189-92de-54b8f6654593 4a3a8e237cc3402191178c26a9d3f8e3 e66d6cffd8b9448aacd33fbf755776e7 - - -] Extend volume failed.
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager Traceback (most recent call last):
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager File "/usr/lib/python2.7/site-packages/cinder/volume/manager.py", line 1992, in extend_volume
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager self.driver.extend_volume(volume, new_size)
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager File "/usr/lib/python2.7/site-packages/osprofiler/profiler.py", line 105, in wrapper
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager return f(*args, **kwargs)
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager File "/usr/lib/python2.7/site-packages/cinder/volume/drivers/vmware/vmdk.py", line 1576, in extend_volume
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager (host, rp, folder, summary) = self._select_ds_for_volume(new_size)
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager File "/usr/lib/python2.7/site-packages/cinder/volume/drivers/vmware/vmdk.py", line 536, in _select_ds_for_volume
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager req[hub.DatastoreSelector.SIZE_BYTES] = (volume['size'] * units.Gi)
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager TypeError: 'int' object has no attribute '__getitem__'
2017-08-22 16:57:01.285 2275061 ERROR cinder.volume.manager

The steps to reproduce the bug:
1)create vm1 by "nova booot" command line
2)cinder create volume1 by "cinder create" command line
3)attach volume1 to vm1 by "nova volume-attach"
4)snapshot vm1 by "nova image-create"
5)detach volume1 from vm1 by "nova volume-detach"
6)extend volume1 by "cinder extend"

The branch of OpenStack that I use is "liberty"

I think this bug is related with the bug:
https://bugs.launchpad.net/cinder/+bug/1712329