HNAS iSCSI Driver errors in concurrent scenarios

It might happen that multiple cinder nodes are trying to add volumes to the same target. In this situation, the driver can get an error trying to add a volume to a hlun just taken by other instance. In this case we backoff and retry ISCSI_ATTACH_RETRIES times.

How to reproduce:
1 - Create 10 or more instances at the same time (boot from image, creates a new volume)

The following error will be shown:

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1b57d45-f97b-4dae-9c17-b02c24f5b92b to volume: 9a3dc42a-5a3a-4e9c-889a-1383d412c803
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume Traceback (most recent call last):
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume File "/opt/stack/cinder/cinder/volume/flows/manager/create_volume.py", line 501, in _copy_image_to_volume
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume copy_image_to_volume(context, volume_ref, image_service, image_id)
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume File "/usr/local/lib/python2.7/dist-packages/osprofiler/profiler.py", line 105, in wrapper
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume return f(*args, **kwargs)
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume File "/opt/stack/cinder/cinder/volume/driver.py", line 558, in copy_image_to_volume
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume attach_info, volume = self._attach_volume(context, volume, properties)
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume File "/opt/stack/cinder/cinder/volume/driver.py", line 701, in _attach_volume
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume raise exception.VolumeBackendAPIException(data=err_msg)
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume VolumeBackendAPIException: Bad or unexpected response from the storage volume backend API: Unable to fetch connection information from backend: Unexpected error while running command.
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume Command: ssc -u supervisor -p supervisor 172.24.44.15 console-context --evs 2 iscsi-target addlu cinder-default volume-9a3dc42a-5a3a-4e9c-889a-1383d412c803 4
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume Exit code: 1
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume Stdout: u"Couldn't add logical unit to target, error: Logical unit in use\n"
2015-07-15 13:56:11.223 TRACE cinder.volume.flows.manager.create_volume