Comment 6 for bug 1752986

I got this issue in my environment:

Add debug message in code:

nova/compute/manager.py

def _resize_instance:
    LOG.debug("-------instance.task_state is %s----",instance.task_state)
    instance.task_state = task_states.RESIZE_MIGRATING
    instance.save(expected_task_state=task_states.RESIZE_PREP)

compute.log:
2019-03-21 15:02:44.797 7 DEBUG nova.compute.manager [req-bc599214-d28f-46f3-b889-3d5abac4830b a6b2a018df8844a58bc7896d2cf91a16 4b37919a7bb7450db35b54ebb20ec29b - default default] -------instance.task_state is resize_prep---- _resize_instance /usr/lib/python2.7/site-packages/nova/compute/manager.py:4498

2019-03-21 15:02:44.927 7 ERROR nova.compute.manager [req-bc599214-d28f-46f3-b889-3d5abac4830b a6b2a018df8844a58bc7896d2cf91a16 4b37919a7bb7450db35b54ebb20ec29b - default default] [instance: b0948982-324c-4991-ab93-bd56f6e4ff52] Setting instance vm_state to ERROR: UnexpectedTaskStateError_Remote: Conflict updating instance b0948982-324c-4991-ab93-bd56f6e4ff52. Expected: {'task_state': [u'resize_prep']}. Actual: {'task_state': None}
Traceback (most recent call last):

The task_state was resize_prep already，but still report: Actual: {'task_state': None}