Error 500 occurs when requesting API simultaneously

[Yonghyun Kim]

Description
===========

If you make multiple reboot API requests simultaneously, a 500 error response will occur.

Steps to reproduce
==================

test_reboot.sh
-------------------------------------------
#!/bin/bash

INSTANCE=$1

for i in $(seq 1 2)
do
    echo "reboot instance: $INSTANCE"
    openstack server reboot $INSTANCE &
done
-------------------------------------------

run script
-------------------------------------------
$ bash test_reboot.sh
-------------------------------------------

Expected result
===============

I expect a 202 or 409 response to be output.

Actual result
=============

Of the two requests, one was performed normally with a 202 response, and the other responded with a 500 error.

-------------------------------------------
Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
<class 'nova.exception.UnexpectedTaskStateError'> (HTTP 500) (Request-ID: req-062f90da-d980-4d23-9624-2c37874fc9ef)
-------------------------------------------

nova-api log
-------------------------------------------
2023-09-25 10:38:05.028 9285 ERROR nova.api.openstack.wsgi [req-062f90da-d980-4d23-9624-2c37874fc9ef e306fe82332941d8a5317738a21cf82c 0f6727c6cc434a219ed84d076d4b0e05 - default default] Unexpected exception in API method: UnexpectedTaskStateError: Conflict updating instance 88ca51ce-f75e-4782-b6cd-85cdc4eb461b. Expected: {'task_state': [None]}. Actual: {'task_state': u'rebooting'}
2023-09-25 10:38:05.038 9285 INFO nova.api.openstack.wsgi [req-062f90da-d980-4d23-9624-2c37874fc9ef e306fe82332941d8a5317738a21cf82c 0f6727c6cc434a219ed84d076d4b0e05 - default default] HTTP exception thrown: Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
2023-09-25 10:38:05.039 9285 DEBUG nova.api.openstack.wsgi [req-062f90da-d980-4d23-9624-2c37874fc9ef e306fe82332941d8a5317738a21cf82c 0f6727c6cc434a219ed84d076d4b0e05 - default default] Returning 500 to user: Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
2023-09-25 10:38:05.043 9285 INFO nova.osapi_compute.wsgi.server [req-062f90da-d980-4d23-9624-2c37874fc9ef e306fe82332941d8a5317738a21cf82c 0f6727c6cc434a219ed84d076d4b0e05 - default default] 192.168.0.1 "POST /v2.1/servers/88ca51ce-f75e-4782-b6cd-85cdc4eb461b/action HTTP/1.1" status: 500 len: 646 time: 1.3927691
-------------------------------------------

Environment
===========

* openstack version: queens
* libvirt + kvm

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/896320