Copy/paste of commands to check before/after:

... Reproducer (15-second startup delay in tgt.service)

	FILE=/etc/systemd/system/tgt.service.d/start-delay.conf
	mkdir -p $(dirname $FILE)
	cat <<EOF > $FILE
	[Service]
	ExecStartPre=$(which sleep) 15
	EOF
	systemctl daemon-reload

Before:
---

... Restart both services at the same time:

	# date; systemctl restart cinder-volume.service tgt.service; date
	Mon Oct 16 21:57:12 UTC 2023
	Mon Oct 16 21:57:27 UTC 2023

... Notice that cinder-volume.service is Started _BEFORE_ tgt.service

	# journalctl -b -u cinder-volume.service -u tgt.service | grep Start | tail -3
	Oct 16 21:57:12 cinder-mantic systemd[1]: Started cinder-volume.service - OpenStack Cinder Volume.
	Oct 16 21:57:12 cinder-mantic systemd[1]: Starting tgt.service - (i)SCSI target daemon...
	Oct 16 21:57:27 cinder-mantic systemd[1]: Started tgt.service - (i)SCSI target daemon.

.. Log error:

	# grep 'ERROR cinder.volume.manager' /var/log/cinder/cinder-volume.log
	...
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager [None req-ce03264a-6765-41de-8016-a6f27d2685e4 - - - - - -] Failed to re-export volume, setting to ERROR.: oslo_concurrency.processutils.ProcessExecutionError: Unexpected error while running command.
	...
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager oslo_concurrency.processutils.ProcessExecutionError: Unexpected error while running command.
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager Command: tgtadm --lld iscsi --op show --mode target
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager Exit code: 107
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager Stdout: ''
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager Stderr: 'tgtadm: failed to send request hdr to tgt daemon, Transport endpoint is not connected\n'
	2023-10-16 21:57:18.301 1658 ERROR cinder.volume.manager

... Volume error:

	# cinder list
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| ID                                   | Status | Name        | Size | Consumes Quota | Volume Type | Bootable | Attached to |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| 17c4f736-058b-4cfc-9864-a64ab0995957 | error  | test-volume | 1    | True           | __DEFAULT__ | false    |             |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+

... Undo error state:

	# systemctl restart cinder-volume.service
	# cinder reset-state --state in-use test-volume
	# cinder list

	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| ID                                   | Status | Name        | Size | Consumes Quota | Volume Type | Bootable | Attached to |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| 17c4f736-058b-4cfc-9864-a64ab0995957 | in-use | test-volume | 1    | True           | __DEFAULT__ | false    |             |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+


After:
---

... Patched with Wants=

	# systemctl show cinder-volume.service | grep Wants=
	Wants=network-online.target tgt.service

	# cinder list
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| ID                                   | Status | Name        | Size | Consumes Quota | Volume Type | Bootable | Attached to |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| 17c4f736-058b-4cfc-9864-a64ab0995957 | in-use | test-volume | 1    | True           | __DEFAULT__ | false    |             |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+

... Restart both services at the same time:

	# date; systemctl restart cinder-volume.service tgt.service; date
	Wed Oct 18 15:29:01 UTC 2023
	Wed Oct 18 15:29:17 UTC 2023

... Notice that cinder-volume.service is Started _AFTER_ tgt.service

	# journalctl -b -u cinder-volume.service -u tgt.service | grep Start | tail -3
	Oct 18 15:29:01 cinder-mantic systemd[1]: Starting tgt.service - (i)SCSI target daemon...
	Oct 18 15:29:17 cinder-mantic systemd[1]: Started tgt.service - (i)SCSI target daemon.
	Oct 18 15:29:17 cinder-mantic systemd[1]: Started cinder-volume.service - OpenStack Cinder Volume.

... Volume not in error:

	# cinder list
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| ID                                   | Status | Name        | Size | Consumes Quota | Volume Type | Bootable | Attached to |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+
	| 17c4f736-058b-4cfc-9864-a64ab0995957 | in-use | test-volume | 1    | True           | __DEFAULT__ | false    |             |
	+--------------------------------------+--------+-------------+------+----------------+-------------+----------+-------------+

... Without tgt.service installed:

	# apt remove --purge --yes tgt

	# systemctl status tgt.service
	Unit tgt.service could not be found.

	# systemctl show cinder-volume.service | grep Wants=
	Wants=tgt.service network-online.target

	# systemctl restart cinder-volume.service
	# echo $?
	0

	# systemctl status cinder-volume.service | grep Active:
	     Active: active (running) since Wed 2023-10-18 16:47:14 UTC; 11s ago

	# journalctl -b -u cinder-volume.service -u tgt.service | grep Start | tail -1
	Oct 18 16:47:14 cinder-mantic systemd[1]: Started cinder-volume.service - OpenStack Cinder Volume.