pre-series-upgrade fails with 'swift-account didn't stop cleanly'

I am upgrading a trusty machine to xenial, the swift storage charm is 20.05.

The upgrade-series prepare step hangs forever.

The charm logs show:

2020-10-02 19:20:23 DEBUG pre-series-upgrade stop: Unknown instance:
2020-10-02 19:20:23 DEBUG pre-series-upgrade Traceback (most recent call last):
2020-10-02 19:20:23 DEBUG pre-series-upgrade File "/var/lib/juju/agents/unit-swift-storage-z1-1/charm/hooks/pre-series-upgrade", line 503, in <module>
2020-10-02 19:20:23 DEBUG pre-series-upgrade main()
2020-10-02 19:20:23 DEBUG pre-series-upgrade File "/var/lib/juju/agents/unit-swift-storage-z1-1/charm/hooks/pre-series-upgrade", line 488, in main
2020-10-02 19:20:23 DEBUG pre-series-upgrade hooks.execute(sys.argv)
2020-10-02 19:20:23 DEBUG pre-series-upgrade File "/var/lib/juju/agents/unit-swift-storage-z1-1/charm/charmhelpers/core/hookenv.py", line 945, in execute
2020-10-02 19:20:23 DEBUG pre-series-upgrade self._hooks[hook_name]()
2020-10-02 19:20:23 DEBUG pre-series-upgrade File "/var/lib/juju/agents/unit-swift-storage-z1-1/charm/hooks/pre-series-upgrade", line 469, in pre_series_upgrade
2020-10-02 19:20:23 DEBUG pre-series-upgrade raise Exception("{} didn't stop cleanly.".format(service))
2020-10-02 19:20:23 DEBUG pre-series-upgrade Exception: swift-account didn't stop cleanly.
2020-10-02 19:20:23 ERROR juju.worker.uniter.operation runhook.go:132 hook "pre-series-upgrade" failed: exit status 1

I suspect this happens because swift-account is stopped more than once. In fact if I try to manually stop an upstart service that is already stopped I get the following:

# stop swift-account; echo $?
stop: Unknown instance:
1

Workaround: comment out the if not stopped / raise in swift_storage_hooks.py, lines 468 and 469