Charm may skip installation of required deb package

[Frode Nordahl]

The charm lists ``distro-info`` as a required package, however during a gate test we ended up with this situation:

2020-05-11 08:18:53 INFO juju-log Initializing Snap Layer
2020-05-11 08:18:53 DEBUG retrofit-image none
2020-05-11 08:18:54 DEBUG retrofit-image All snaps up to date.
2020-05-11 08:18:54 DEBUG juju-log Running action "retrofit-image"
2020-05-11 08:18:54 INFO juju-log DEPRECATION WARNING: Function action_get is being removed : moved to function_get()
2020-05-11 08:18:55 ERROR juju-log action "retrofit-image" failed: "[Errno 2] No such file or directory: 'distro-info': 'distro-info'" "Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-octavia-diskimage-retrofit-0/charm/actions/retrofit-image", line 67, in main
    action(args)
  File "/var/lib/juju/agents/unit-octavia-diskimage-retrofit-0/charm/actions/retrofit-image", line 49, in retrofit_image
    ch_core.hookenv.action_get('source-image'))
  File "lib/charm/openstack/octavia_diskimage_retrofit.py", line 94, in retrofit
    series=self.config['retrofit-series'])
  File "lib/charm/openstack/octavia_diskimage_retrofit.py", line 67, in get_ubuntu_release
    output = subprocess.check_output(cmd, universal_newlines=True)
  File "/usr/lib/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1364, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'distro-info': 'distro-info'
"
2020-05-11 08:18:55 INFO juju-log DEPRECATION WARNING: Function action_fail is being removed : moved to function_fail()

Looking at the reactive handlers of the charm:

charm.use_defaults(
    'charm.installed',
    'config.changed',
    'update-status',
    'upgrade-charm',
)

@reactive.when_not('charm.installed')
def check_snap_installed():
    # Installation is handled by the ``snap`` layer, just update our status.
    with charm.provide_charm_instance() as instance:
        instance.assess_status()
    reactive.set_flag('charm.installed')

While the above code was fine when the charm relied only on the snap to do its work, it does not guarantee that the now required ``distro-info`` package is installed in the system before it announces itself as being ready.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/726795

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/726795
Committed: https://git.openstack.org/cgit/openstack/charm-octavia-diskimage-retrofit/commit/?id=fb3c4593fc30ce29b2da867b9de32b39ee916c7a
Submitter: Zuul
Branch: master

commit fb3c4593fc30ce29b2da867b9de32b39ee916c7a
Author: Frode Nordahl <email address hidden>
Date: Mon May 11 13:55:38 2020 +0200

    Let default install helper do its job

    Change I80809dbe0ae43a87986b33c902e0eb53563cb90d added a required
    deb package to be installed by the charm.

    Before that the only installation done was through the snap layer
    and the charm updated the status and set the ``charm.installed``
    to have the status set correctly.

    Remove this code as this now is at odds with the requirement of
    having a deb installed before charm operation.

    Closes-Bug: #1877972
    Change-Id: Iee82f7ff066e81787bb658bca3dfce4087ab4e00