install hook fails with install_method: "snap" if telegraf package is missing

[Chris Johnston]

On bionic and focal, where the telegraf package doesn't exist, deploying the charm in an environment where there isn't access to the telegraf ppa the install fails.

2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 Traceback (most recent call last):
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/charm/hooks/install", line 22, in <module>
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 main()
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 bus.dispatch(restricted=restricted_mode)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 _invoke(other_handlers)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 handler.invoke()
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 self._action(*args)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/charm/reactive/telegraf.py", line 975, in install_telegraf
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 apt.purge("telegraf")
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/charm/lib/charms/apt.py", line 92, in purge
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 fetch.apt_purge(packages, fatal=True)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charmhelpers/fetch/ubuntu.py", line 419, in apt_purge
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 _run_apt_command(cmd, fatal)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charmhelpers/fetch/ubuntu.py", line 945, in _run_apt_command
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 _run_with_retries(
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.8/site-packages/charmhelpers/fetch/ubuntu.py", line 922, in _run_with_retries
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 result = subprocess.check_call(cmd, env=env, **kwargs)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 raise CalledProcessError(retcode, cmd)
2023-04-16 19:48:05 WARNING unit.telegraf/0.install logger.go:60 subprocess.CalledProcessError: Command '['apt-get', '--assume-yes', 'purge', 'telegraf']' returned non-zero exit status 100.

ubuntu@juju-f784d2-0:~$ sudo apt-get --assume-yes purge telegraf
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package telegraf
ubuntu@juju-f784d2-0:~$ echo $?
100