[JUJU-573] Fix charm resolution for Juju 2.8.11

[cderici]

Description

This is in response to some charm deployment issues with Juju 2.8.11. It changes two things:

1 - It patches the deployment of CharmStore charms
2 - Normally pylibjuju assumes the CharmHub when it receives a charm url without a prefix. This change adds an override for the case where the CharmsFacade version is too low to resolve the charm.

Fixes #623

QA Steps

You'll need the juju 2.8.11, and to put the following script in a file (slightly modified version of what is provided in the reporting issue #623):

import sys
import asyncio
import juju.controller

async def controller():
    c = juju.controller.Controller()
    await c.connect_current()
    return c

async def model(controller):
    return await controller.add_model('test')

loop = asyncio.get_event_loop()

try:
    c = loop.run_until_complete(controller())
    m = loop.run_until_complete(model(c))
    loop.run_until_complete(m.deploy(sys.argv[1], channel='stable'))

    loop.run_until_complete(m.disconnect())
    loop.run_until_complete(c.destroy_model('test'))
    loop.run_until_complete(c.disconnect())
    print("success")
except:
    loop.run_until_complete(c.destroy_model('test'))
    loop.run_until_complete(c.disconnect())
    raise

Bootstrap with Juju 2.8.11 and run the file containing the script above like python3 test.py ubuntu, you should see success.

Notes & Discussion

_resolve_charm method literally has a failing check for this because lower versions (such as Juju 2.8) cannot deploy from the CharmHub in the first place, so it's safe to put in this override and try to get the charm from the CharmStore instead of failing.

[juanmanuel-tirado]

LGTM

[SimonRichardson]

I'm not fond of the URL.parse having force_v1. I think we should have URL.defaultStore that we can then pass into URL.parse. If the return type of URL.defaultStore is None, then fallback to charmhub.

[cderici]

I'm not fond of the URL.parse having force_v1. I think we should have URL.defaultStore that we can then pass into URL.parse. If the return type of URL.defaultStore is None, then fallback to charmhub.

Agreed, I think that's a better way of handling this, though parse being a static method doesn't help. Check out the changes. First I actually put a URL.default_store variable and changed that but I don't think modifying a global static class constant is a good idea, so I just made a new variable default_store to parse (defaulting to Schema.CHARM_HUB).

[cderici]

!!build!!

[SimonRichardson]

I think we should write this like the following:

if Schema.LOCAL.matches(u.scheme):
            c = URL(Schema.LOCAL, name=u.path)
elif Schema.CHARM_STORE.matches(u.scheme) or (u.Scheme == "" and default_store == Schema.CHARM_STORE):
            c = parse_v1_url(Schema.CHARM_STORE, u, s)
else:
            c = parse_v2_url(u, s)

[cderici]

$$merge$$