Improve k8s canonical DNS lookup

[gboutry]

Issue

Fixes #137.

Solution

Replace getfqdn-based Kubernetes hostname resolution with a canonical-name
lookup built on socket.getaddrinfo(..., AI_CANONNAME).

The helper now scans resolver results for a canonical name, handles resolver
failures explicitly, preserves the trailing dot returned by DNS, and updates
the unit tests covering the charm, provider, upgrade, and utility paths.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@             Coverage Diff             @@
##             8.0/edge     #138   +/-   ##
===========================================
  Coverage            ?   59.91%           
===========================================
  Files               ?       34           
  Lines               ?     6364           
  Branches            ?     1069           
===========================================
  Hits                ?     3813           
  Misses              ?     2233           
  Partials            ?      318           

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[astrojuanlu]

Thank you @gboutry . We'll nudge the CI so that the s390x builds are retried and the integration tests are triggered.

I see you added new unit tests 👍🏼 Any chance to validate whether this PR fixes the original issue?

[astrojuanlu]

Backups and subordinate tests are failing, as expected (see #119 (comment))

I will retry the 2 remaining tests a few more times just in case.

[paulomach]

Exquisite

[astrojuanlu]

Been retrying the last test a few times. Despite not passing, the timeouts seem to be happening in different places, and there's nothing in the logs that suggests that they're caused by this PR. So I'm going to go ahead and merge.

Thanks again @gboutry !

[delgod]

I don't sure that logic is robust enough with different DNS engines for kubernetes.

[astrojuanlu]

@delgod Are you concerned about any of those in particular? Tests have been passing on CI with microk8s and on my laptop with k3s. We're eyeing Canonical K8s but haven't finished it yet #142

[delgod]

I think we should add the following function

import re

def is_srv_dns_record(s: str) -> bool:
    """Match pattern: pod-name.service-name.srv.some.domain.name."""
    pattern = r'^[a-z0-9]([a-z0-9-]*[a-z0-9])?\.([a-z0-9]([a-z0-9-]*[a-z0-9])?\.)+srv(\.[a-z0-9]([a-z0-9-]*[a-z0-9])?){2,}\.?$'
    return bool(re.match(pattern, s, re.IGNORECASE))

and use it here

Suggested change

	for entry in info:
	if canonname := entry[3]:
	return canonname
	for entry in info:
	if canonname := entry[3] and is_srv_dns_record(canonname):
	return canonname