Use ThreadedResolver to honor /etc/hosts and nsswitch.conf

[dralley]

Pulpcore depended on aiodns, which causes aiohttp to default to AsyncResolver (c-ares) for DNS resolution. c-ares is a DNS client that speaks the wire protocol directly -- it does not go through glibc's getaddrinfo() and therefore ignores nsswitch.conf and /etc/hosts. This causes sync to fail when the remote hostname is only resolvable via /etc/hosts or other non-DNS nsswitch sources.

Explicitly use aiohttp.resolver.ThreadedResolver, which wraps socket.getaddrinfo() in a thread pool. Since getaddrinfo() uses the system resolver, it respects nsswitch.conf ordering and reads /etc/hosts when configured.

The performance difference is negligible for Pulp's use case: DNS resolution happens once per host (aiohttp caches resolved addresses on the connector), and syncs typically talk to a small number of distinct hosts.

Since we now bypass c-ares entirely, drop the aiodns dependency and the related pycares version constraint from CI.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough

[dralley]

My understanding is that we've had a number of issues with aiodns and pycares - as best I can tell we only ever depended on aiodns for purely theoretical performance reasons. But at least for syncs, you really only expect to talk to one host, or maybe a small handful, so aiodns provides no benefit.

Original issue: https://pulp.plan.io/issues/6190

I don't think we ever measured a practical benefit nor would one really be expected. But in this case we're being asked explicitly to follow system configurations which aiodns / c-ares does not support.

https://redhat.atlassian.net/browse/PULP-1788
https://redhat.atlassian.net/browse/SAT-45512

So, we can ditch aiodns and also ditch the pinning of pycares.

[mdellweg]

Transcending the problem, I love it!

[mdellweg]

This will come in via the plugin template, but also it is not-a-problem, since constraints will not pull a package, but just restrict it's version range when selected elsewhere.