Add net.LookipIP DNS provider implementation

[zserge]

As we are looking forward to using dskit inside Grafana for HA, we would require an implementation for the kv/memberlist.DNSProvider. Currently it looks like the interface closely matches the API from Thanos, but we would need something lighter, possibly built over net.LookupIP. This PR introduces the "builtin" DNSProvider implementation and a basic test to make sure it works.

[CLAassistant]

All committers have signed the CLA.

[56quarters]

Seems like a good change to me. A few safety related comments.

[56quarters]

I think you need to make a copy of this otherwise the underlying storage of the slice can still be modified after the caller starts using it (I could be wrong, we do run tests with -race but this code feels like a race condition).

[56quarters]

Instead of doing an append(d.addr) in a loop, I'd rather see a new slice of addresses constructed and then swapped at the end. That prevents cases where an error is returned and d.addr is only half updated. This would also allow you to avoid holding the lock while doing network operations. Something like:

var addrs []string
for _, a: range addrs {
    // do the lookup
    addrs = append(addrs, ip.String())
}
d.Lock()
d.addrs = addrs
d.Unlock()
return nil

[56quarters]

Should the context be checked for cancellation on each iteration? That seems like it'd make this more responsive.

[56quarters]

Or perhaps just once before doing any DNS lookups?

[pracucci]

Hi! 👋 We recently merged #947 which changed the CI configuration for unit tests. To get CI passing on this PR, please rebase on the latest main branch:

git fetch origin main
git rebase origin/main
git push --force-with-lease

Thanks!