Narrow Resolv::DNS resource methods by typeclass

[zonuexe]

Summary

Resolv::DNS#getresource / #getresources / #each_resource / #extract_resources accept a typeclass argument (Resolv::DNS::Resource::IN::MX, IN::A, IN::AAAA, …) that fully determines the returned resource subclass. The current signatures return the upper bound Resolv::DNS::Resource, dropping that information — so callers using subclass-specific methods like MX#exchange have to downcast.

This adds per-subclass overloads for the leaf Resolv::DNS::Resource::IN::* types and Resolv::DNS::Resource::Generic, with the previous wide signature kept as the trailing fallback (which also covers Resource::ANY and other classes under Resolv::DNS::Query).

Motivation

A typical caller pattern, from Mastodon (app/validators/email_mx_validator.rb:49):

records = dns.getresources(domain, Resolv::DNS::Resource::IN::MX).to_a.map { |e| e.exchange.to_s }

#exchange is defined on Resolv::DNS::Resource::MX (inherited by IN::MX), not on the base Resolv::DNS::Resource. Under the existing signature, e is typed Resolv::DNS::Resource and .exchange does not resolve.

Approach considered

A bounded generic would be the cleanest shape:

def getresources: [T < Resolv::DNS::Resource] (dns_name name, singleton(T) typeclass) -> Array[T]

but RBS does not currently accept a type variable inside singleton(...):

ERROR -- rbs: Could not find ::T (RBS::NoTypeFoundError)
    def getresources: [T < Resolv::DNS::Resource] (dns_name name, singleton(T) typeclass) -> Array[T]
                                                                  ^^^^^^^^^^^^

Falling back to per-subclass overloads — verbose but the precision is concentrated in the four methods.

Change

For each of the four methods, 15 overloads:

• 13 leaf Resolv::DNS::Resource::IN::{A, AAAA, CNAME, HINFO, LOC, MINFO, MX, NS, PTR, SOA, SRV, TXT, WKS} overloads — narrowed return / block-parameter type.
• 1 Resolv::DNS::Resource::Generic overload — narrowed return / block-parameter type.
• 1 trailing wide singleton(Resolv::DNS::Query) fallback — keeps the previous behaviour for Resolv::DNS::Resource::ANY, the non-IN resource bases (MX, NS, CNAME, DomainName, SOA, MINFO, HINFO, LOC, TXT), and any other Query subclass.

Ruby Method	Implementation
Resolv::DNS#getresource	Resolv::DNS#getresource
Resolv::DNS#getresources	Resolv::DNS#getresources
Resolv::DNS#each_resource	Resolv::DNS#each_resource
Resolv::DNS#extract_resources	Resolv::DNS#extract_resources

Out of scope

• Resolv::DNS#fetch_resource also accepts a typeclass argument, but its block yields (Message, Name) rather than a Resource, so it does not benefit from the same narrowing.

Test plan

• bundle exec rbs -r resolv validate
• bundle exec ruby -Ilib bin/test_runner.rb test/stdlib/resolv/DNS_test.rb (67 tests pass)