Catch NoResponseError from Net::DNS in LoadBalancing::Resolver

Catch the `NoResponseError` rendered from `Net::DNS::Resolver` when it
detects no repsonse from the DNS server. We do this so that we can
`raise` our own `UnresolvableNameserverError` stating that there was
no repsonse. This allows us to differentiate between getting no answers
and getting no response from the system's DNS servers.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/271575

Changelog: fixed

ee/changelogs/unreleased/dblb-split-ns-resolution.yml

+---
+title: Catch NoResponseError from Net::DNS in LoadBalancing::Resolver
+merge_request: 57187
+author:
+type: fixed

ee/lib/gitlab/database/load_balancing/resolver.rb

@@ -43,6 +43,8 @@ module Gitlab
           return if answer.empty?
 
           answer.first.address
+        rescue Net::DNS::Resolver::NoResponseError
+          raise UnresolvableNameserverError, "no response from DNS server(s)"
         end
       end
     end

ee/spec/lib/gitlab/database/load_balancing/resolver_spec.rb

@@ -57,6 +57,23 @@ RSpec.describe Gitlab::Database::LoadBalancing::Resolver do
             )
           end
         end
+
+        context 'when DNS does not respond' do
+          it 'raises an exception' do
+            allow_next_instance_of(Resolv::Hosts) do |instance|
+              allow(instance).to receive(:getaddress).with('localhost').and_raise(Resolv::ResolvError)
+            end
+
+            allow(Net::DNS::Resolver).to receive(:start)
+              .with('localhost', Net::DNS::A)
+              .and_raise(Net::DNS::Resolver::NoResponseError)
+
+            expect { subject }.to raise_exception(
+              described_class::UnresolvableNameserverError,
+              'no response from DNS server(s)'
+            )
+          end
+        end
       end
     end
   end