Fix NULL MX handling and tighten DNS resolving specs (#28607)

app/validators/email_mx_validator.rb

@@ -47,6 +47,7 @@ class EmailMxValidator < ActiveModel::Validator
       dns.timeouts = 5
 
       records = dns.getresources(domain, Resolv::DNS::Resource::IN::MX).to_a.map { |e| e.exchange.to_s }
+      next if records == [''] # This domain explicitly rejects emails
 
       ([domain] + records).uniq.each do |hostname|
         ips.concat(dns.getresources(hostname, Resolv::DNS::Resource::IN::A).to_a.map { |e| e.address.to_s })

spec/controllers/admin/email_domain_blocks_controller_spec.rb

@@ -35,6 +35,16 @@ RSpec.describe Admin::EmailDomainBlocksController do
   describe 'POST #create' do
     context 'when resolve button is pressed' do
       before do
+        resolver = instance_double(Resolv::DNS)
+
+        allow(resolver).to receive(:getresources)
+          .with('example.com', Resolv::DNS::Resource::IN::MX)
+          .and_return([])
+        allow(resolver).to receive(:getresources).with('example.com', Resolv::DNS::Resource::IN::A).and_return([])
+        allow(resolver).to receive(:getresources).with('example.com', Resolv::DNS::Resource::IN::AAAA).and_return([])
+        allow(resolver).to receive(:timeouts=).and_return(nil)
+        allow(Resolv::DNS).to receive(:open).and_yield(resolver)
+
         post :create, params: { email_domain_block: { domain: 'example.com' } }
       end
 

spec/rails_helper.rb

@@ -128,6 +128,10 @@ RSpec.configure do |config|
     self.use_transactional_tests = true
   end
 
+  config.before do |example|
+    allow(Resolv::DNS).to receive(:open).and_raise('Real DNS queries are disabled, stub Resolv::DNS as needed') unless example.metadata[:type] == :system
+  end
+
   config.before do |example|
     unless example.metadata[:paperclip_processing]
       allow_any_instance_of(Paperclip::Attachment).to receive(:post_process).and_return(true) # rubocop:disable RSpec/AnyInstance

spec/validators/email_mx_validator_spec.rb

@@ -111,7 +111,7 @@ describe EmailMxValidator do
       allow(resolver).to receive(:getresources).with('example.com', Resolv::DNS::Resource::IN::A).and_return([])
       allow(resolver).to receive(:getresources).with('example.com', Resolv::DNS::Resource::IN::AAAA).and_return([])
       allow(resolver).to receive(:getresources).with('mail.example.com', Resolv::DNS::Resource::IN::A).and_return([instance_double(Resolv::DNS::Resource::IN::A, address: '2.3.4.5')])
-      allow(resolver).to receive(:getresources).with('mail.example.com', Resolv::DNS::Resource::IN::AAAA).and_return([instance_double(Resolv::DNS::Resource::IN::A, address: 'fd00::2')])
+      allow(resolver).to receive(:getresources).with('mail.example.com', Resolv::DNS::Resource::IN::AAAA).and_return([instance_double(Resolv::DNS::Resource::IN::AAAA, address: 'fd00::2')])
       allow(resolver).to receive(:timeouts=).and_return(nil)
       allow(Resolv::DNS).to receive(:open).and_yield(resolver)