Allow numeric pages domain

Previously, `PagesDomain` would not allow a domain such as
123.example.com. With this change, this is now allowed, because
it is a perfectly valid domain.

app/models/pages_domain.rb

 class PagesDomain < ActiveRecord::Base
   belongs_to :project
 
-  validates :domain, hostname: true
+  validates :domain, hostname: { allow_numeric_hostname: true }
   validates :domain, uniqueness: { case_sensitive: false }
   validates :certificate, certificate: true, allow_nil: true, allow_blank: true
   validates :key, certificate_key: true, allow_nil: true, allow_blank: true
@@ -98,7 +98,7 @@ class PagesDomain < ActiveRecord::Base
 
   def validate_pages_domain
     return unless domain
-    if domain.downcase.ends_with?(".#{Settings.pages.host}".downcase)
+    if domain.downcase.ends_with?(Settings.pages.host.downcase)
       self.errors.add(:domain, "*.#{Settings.pages.host} is restricted")
     end
   end

changelogs/unreleased/allow_numeric_pages_domain.yml

+---
+title: Allow numeric pages domain
+merge_request: 11550
+author:

spec/models/pages_domain_spec.rb

@@ -6,7 +6,7 @@ describe PagesDomain, models: true do
   end
 
   describe 'validate domain' do
-    subject { build(:pages_domain, domain: domain) }
+    subject(:pages_domain) { build(:pages_domain, domain: domain) }
 
     context 'is unique' do
       let(:domain) { 'my.domain.com' }
@@ -14,36 +14,25 @@ describe PagesDomain, models: true do
       it { is_expected.to validate_uniqueness_of(:domain) }
     end
 
-    context 'valid domain' do
-      let(:domain) { 'my.domain.com' }
-
-      it { is_expected.to be_valid }
-    end
-
-    context 'valid hexadecimal-looking domain' do
-      let(:domain) { '0x12345.com'}
-
-      it { is_expected.to be_valid }
-    end
-
-    context 'no domain' do
-      let(:domain) { nil }
-
-      it { is_expected.not_to be_valid }
-    end
-
-    context 'invalid domain' do
-      let(:domain) { '0123123' }
-
-      it { is_expected.not_to be_valid }
-    end
-
-    context 'domain from .example.com' do
-      let(:domain) { 'my.domain.com' }
-
-      before { allow(Settings.pages).to receive(:host).and_return('domain.com') }
-
-      it { is_expected.not_to be_valid }
+    {
+      'my.domain.com'    => true,
+      '123.456.789'      => true,
+      '0x12345.com'      => true,
+      '0123123'          => true,
+      '_foo.com'         => false,
+      'reserved.com'     => false,
+      'a.reserved.com'   => false,
+      nil                => false
+    }.each do |value, validity|
+      context "domain #{value.inspect} validity" do
+        before do
+          allow(Settings.pages).to receive(:host).and_return('reserved.com')
+        end
+
+        let(:domain) { value }
+
+        it { expect(pages_domain.valid?).to eq(validity) }
+      end
     end
   end