Improve email address validation (#29838)

Gemfile

@@ -158,3 +158,4 @@ gem 'concurrent-ruby', require: false
 gem 'connection_pool', require: false
 gem 'xorcist', '~> 1.1'
 gem 'cocoon', '~> 1.2'
+gem 'mail', '~> 2.8'

Gemfile.lock

@@ -816,6 +816,7 @@ DEPENDENCIES
   letter_opener_web (~> 2.0)
   link_header (~> 0.0)
   lograge (~> 0.12)
+  mail (~> 2.8)
   makara (~> 0.5)
   mario-redis-lock (~> 1.2)
   memory_profiler

app/models/user.rb

@@ -94,6 +94,9 @@ class User < ApplicationRecord
   validates :invite_request, presence: true, on: :create, if: :invite_text_required?
 
   validates :locale, inclusion: I18n.available_locales.map(&:to_s), if: :locale?
+
+  validates :email, presence: true, email_address: true
+
   validates_with BlacklistedEmailValidator, if: -> { ENV['EMAIL_DOMAIN_LISTS_APPLY_AFTER_CONFIRMATION'] == 'true' || !confirmed? }
   validates_with EmailMxValidator, if: :validate_email_dns?
   validates :agreement, acceptance: { allow_nil: false, accept: [true, 'true', '1'] }, on: :create

app/validators/email_address_validator.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# NOTE: I initially wrote this as `EmailValidator` but it ended up clashing
+# with an indirect dependency of ours, `validate_email`, which, turns out,
+# has the same approach as we do, but with an extra check disallowing
+# single-label domains. Decided to not switch to `validate_email` because
+# we do want to allow at least `localhost`.
+
+class EmailAddressValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    value = value.strip
+
+    address = Mail::Address.new(value)
+    record.errors.add(attribute, :invalid) if address.address != value
+  rescue Mail::Field::FieldError
+    record.errors.add(attribute, :invalid)
+  end
+end

spec/models/user_spec.rb

@@ -40,6 +40,12 @@ RSpec.describe User, type: :model do
       expect(user.valid?).to be true
     end
 
+    it 'is valid with a localhost e-mail address' do
+      user = Fabricate.build(:user, email: 'admin@localhost')
+      user.valid?
+      expect(user.valid?).to be true
+    end
+
     it 'cleans out empty string from languages' do
       user = Fabricate.build(:user, chosen_languages: [''])
       user.valid?