Add ability to manage which websites can credit you in link previews (#31819)

app/validators/domain_validator.rb

@@ -1,22 +1,29 @@
 # frozen_string_literal: true
 
 class DomainValidator < ActiveModel::EachValidator
+  MAX_DOMAIN_LENGTH = 256
+  MIN_LABEL_LENGTH = 1
+  MAX_LABEL_LENGTH = 63
+  ALLOWED_CHARACTERS_RE = /^[a-z0-9\-]+$/i
+
   def validate_each(record, attribute, value)
     return if value.blank?
 
-    domain = if options[:acct]
-               value.split('@').last
-             else
-               value
-             end
+    (options[:multiline] ? value.split : [value]).each do |domain|
+      _, domain = domain.split('@') if options[:acct]
 
-    record.errors.add(attribute, I18n.t('domain_validator.invalid_domain')) unless compliant?(domain)
+      next if domain.blank?
+
+      record.errors.add(attribute, options[:multiline] ? :invalid_domain_on_line : :invalid, value: domain) unless compliant?(domain)
+    end
   end
 
   private
 
   def compliant?(value)
-    Addressable::URI.new.tap { |uri| uri.host = value }
+    uri = Addressable::URI.new
+    uri.host = value
+    uri.normalized_host.size < MAX_DOMAIN_LENGTH && uri.normalized_host.split('.').all? { |label| label.size.between?(MIN_LABEL_LENGTH, MAX_LABEL_LENGTH) && label =~ ALLOWED_CHARACTERS_RE }
   rescue Addressable::URI::InvalidURIError, IDN::Idna::IdnaError
     false
   end

app/validators/lines_validator.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class LinesValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    return if value.blank?
+
+    record.errors.add(attribute, :too_many_lines, limit: options[:maximum]) if options[:maximum].present? && value.split.size > options[:maximum]
+  end
+end