Use Rails tag API to build RSS feed for spoilers and polls (#20163)

* Use Rails tag API to build RSS feed for spoilers and polls

While the previous method did not contain a bug or a potential issue,
the tag API can be very resilient against future problems and reduces the
amount of manual management of the escape status of the content.

I've added tests to ensure that the formatting is broken and still
escapes control characters correctly.

* this seems cleaner and passes

* Incorporate feedback by moving the br to its own line and using the tag helper over the string constant for the br tag itself

* whoops, tag helper doesn't use a self-closing tag

app/helpers/formatting_helper.rb

@@ -23,19 +23,28 @@ module FormattingHelper
 
     before_html = begin
       if status.spoiler_text?
-        "<p><strong>#{I18n.t('rss.content_warning', locale: available_locale_or_nil(status.language) || I18n.default_locale)}</strong> #{h(status.spoiler_text)}</p><hr />"
+        tag.p do
-      else
+          tag.strong do
-        ''
+            I18n.t('rss.content_warning', locale: available_locale_or_nil(status.language) || I18n.default_locale)
+          end
+
+          status.spoiler_text
+        end + tag.hr
       end
-    end.html_safe # rubocop:disable Rails/OutputSafety
+    end
 
     after_html = begin
       if status.preloadable_poll
-        "<p>#{status.preloadable_poll.options.map { |o| "<input type=#{status.preloadable_poll.multiple? ? 'checkbox' : 'radio'} disabled /> #{h(o)}" }.join('<br />')}</p>"
+        tag.p do
-      else
+          safe_join(
-        ''
+            status.preloadable_poll.options.map do |o|
+              tag.send(status.preloadable_poll.multiple? ? 'checkbox' : 'radio', o, disabled: true)
+            end,
+            tag.br
+          )
+        end
       end
-    end.html_safe # rubocop:disable Rails/OutputSafety
+    end
 
     prerender_custom_emojis(
       safe_join([before_html, html, after_html]),

spec/helpers/formatting_helper_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe FormattingHelper, type: :helper do
+  include Devise::Test::ControllerHelpers
+
+  describe '#rss_status_content_format' do
+    let(:status) { Fabricate(:status, text: 'Hello world<>', spoiler_text: 'This is a spoiler<>', poll: Fabricate(:poll, options: %w(Yes<> No))) }
+    let(:html) { helper.rss_status_content_format(status) }
+
+    it 'renders the spoiler text' do
+      expect(html).to include('<p>This is a spoiler&lt;&gt;</p><hr>')
+    end
+
+    it 'renders the status text' do
+      expect(html).to include('<p>Hello world&lt;&gt;</p>')
+    end
+
+    it 'renders the poll' do
+      expect(html).to include('<radio disabled="disabled">Yes&lt;&gt;</radio><br>')
+    end
+  end
+end