Fix a potential frozen string error in app/mailers/notify.rb

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/mailers/notify.rb

@@ -130,7 +130,7 @@ class Notify < BaseMailer
         address.display_name = reply_display_name(model)
       end
 
-      fallback_reply_message_id = "<reply-#{reply_key}@#{Gitlab.config.gitlab.host}>".freeze
+      fallback_reply_message_id = "<reply-#{reply_key}@#{Gitlab.config.gitlab.host}>"
       headers['References'] ||= []
       headers['References'].unshift(fallback_reply_message_id)
 
@@ -180,7 +180,7 @@ class Notify < BaseMailer
 
     headers['X-GitLab-Discussion-ID'] = note.discussion.id if note.part_of_discussion?
 
-    headers[:subject]&.prepend('Re: ')
+    headers[:subject] = "Re: #{headers[:subject]}" if headers[:subject]
 
     mail_thread(model, headers)
   end

changelogs/unreleased/55183-frozenerror-can-t-modify-frozen-string-in-app-mailers-notify-rb.yml

+---
+title: Fix a potential frozen string error in app/mailers/notify.rb
+merge_request: 23728
+author:
+type: fixed

spec/mailers/notify_spec.rb

@@ -4,6 +4,7 @@ require 'email_spec'
 describe Notify do
   include EmailSpec::Helpers
   include EmailSpec::Matchers
+  include EmailHelpers
   include RepoHelpers
 
   include_context 'gitlab email notification'
@@ -27,15 +28,6 @@ describe Notify do
                    description: 'My awesome description!')
   end
 
-  def have_referable_subject(referable, reply: false)
-    prefix = (referable.project ? "#{referable.project.name} | " : '').freeze
-    prefix = "Re: #{prefix}" if reply
-
-    suffix = "#{referable.title} (#{referable.to_reference})"
-
-    have_subject [prefix, suffix].compact.join
-  end
-
   context 'for a project' do
     shared_examples 'an assignee email' do
       it 'is sent to the assignee as the author' do

spec/support/helpers/email_helpers.rb

@@ -34,4 +34,13 @@ module EmailHelpers
   def find_email_for(user)
     ActionMailer::Base.deliveries.find { |d| d.to.include?(user.notification_email) }
   end
+
+  def have_referable_subject(referable, include_project: true, reply: false)
+    prefix = (include_project && referable.project ? "#{referable.project.name} | " : '').freeze
+    prefix = "Re: #{prefix}" if reply
+
+    suffix = "#{referable.title} (#{referable.to_reference})"
+
+    have_subject [prefix, suffix].compact.join
+  end
 end