Make mail notifications of discussion notes In-Reply-To of each other

When a note is part of a discussion, the email sent out should be
`In-Reply-To` the previous note in that discussion.

Closes gitlab-org/gitlab-ce#36054

1 files changed, 12 insertions, 0 deletions

diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 9efabe3f44e..250583d2d28 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -156,6 +156,18 @@ class Notify < BaseMailer
     mail_thread(model, headers)
   end
 
+  def mail_answer_note_thread(model, note, headers = {})
+    headers['Message-ID'] = message_id(note)
+    headers['In-Reply-To'] = message_id(note.replies_to)
+    headers['References'] = message_id(model)
+
+    headers['X-GitLab-Discussion-ID'] = note.discussion.id if note.part_of_discussion?
+
+    headers[:subject]&.prepend('Re: ')
+
+    mail_thread(model, headers)
+  end
+
   def reply_key
     @reply_key ||= SentNotification.reply_key
   end