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

2 files changed, 17 insertions, 5 deletions

diff --git a/app/mailers/emails/notes.rb b/app/mailers/emails/notes.rb
index 77a82b895ce..50e17fe7717 100644
--- a/app/mailers/emails/notes.rb
+++ b/app/mailers/emails/notes.rb
@@ -5,7 +5,7 @@ module Emails
 
       @commit = @note.noteable
       @target_url = project_commit_url(*note_target_url_options)
-      mail_answer_thread(@commit, note_thread_options(recipient_id))
+      mail_answer_note_thread(@commit, @note, note_thread_options(recipient_id))
     end
 
     def note_issue_email(recipient_id, note_id)
@@ -13,7 +13,7 @@ module Emails
 
       @issue = @note.noteable
       @target_url = project_issue_url(*note_target_url_options)
-      mail_answer_thread(@issue, note_thread_options(recipient_id))
+      mail_answer_note_thread(@issue, @note, note_thread_options(recipient_id))
     end
 
     def note_merge_request_email(recipient_id, note_id)
@@ -21,7 +21,7 @@ module Emails
 
       @merge_request = @note.noteable
       @target_url = project_merge_request_url(*note_target_url_options)
-      mail_answer_thread(@merge_request, note_thread_options(recipient_id))
+      mail_answer_note_thread(@merge_request, @note, note_thread_options(recipient_id))
     end
 
     def note_snippet_email(recipient_id, note_id)
@@ -29,7 +29,7 @@ module Emails
 
       @snippet = @note.noteable
       @target_url = project_snippet_url(*note_target_url_options)
-      mail_answer_thread(@snippet, note_thread_options(recipient_id))
+      mail_answer_note_thread(@snippet, @note, note_thread_options(recipient_id))
     end
 
     def note_personal_snippet_email(recipient_id, note_id)
@@ -37,7 +37,7 @@ module Emails
 
       @snippet = @note.noteable
       @target_url = snippet_url(@note.noteable)
-      mail_answer_thread(@snippet, note_thread_options(recipient_id))
+      mail_answer_note_thread(@snippet, @note, note_thread_options(recipient_id))
     end
 
     private
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