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

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

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

app/models/note.rb

@@ -344,6 +344,15 @@ class Note < ActiveRecord::Base
     end
   end
+  def replies_to
+    if part_of_discussion?
+      previous_note = to_discussion.notes.where('id < ?', id).last
+      return previous_note if previous_note
+    end
+
+    noteable
+  end
+
   def expire_etag_cache
     return unless noteable&.discussions_rendered_on_frontend?

changelogs/unreleased/tc-correct-email-in-reply-to.yml

+---
+title: Make mail notifications of discussion notes In-Reply-To of each other
+merge_request: 14289
+author:
+type: changed

spec/models/note_spec.rb

@@ -725,6 +725,18 @@ describe Note do
     end
   end
+  describe '#replies_to' do
+    context 'when part of a discussion' do
+      subject { create(:discussion_note_on_issue) }
+      let(:note) { create(:discussion_note_on_issue, in_reply_to: subject) }
+    end
+
+    context 'when not part of a discussion' do
+      subject { create(:note) }
+      let(:note) { create(:note, in_reply_to: subject) }
+    end
+  end
+
   describe 'expiring ETag cache' do
     let(:note) { build(:note_on_issue) }