Make discussion mail References all notes in the discussion

When a note is part of a discussion, the email sent out will be
`In-Reply-To` the previous note in that discussion. It also
`References` all the previous notes in that discussion, and the
original issue.

Closes gitlab-org/gitlab-ce#36054.

app/mailers/notify.rb

@@ -119,8 +119,8 @@ class Notify < BaseMailer
       headers['Reply-To'] = address
 
       fallback_reply_message_id = "<reply-#{reply_key}@#{Gitlab.config.gitlab.host}>".freeze
-      headers['References'] ||= ''
-      headers['References'] << ' ' << fallback_reply_message_id
+      headers['References'] ||= []
+      headers['References'] << fallback_reply_message_id
 
       @reply_by_email = true
     end
@@ -158,8 +158,8 @@ class Notify < BaseMailer
 
   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['In-Reply-To'] = message_id(note.references.last)
+    headers['References'] = note.references.map { |ref| message_id(ref) }
 
     headers['X-GitLab-Discussion-ID'] = note.discussion.id if note.part_of_discussion?
 

app/models/note.rb

@@ -360,13 +360,14 @@ class Note < ActiveRecord::Base
     end
   end
 
-  def replies_to
+  def references
+    refs = [noteable]
+
     if part_of_discussion?
-      previous_note = discussion.notes.take_while { |n| n.id < id }.last
-      return previous_note if previous_note
+      refs += discussion.notes.take_while { |n| n.id < id }
     end
 
-    noteable
+    refs
   end
 
   def expire_etag_cache

spec/mailers/notify_spec.rb

@@ -342,6 +342,46 @@ describe Notify do
       end
     end
 
+    context 'for issue notes' do
+      let(:host) { Gitlab.config.gitlab.host }
+
+      context 'in discussion' do
+        set(:first_note) { create(:discussion_note_on_issue) }
+        set(:second_note) { create(:discussion_note_on_issue, in_reply_to: first_note) }
+        set(:third_note) { create(:discussion_note_on_issue, in_reply_to: second_note) }
+
+        subject { described_class.note_issue_email(recipient.id, third_note.id) }
+
+        it 'has In-Reply-To header pointing to previous note in discussion' do
+          expect(subject.header['In-Reply-To'].message_ids).to eq(["note_#{second_note.id}@#{host}"])
+        end
+
+        it 'has References header including the notes and issue of the discussion' do
+          expect(subject.header['References'].message_ids).to include("issue_#{first_note.noteable.id}@#{host}",
+                                                                   "note_#{first_note.id}@#{host}",
+                                                                   "note_#{second_note.id}@#{host}")
+        end
+
+        it 'has X-GitLab-Discussion-ID header' do
+          expect(subject.header['X-GitLab-Discussion-ID'].value).to eq(third_note.discussion.id)
+        end
+      end
+
+      context 'individual issue comments' do
+        set(:note) { create(:note_on_issue) }
+
+        subject { described_class.note_issue_email(recipient.id, note.id) }
+
+        it 'has In-Reply-To header pointing to the issue' do
+          expect(subject.header['In-Reply-To'].message_ids).to eq(["issue_#{note.noteable.id}@#{host}"])
+        end
+
+        it 'has References header including the notes and issue of the discussion' do
+          expect(subject.header['References'].message_ids).to include("issue_#{note.noteable.id}@#{host}")
+        end
+      end
+    end
+
     context 'for snippet notes' do
       let(:project_snippet) { create(:project_snippet, project: project) }
       let(:project_snippet_note) { create(:note_on_project_snippet, project: project, noteable: project_snippet) }

spec/models/note_spec.rb

@@ -756,18 +756,15 @@ describe Note do
     end
   end
 
-  describe '#replies_to' do
+  describe '#references' do
     context 'when part of a discussion' do
-      let(:note) { create(:discussion_note_on_issue) }
+      it 'references all earlier notes in the discussion' do
+        first_note = create(:discussion_note_on_issue)
+        second_note = create(:discussion_note_on_issue, in_reply_to: first_note)
+        third_note = create(:discussion_note_on_issue, in_reply_to: second_note)
+        create(:discussion_note_on_issue, in_reply_to: third_note)
 
-      it 'returns noteable when there are not earlier notes in the discussion' do
-        expect(note.replies_to).to eq(note.noteable)
-      end
-
-      it 'returns previous note in discussion' do
-        reply = create(:discussion_note_on_issue, in_reply_to: note)
-
-        expect(reply.replies_to).to eq(note)
+        expect(third_note.references).to eq([first_note.noteable, first_note, second_note])
       end
     end
 
@@ -776,7 +773,7 @@ describe Note do
       let(:note) { create(:note, in_reply_to: subject) }
 
       it 'returns the noteable' do
-        expect(note.replies_to).to eq(note.noteable)
+        expect(note.references).to eq([note.noteable])
       end
     end
   end