Put fallback reply-key address first in the References header

The References header is an ordered list of mails, and some clients
only look at the last mail in the list to determine to which mail the
mail is in reply to.

Therefore, to be a less obtrusive as possible, put the fallback
address first.

Some info about the reply key lookup in the References header:
https://docs.gitlab.com/ee/administration/reply_by_email.html#how-it-works

app/mailers/notify.rb

@@ -124,7 +124,7 @@ class Notify < BaseMailer
 
       fallback_reply_message_id = "<reply-#{reply_key}@#{Gitlab.config.gitlab.host}>".freeze
       headers['References'] ||= []
-      headers['References'] << fallback_reply_message_id
+      headers['References'].unshift(fallback_reply_message_id)
 
       @reply_by_email = true
     end
@@ -158,7 +158,7 @@ class Notify < BaseMailer
   def mail_answer_thread(model, headers = {})
     headers['Message-ID'] = "<#{SecureRandom.hex}@#{Gitlab.config.gitlab.host}>"
     headers['In-Reply-To'] = message_id(model)
-    headers['References'] = message_id(model)
+    headers['References'] = [message_id(model)]
 
     headers[:subject]&.prepend('Re: ')
 

changelogs/unreleased/tc-reorder-mail-notify-references.yml

+---
+title: Put fallback reply-key address first in the References header
+merge_request: 20871
+author:
+type: changed

spec/fixtures/emails/commands_in_reply.eml

@@ -8,7 +8,7 @@ From: Jake the Dog <jake@adventuretime.ooo>
 To: reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
 In-Reply-To: <issue_1@localhost>
-References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
+References: <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost> <issue_1@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;

spec/fixtures/emails/commands_only_reply.eml

@@ -8,7 +8,7 @@ From: Jake the Dog <jake@adventuretime.ooo>
 To: reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
 In-Reply-To: <issue_1@localhost>
-References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
+References: <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost> <issue_1@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;

spec/fixtures/emails/reply_without_subaddressing_and_key_inside_references.eml

@@ -8,7 +8,7 @@ From: Jake the Dog <jake@adventuretime.ooo>
 To: reply@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
 In-Reply-To: <issue_1@localhost>
-References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
+References: <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost> <issue_1@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;

spec/fixtures/emails/reply_without_subaddressing_and_key_inside_references_with_a_comma.eml

@@ -8,7 +8,7 @@ From: Jake the Dog <jake@adventuretime.ooo>
 To: reply@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
 In-Reply-To: <issue_1@localhost>
-References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>,<exchange@microsoft.com>
+References: <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost> <issue_1@localhost>,<exchange@microsoft.com>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;

spec/fixtures/emails/update_commands_only_reply.eml

@@ -8,7 +8,7 @@ From: Jake the Dog <jake@adventuretime.ooo>
 To: reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
 In-Reply-To: <issue_1@localhost>
-References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
+References: <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost> <issue_1@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;

spec/fixtures/emails/valid_reply.eml

@@ -8,7 +8,7 @@ From: Jake the Dog <jake@adventuretime.ooo>
 To: reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
 In-Reply-To: <issue_1@localhost>
-References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
+References: <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost> <issue_1@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;

spec/support/shared_examples/notify_shared_examples.rb

@@ -77,7 +77,7 @@ shared_examples 'a thread answer email with reply-by-email enabled' do
     aggregate_failures do
       is_expected.to have_header('Message-ID', /\A<.*@#{host}>\Z/)
       is_expected.to have_header('In-Reply-To', "<#{route_key}@#{host}>")
-      is_expected.to have_header('References',  /\A<#{route_key}@#{host}> <reply\-.*@#{host}>\Z/ )
+      is_expected.to have_header('References', /\A<reply\-.*@#{host}> <#{route_key}@#{host}>\Z/ )
       is_expected.to have_subject(/^Re: /)
     end
   end