Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature2364-fallback-to-in-reply-to-header

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

1 files changed, 10 insertions, 6 deletions

diff --git a/lib/gitlab/incoming_email.rb b/lib/gitlab/incoming_email.rb
index 9068d79c95e..8ce9d32abe0 100644
--- a/lib/gitlab/incoming_email.rb
+++ b/lib/gitlab/incoming_email.rb
@@ -1,13 +1,10 @@
 module Gitlab
   module IncomingEmail
     class << self
-      def enabled?
-        config.enabled && address_formatted_correctly?
-      end
+      FALLBACK_REPLY_MESSAGE_ID_REGEX = /\Areply\-(.+)@#{Gitlab.config.gitlab.host}\Z/.freeze
 
-      def address_formatted_correctly?
-        config.address &&
-          config.address.include?("%{key}")
+      def enabled?
+        config.enabled && config.address
       end
 
       def reply_address(key)
@@ -24,6 +21,13 @@ module Gitlab
         match[1]
       end
 
+      def key_from_fallback_reply_message_id(message_id)
+        match = message_id.match(FALLBACK_REPLY_MESSAGE_ID_REGEX)
+        return unless match
+
+        match[1]
+      end
+
       def config
         Gitlab.config.incoming_email
       end