Allow more mail clients to group emails by thread

* send a 'In-Reply-To' header along the 'References' header
* subject of answers to an existing thread begins with 'Re: '

1 files changed, 38 insertions, 8 deletions

diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 84a0da0129d..bd438bab89a 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -1,4 +1,6 @@
 class Notify < ActionMailer::Base
+  include ActionDispatch::Routing::PolymorphicRoutes
+
   include Emails::Issues
   include Emails::MergeRequests
   include Emails::Notes
@@ -53,14 +55,6 @@ class Notify < ActionMailer::Base
     end
   end
 
-  # Set the Message-ID header field
-  #
-  # local_part - The local part of the message ID
-  #
-  def set_message_id(local_part)
-    headers["Message-ID"] = "<#{local_part}@#{Gitlab.config.gitlab.host}>"
-  end
-
   # Set the References header field
   #
   # local_part - The local part of the referenced message ID
@@ -93,4 +87,40 @@ class Notify < ActionMailer::Base
     subject << extra.join(' | ') if extra.present?
     subject
   end
+
+  # Return a string suitable for inclusion in the 'Message-Id' mail header.
+  #
+  # The message-id is generated from the unique URL to a model object.
+  def message_id(model)
+    model_name = model.class.model_name.singular_route_key
+    "<#{model_name}_#{model.id}@#{Gitlab.config.gitlab.host}>"
+  end
+
+  # Send an email that starts a new conversation thread,
+  # with headers suitable for grouping by thread in email clients.
+  #
+  # See: mail_answer_thread
+  def mail_new_thread(model, headers = {}, &block)
+    headers['Message-ID'] = message_id(model)
+    mail(headers, &block)
+  end
+
+  # Send an email that responds to an existing conversation thread,
+  # with headers suitable for grouping by thread in email clients.
+  #
+  # For grouping emails by thread, email clients heuristics require the answers to:
+  #
+  #  * have a subject that begin by 'Re: '
+  #  * have a 'In-Reply-To' or 'References' header that references the original 'Message-ID'
+  #
+  def mail_answer_thread(model, headers = {}, &block)
+    headers['In-Reply-To'] = message_id(model)
+    headers['References'] = message_id(model)
+
+    if (headers[:subject])
+      headers[:subject].prepend('Re: ')
+    end
+
+    mail(headers, &block)
+  end
 end