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: ’

This fixes threading with at least Mail.app and Airmail.

1 files changed, 44 insertions, 8 deletions

diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 8eea8cf25e0..d65bead9cfe 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -67,14 +67,6 @@ 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
@@ -107,4 +99,48 @@ 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)
+    raise ArgumentError, '"To:" header will be overwritten; use "Cc:" or "Bcc:"' unless headers[:to].nil?
+    headers[:to] = project_sender_address.format
+
+    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'
+  #  * have stable 'From' and 'To' headers between messages of the same thread
+  #
+  def mail_answer_thread(model, headers = {}, &block)
+    raise ArgumentError, '"To:" header will be overwritten; use "Cc:" or "Bcc:"' unless headers[:to].nil?
+    headers[:to] = project_sender_address.format
+
+    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