Added email threading for update emails on issues and merge requests (if the…

app/mailers/emails/issues.rb

@@ -4,6 +4,7 @@ module Emails
       @issue = Issue.find(issue_id)
       @project = @issue.project
       @target_url = project_issue_url(@project, @issue)
+      set_message_id("issue_#{issue_id}")
       mail(from: sender(@issue.author_id),
            to: recipient(recipient_id),
            subject: subject("#{@issue.title} (##{@issue.iid})"))
@@ -14,6 +15,7 @@ module Emails
       @previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
       @project = @issue.project
       @target_url = project_issue_url(@project, @issue)
+      set_reference("issue_#{issue_id}")
       mail(from: sender(updated_by_user_id),
            to: recipient(recipient_id),
            subject: subject("#{@issue.title} (##{@issue.iid})"))
@@ -24,6 +26,7 @@ module Emails
       @project = @issue.project
       @updated_by = User.find updated_by_user_id
       @target_url = project_issue_url(@project, @issue)
+      set_reference("issue_#{issue_id}")
       mail(from: sender(updated_by_user_id),
            to: recipient(recipient_id),
            subject: subject("#{@issue.title} (##{@issue.iid})"))
@@ -35,6 +38,7 @@ module Emails
       @project = @issue.project
       @updated_by = User.find updated_by_user_id
       @target_url = project_issue_url(@project, @issue)
+      set_reference("issue_#{issue_id}")
       mail(from: sender(updated_by_user_id),
            to: recipient(recipient_id),
            subject: subject("#{@issue.title} (##{@issue.iid})"))

app/mailers/emails/merge_requests.rb

@@ -4,6 +4,7 @@ module Emails
       @merge_request = MergeRequest.find(merge_request_id)
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
+      set_message_id("merge_request_#{merge_request_id}")
       mail(from: sender(@merge_request.author_id),
            to: recipient(recipient_id),
            subject: subject("#{@merge_request.title} (!#{@merge_request.iid})"))
@@ -14,6 +15,7 @@ module Emails
       @previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
+      set_reference("merge_request_#{merge_request_id}")
       mail(from: sender(updated_by_user_id),
            to: recipient(recipient_id),
            subject: subject("#{@merge_request.title} (!#{@merge_request.iid})"))
@@ -24,6 +26,7 @@ module Emails
       @updated_by = User.find updated_by_user_id
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
+      set_reference("merge_request_#{merge_request_id}")
       mail(from: sender(updated_by_user_id),
            to: recipient(recipient_id),
            subject: subject("#{@merge_request.title} (!#{@merge_request.iid})"))
@@ -33,6 +36,7 @@ module Emails
       @merge_request = MergeRequest.find(merge_request_id)
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
+      set_reference("merge_request_#{merge_request_id}")
       mail(from: sender(updated_by_user_id),
            to: recipient(recipient_id),
            subject: subject("#{@merge_request.title} (!#{@merge_request.iid})"))

app/mailers/emails/notes.rb

@@ -15,6 +15,7 @@ module Emails
       @issue = @note.noteable
       @project = @note.project
       @target_url = project_issue_url(@project, @issue, anchor: "note_#{@note.id}")
+      set_reference("issue_#{@issue.iid}")
       mail(from: sender(@note.author_id),
            to: recipient(recipient_id),
            subject: subject("#{@issue.title} (##{@issue.iid})"))
@@ -25,6 +26,7 @@ module Emails
       @merge_request = @note.noteable
       @project = @note.project
       @target_url = project_merge_request_url(@project, @merge_request, anchor: "note_#{@note.id}")
+      set_reference("merge_request_#{@merge_request.iid}")
       mail(from: sender(@note.author_id),
            to: recipient(recipient_id),
            subject: subject("#{@merge_request.title} (!#{@merge_request.iid})"))

app/mailers/notify.rb

@@ -53,6 +53,22 @@ 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
+  #
+  def set_reference(local_part)
+    headers["References"] = "<#{local_part}@#{Gitlab.config.gitlab.host}>"
+  end
+
   # Formats arguments into a String suitable for use as an email subject
   #
   # extra - Extra Strings to be inserted into the subject