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

Added email threading for update emails on issues and merge requests (if the mail client support the References: mail header)

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