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: '

3 files changed, 44 insertions, 43 deletions

diff --git a/app/mailers/emails/issues.rb b/app/mailers/emails/issues.rb
index a096df9dc0d..e5346235963 100644
--- a/app/mailers/emails/issues.rb
+++ b/app/mailers/emails/issues.rb
@@ -4,10 +4,10 @@ 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})"))
+      mail_new_thread(@issue,
+                      from: sender(@issue.author_id),
+                      to: recipient(recipient_id),
+                      subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def reassigned_issue_email(recipient_id, issue_id, previous_assignee_id, updated_by_user_id)
@@ -15,10 +15,10 @@ 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})"))
+      mail_answer_thread(@issue,
+                         from: sender(updated_by_user_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def closed_issue_email(recipient_id, issue_id, updated_by_user_id)
@@ -26,10 +26,10 @@ 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})"))
+      mail_answer_thread(@issue,
+                         from: sender(updated_by_user_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def issue_status_changed_email(recipient_id, issue_id, status, updated_by_user_id)
@@ -38,10 +38,10 @@ 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})"))
+      mail_answer_thread(@issue,
+                         from: sender(updated_by_user_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
   end
 end
diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index ea5671c4502..dfc3c12299a 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -4,10 +4,10 @@ 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})"))
+      mail_new_thread(@merge_request,
+                      from: sender(@merge_request.author_id),
+                      to: recipient(recipient_id),
+                      subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_id, updated_by_user_id)
@@ -15,10 +15,10 @@ 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})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def closed_merge_request_email(recipient_id, merge_request_id, updated_by_user_id)
@@ -26,20 +26,20 @@ 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})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def merged_merge_request_email(recipient_id, merge_request_id, updated_by_user_id)
       @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})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
   end
 
diff --git a/app/mailers/emails/notes.rb b/app/mailers/emails/notes.rb
index b7fedbd3707..ef9af726a6c 100644
--- a/app/mailers/emails/notes.rb
+++ b/app/mailers/emails/notes.rb
@@ -5,9 +5,10 @@ module Emails
       @commit = @note.noteable
       @project = @note.project
       @target_url = project_commit_url(@project, @commit, anchor: "note_#{@note.id}")
-      mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@commit.title} (#{@commit.short_id})"))
+      mail_answer_thread(@commit,
+                         from: sender(@note.author_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@commit.title} (#{@commit.short_id})"))
     end
 
     def note_issue_email(recipient_id, note_id)
@@ -15,10 +16,10 @@ module Emails
       @issue = @note.noteable
       @project = @note.project
       @target_url = project_issue_url(@project, @issue, anchor: "note_#{@note.id}")
-      set_reference("issue_#{@issue.id}")
-      mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_answer_thread(@issue,
+                         from: sender(@note.author_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def note_merge_request_email(recipient_id, note_id)
@@ -26,10 +27,10 @@ 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.id}")
-      mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(@note.author_id),
+                         to: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
   end
 end