Original implementation to allow users to subscribe to labels

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

4 files changed, 57 insertions, 3 deletions

diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index ca87dca4a70..971c074882e 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -95,7 +95,7 @@ class IssuableBaseService < BaseService
 
     old_labels = options[:old_labels]
     if old_labels && (issuable.labels != old_labels)
-      create_labels_note(issuable, issuable.labels - old_labels, old_labels - issuable.labels)
+      create_labels_note(issuable, issuable.added_labels(old_labels), old_labels - issuable.labels)
     end
   end
 end
diff --git a/app/services/issues/update_service.rb b/app/services/issues/update_service.rb
index 51ef9dfe610..b2e63a4e1af 100644
--- a/app/services/issues/update_service.rb
+++ b/app/services/issues/update_service.rb
@@ -4,7 +4,7 @@ module Issues
       update(issue)
     end
 
-    def handle_changes(issue, options = {})
+    def handle_changes(issue, old_labels: [], new_labels: [])
       if has_changes?(issue, options)
         todo_service.mark_pending_todos_as_done(issue, current_user)
       end
@@ -23,6 +23,11 @@ module Issues
         notification_service.reassigned_issue(issue, current_user)
         todo_service.reassigned_issue(issue, current_user)
       end
+
+      new_labels = issue.added_labels(old_labels)
+      if new_labels.present?
+        notification_service.relabeled_issue(issue, new_labels, current_user)
+      end
     end
 
     def reopen_service
diff --git a/app/services/merge_requests/update_service.rb b/app/services/merge_requests/update_service.rb
index 6319ad805b6..6fd569dc302 100644
--- a/app/services/merge_requests/update_service.rb
+++ b/app/services/merge_requests/update_service.rb
@@ -14,7 +14,7 @@ module MergeRequests
       update(merge_request)
     end
 
-    def handle_changes(merge_request, options = {})
+    def handle_changes(issue, old_labels: [], new_labels: [])
       if has_changes?(merge_request, options)
         todo_service.mark_pending_todos_as_done(merge_request, current_user)
       end
@@ -44,6 +44,11 @@ module MergeRequests
           merge_request.previous_changes.include?('source_branch')
         merge_request.mark_as_unchecked
       end
+
+      new_labels = merge_request.added_labels(old_labels)
+      if new_labels.present?
+        notification_service.relabeled_merge_request(merge_request, new_labels, current_user)
+      end
     end
 
     def reopen_service
diff --git a/app/services/notification_service.rb b/app/services/notification_service.rb
index ca8a41d93b8..e9955cd3e2d 100644
--- a/app/services/notification_service.rb
+++ b/app/services/notification_service.rb
@@ -52,6 +52,14 @@ class NotificationService
     reassign_resource_email(issue, issue.project, current_user, 'reassigned_issue_email')
   end
 
+  # When we change labels on an issue we should send emails.
+  #
+  # We pass in the labels, here, because we only want the labels that
+  # have been *added* during this relabel, not all of them.
+  def relabeled_issue(issue, labels, current_user)
+    relabel_resource_email(issue, issue.project, labels, current_user, 'relabeled_issue_email')
+  end
+
 
   # When create a merge request we should send next emails:
   #
@@ -70,6 +78,14 @@ class NotificationService
     reassign_resource_email(merge_request, merge_request.target_project, current_user, 'reassigned_merge_request_email')
   end
 
+  # When we change labels on a merge request we should send emails.
+  #
+  # We pass in the labels, here, because we only want the labels that
+  # have been *added* during this relabel, not all of them.
+  def relabeled_merge_request(merge_request, labels, current_user)
+    relabel_resource_email(merge_request, merge_request.project, labels, current_user, 'relabeled_merge_request_email')
+  end
+
   def close_mr(merge_request, current_user)
     close_resource_email(merge_request, merge_request.target_project, current_user, 'closed_merge_request_email')
   end
@@ -142,6 +158,7 @@ class NotificationService
     recipients = reject_muted_users(recipients, note.project)
 
     recipients = add_subscribed_users(recipients, note.noteable)
+    recipients = add_label_subscriptions(recipients, note.noteable)
     recipients = reject_unsubscribed_users(recipients, note.noteable)
 
     recipients.delete(note.author)
@@ -359,6 +376,16 @@ class NotificationService
     end
   end
 
+  def add_label_subscriptions(recipients, target)
+    return recipients unless target.respond_to? :labels
+
+    target.labels.each do |label|
+      recipients += label.subscriptions.where(subscribed: true).map(&:user)
+    end
+
+    recipients
+  end
+
   def new_resource_email(target, project, method)
     recipients = build_recipients(target, project, target.author)
 
@@ -392,6 +419,15 @@ class NotificationService
     end
   end
 
+  def relabel_resource_email(target, project, labels, current_user, method)
+    recipients = build_relabel_recipients(target, project, labels, current_user)
+    label_names = labels.map(&:name)
+
+    recipients.each do |recipient|
+      mailer.send(method, recipient.id, target.id, current_user.id, label_names).deliver_later
+    end
+  end
+
   def reopen_resource_email(target, project, current_user, method, status)
     recipients = build_recipients(target, project, current_user)
 
@@ -416,6 +452,7 @@ class NotificationService
 
     recipients = reject_muted_users(recipients, project)
     recipients = add_subscribed_users(recipients, target)
+    recipients = add_label_subscriptions(recipients, target)
     recipients = reject_unsubscribed_users(recipients, target)
 
     recipients.delete(current_user)
@@ -423,6 +460,13 @@ class NotificationService
     recipients.uniq
   end
 
+  def build_relabel_recipients(target, project, labels, current_user)
+    recipients = add_label_subscriptions([], target)
+    recipients = reject_unsubscribed_users(recipients, target)
+    recipients.delete(current_user)
+    recipients.uniq
+  end
+
   def mailer
     Notify
   end