Send notification emails when users are newly mentioned in issue edits

app/mailers/emails/issues.rb

@@ -6,6 +6,11 @@ module Emails
       mail_new_thread(@issue, issue_thread_options(@issue.author_id, recipient_id))
     end
 
+    def new_mention_in_issue_email(recipient_id, issue_id, updated_by_user_id)
+      setup_issue_mail(issue_id, recipient_id)
+      mail_answer_thread(@issue, issue_thread_options(updated_by_user_id, recipient_id))
+    end
+
     def reassigned_issue_email(recipient_id, issue_id, previous_assignee_id, updated_by_user_id)
       setup_issue_mail(issue_id, recipient_id)
 

app/services/issuable_base_service.rb

@@ -104,11 +104,12 @@ class IssuableBaseService < BaseService
     change_subscription(issuable)
     filter_params
     old_labels = issuable.labels.to_a
+    old_mentioned_users = issuable.mentioned_users.to_a
 
     if params.present? && update_issuable(issuable, params)
       issuable.reset_events_cache
       handle_common_system_notes(issuable, old_labels: old_labels)
-      handle_changes(issuable, old_labels: old_labels)
+      handle_changes(issuable, old_labels: old_labels, old_mentioned_users: old_mentioned_users)
       issuable.create_new_cross_references!(current_user)
       execute_hooks(issuable, 'update')
     end

app/services/issues/update_service.rb

@@ -4,7 +4,7 @@ module Issues
       update(issue)
     end
 
-    def handle_changes(issue, old_labels: [])
+    def handle_changes(issue, old_labels: [], old_mentioned_users: [])
       if has_changes?(issue, old_labels: old_labels)
         todo_service.mark_pending_todos_as_done(issue, current_user)
       end
@@ -32,6 +32,11 @@ module Issues
       if added_labels.present?
         notification_service.relabeled_issue(issue, added_labels, current_user)
       end
+
+      added_mentions = issue.mentioned_users - old_mentioned_users
+      if added_mentions.present?
+        notification_service.new_mentions_in_issue(issue, added_mentions, current_user)
+      end
     end
 
     def reopen_service

app/services/notification_service.rb

@@ -35,6 +35,20 @@ class NotificationService
     new_resource_email(issue, issue.project, :new_issue_email)
   end
 
+  # When issue text is updated, we should send an email to:
+  #
+  #  * newly mentioned project team members with notification level higher than Participating
+  #
+  def new_mentions_in_issue(issue, new_mentioned_users, current_user)
+    new_mentions_in_resource_email(
+      issue,
+      issue.project,
+      new_mentioned_users,
+      current_user,
+      :new_mention_in_issue_email
+    )
+  end
+
   # When we close an issue we should send an email to:
   #
   #  * issue author if their notification level is not Disabled
@@ -471,6 +485,14 @@ class NotificationService
     end
   end
 
+  def new_mentions_in_resource_email(target, project, new_mentioned_users, current_user, method)
+    recipients = build_recipients(target, project, current_user) & new_mentioned_users
+
+    recipients.each do |recipient|
+      mailer.send(method, recipient.id, target.id, current_user.id).deliver_later
+    end
+  end
+
   def close_resource_email(target, project, current_user, method)
     action = method == :merged_merge_request_email ? "merge" : "close"
     recipients = build_recipients(target, project, current_user, action: action)

app/views/notify/new_mention_in_issue_email.html.haml

+- if current_application_settings.email_author_in_body
+  %div
+    #{link_to @issue.author_name, user_url(@issue.author)} wrote:
+-if @issue.description
+  = markdown(@issue.description, pipeline: :email, author: @issue.author)
+
+- if @issue.assignee_id.present?
+  %p
+    Assignee: #{@issue.assignee_name}

app/views/notify/new_mention_in_issue_email.text.erb

+You have been mentioned in an issue.
+
+Issue <%= @issue.iid %>: <%= url_for(namespace_project_issue_url(@issue.project.namespace, @issue.project, @issue)) %>
+Author:    <%= @issue.author_name %>
+Assignee:  <%= @issue.assignee_name %>
+
+<%= @issue.description %>

spec/services/issues/update_service_spec.rb

@@ -319,5 +319,38 @@ describe Issues::UpdateService, services: true do
         end
       end
     end
+
+    context 'updated user mentions' do
+      let(:user4) { create(:user) }
+      before do
+        project.team << [user4, :developer]
+      end
+
+      context "in title" do
+        before do
+          perform_enqueued_jobs { update_issue(title: user4.to_reference) }
+        end
+
+        it "should email only the newly-mentioned user" do
+          should_not_email(user)
+          should_not_email(user2)
+          should_not_email(user3)
+          should_email(user4)
+        end
+      end
+
+      context "in description" do
+        before do
+          perform_enqueued_jobs { update_issue(description: user4.to_reference) }
+        end
+
+        it "should email only the newly-mentioned user" do
+          should_not_email(user)
+          should_not_email(user2)
+          should_not_email(user3)
+          should_email(user4)
+        end
+      end
+    end
   end
 end

spec/services/notification_service_spec.rb

@@ -399,6 +399,32 @@ describe NotificationService, services: true do
       end
     end
 
+    describe '#new_mentions_in_issue' do
+      def send_notifications(*new_mentions)
+        ActionMailer::Base.deliveries.clear
+        notification.new_mentions_in_issue(issue, new_mentions, @u_disabled)
+      end
+
+      it "should not email anyone unless they are newly mentioned" do
+        send_notifications()
+        expect(ActionMailer::Base.deliveries).to eq []
+      end
+
+      it "should email new mentions with a watch level higher than participant" do
+        send_notifications(@u_watcher, @u_participant_mentioned)
+
+        should_email(@u_watcher)
+        should_email(@u_participant_mentioned)
+
+        expect(ActionMailer::Base.deliveries.count).to eq 2
+      end
+
+      it "should not email new mentions with a watch level equal to or less than participant" do
+        send_notifications(@u_participating, @u_mentioned)
+        expect(ActionMailer::Base.deliveries).to eq []
+      end
+    end
+
     describe '#reassigned_issue' do
       before do
         update_custom_notification(:reassign_issue, @u_guest_custom, project)