Add rudimentary protection against double-send Welcome

Adds a check for prior welcome notifications before delivering the
welcome email. Checking for the presence of a notification record at
may prevent any welcome email being sent if multiple notification
records are created prior to processing the notification event job for
the first.

Author: rossta

app/models/notification_event.rb

@@ -29,16 +29,22 @@ def self.inherited(notifier)
     notifier.const_set :Notification, Class.new(Notification)
   end
 
+  def deliver_to?(recipient)
+    true
+  end
+
   # CommentNotifier.deliver(User.all)
   # CommentNotifier.deliver(User.all, priority: 10)
   # CommentNotifier.deliver(User.all, queue: :low_priority)
   # CommentNotifier.deliver(User.all, wait: 5.minutes)
   # CommentNotifier.deliver(User.all, wait_until: 1.hour.from_now)
-  def deliver(recipients = nil, enqueue_job: true, **options)
+  def deliver(given_recipients = nil, enqueue_job: true, **options)
     validate!
 
+    recipients = Array.wrap(given_recipients)
+
     transaction do
-      recipients_attributes = Array.wrap(recipients).map do |recipient|
+      recipients_attributes = recipients.map do |recipient|
         recipient_attributes_for(recipient)
       end
 

app/notifiers/welcome_notifier.rb

@@ -6,6 +6,16 @@ def self.deliver_to(user, **)
   def deliver_notification(notification)
     user = notification.recipient
 
+    if !deliver_to?(user)
+      Rails.logger.info "#[#{self.class}] Skipping delivery for: #{user.class.name}##{user.id}"
+      Honeybadger.event("empty_notification", notifier: self.class.name, recipient: "#{user.class.name}##{user.id}")
+      return false
+    end
+
     Emails::UserMailer.welcome(user).deliver_later
   end
+
+  def deliver_to?(recipient)
+    Notification.joins(:notification_event).where(recipient: recipient, notification_event: {type: self.class.name}).count < 2
+  end
 end

spec/notifiers/welcome_notifier_spec.rb

@@ -0,0 +1,18 @@
+require "rails_helper"
+
+RSpec.describe WelcomeNotifier do
+  describe ".deliver_to" do
+    it "sends at most once per user" do
+      allow(Emails::UserMailer).to receive(:welcome).and_return(double(deliver_later: nil))
+
+      user = FactoryBot.create(:user, :confirmed)
+
+      WelcomeNotifier.deliver_to(user)
+      perform_enqueued_jobs
+      WelcomeNotifier.deliver_to(user)
+      perform_enqueued_jobs
+
+      expect(Emails::UserMailer).to have_received(:welcome).with(user).once
+    end
+  end
+end