PERF: Quit out of the email job quickly if disabled (#6423)

This prevents sidekiq from doing a bunch of queries when email is
disabled.

Critical emails are a special case and will be sent.

app/jobs/regular/critical_user_email.rb

@@ -6,6 +6,10 @@ module Jobs
 
     sidekiq_options queue: 'critical'
 
+    def quit_email_early?
+      false
+    end
+
     def execute(args)
       super(args)
     end

app/jobs/regular/user_email.rb

@@ -9,10 +9,20 @@ module Jobs
 
     sidekiq_options queue: 'low'
 
+    # Can be overridden by subclass, for example critical email
+    # should always consider being sent
+    def quit_email_early?
+      SiteSetting.disable_emails == 'yes'
+    end
+
     def execute(args)
       raise Discourse::InvalidParameters.new(:user_id) unless args[:user_id].present?
       raise Discourse::InvalidParameters.new(:type)    unless args[:type].present?
 
+      # This is for performance. Quit out fast without doing a bunch
+      # of extra work when emails are disabled.
+      return if quit_email_early?
+
       post = nil
       notification = nil
       type = args[:type]

spec/jobs/user_email_spec.rb

@@ -58,6 +58,26 @@ describe Jobs::UserEmail do
     end
   end
 
+  context "disable_emails setting" do
+    it "sends when no" do
+      SiteSetting.disable_emails = 'no'
+      Email::Sender.any_instance.expects(:send).once
+      Jobs::UserEmail.new.execute(type: :confirm_new_email, user_id: user.id)
+    end
+
+    it "does not send an email when yes" do
+      SiteSetting.disable_emails = 'yes'
+      Email::Sender.any_instance.expects(:send).never
+      Jobs::UserEmail.new.execute(type: :confirm_new_email, user_id: user.id)
+    end
+
+    it "sends when critical" do
+      SiteSetting.disable_emails = 'yes'
+      Email::Sender.any_instance.expects(:send)
+      Jobs::CriticalUserEmail.new.execute(type: :confirm_new_email, user_id: user.id)
+    end
+  end
+
   context "recently seen" do
     let(:post) { Fabricate(:post, user: user) }