Ensure "new SSH key" email do not ends up as dead Sidekiq jobs

Related to #2235.

This is done by:
1. Delaying the notification sending after the SSH key is commited in DB
2. Gracefully exit the mailer method if the record cannot be found

CHANGELOG

@@ -25,6 +25,7 @@ v 8.6.0 (unreleased)
   - Allow to pass name of created artifacts archive in `.gitlab-ci.yml`
   - Refactor and greatly improve search performance
   - Add support for cross-project label references
+  - Ensure "new SSH key" email do not ends up as dead Sidekiq jobs
   - Update documentation to reflect Guest role not being enforced on internal projects
   - Allow search for logged out users
   - Allow to define on which builds the current one depends on

app/mailers/emails/profile.rb

@@ -14,7 +14,10 @@ module Emails
     end
 
     def new_ssh_key_email(key_id)
-      @key = Key.find(key_id)
+      @key = Key.find_by_id(key_id)
+
+      return unless @key
+
       @current_user = @user = @key.user
       @target_url = user_url(@user)
       mail(to: @user.notification_email, subject: subject("SSH key was added to your account"))

app/models/key.rb

@@ -16,6 +16,7 @@
 require 'digest/md5'
 
 class Key < ActiveRecord::Base
+  include AfterCommitQueue
   include Sortable
 
   belongs_to :user
@@ -62,7 +63,7 @@ class Key < ActiveRecord::Base
   end
 
   def notify_user
-    NotificationService.new.new_key(self)
+    run_after_commit { NotificationService.new.new_key(self) }
   end
 
   def post_create_hook

spec/mailers/emails/profile_spec.rb

@@ -77,6 +77,10 @@ describe Notify do
       it 'includes a link to ssh keys page' do
         is_expected.to have_body_text /#{profile_keys_path}/
       end
+
+      context 'with SSH key that does not exist' do
+        it { expect { Notify.new_ssh_key_email('foo') }.not_to raise_error }
+      end
     end
 
     describe 'user added email' do