Move NotificationService calls to Sidekiq

The NotificationService has to do quite a lot of work to calculate the
recipients for an email. Where possible, we should try to avoid doing this in an
HTTP request, because the mail are sent by Sidekiq anyway, so there's no need to
schedule those emails immediately.

This commit creates a generic Sidekiq worker that uses Global ID to serialise
and deserialise its arguments, then forwards them to the NotificationService.
The NotificationService gains an `#async` method, so you can replace:

    notification_service.new_issue(issue, current_user)

With:

    notification_service.async.new_issue(issue, current_user)

And have everything else work as normal, except that calculating the recipients
will be done by Sidekiq, which will then schedule further Sidekiq jobs to send
each email.

1 files changed, 8 insertions, 3 deletions

diff --git a/app/services/merge_requests/update_service.rb b/app/services/merge_requests/update_service.rb
index 8a40ad88182..7350725e223 100644
--- a/app/services/merge_requests/update_service.rb
+++ b/app/services/merge_requests/update_service.rb
@@ -21,6 +21,7 @@ module MergeRequests
       update(merge_request)
     end
 
+    # rubocop:disable Metrics/AbcSize
     def handle_changes(merge_request, options)
       old_associations = options.fetch(:old_associations, {})
       old_labels = old_associations.fetch(:labels, [])
@@ -42,8 +43,11 @@ module MergeRequests
       end
 
       if merge_request.previous_changes.include?('assignee_id')
+        old_assignee_id = merge_request.previous_changes['assignee_id'].first
+        old_assignee = User.find(old_assignee_id) if old_assignee_id
+
         create_assignee_note(merge_request)
-        notification_service.reassigned_merge_request(merge_request, current_user)
+        notification_service.async.reassigned_merge_request(merge_request, current_user, old_assignee)
         todo_service.reassigned_merge_request(merge_request, current_user)
       end
 
@@ -54,7 +58,7 @@ module MergeRequests
 
       added_labels = merge_request.labels - old_labels
       if added_labels.present?
-        notification_service.relabeled_merge_request(
+        notification_service.async.relabeled_merge_request(
           merge_request,
           added_labels,
           current_user
@@ -63,13 +67,14 @@ module MergeRequests
 
       added_mentions = merge_request.mentioned_users - old_mentioned_users
       if added_mentions.present?
-        notification_service.new_mentions_in_merge_request(
+        notification_service.async.new_mentions_in_merge_request(
           merge_request,
           added_mentions,
           current_user
         )
       end
     end
+    # rubocop:enable Metrics/AbcSize
 
     def merge_from_quick_action(merge_request)
       last_diff_sha = params.delete(:merge)