Mailroom fails to start in GitLab 16.6

Summary

!136002 (merged) added support for encrypted secrets for Redis. This relied on GitLab rails environment being available to populate few settings. However, Mailroom also uses Redis, but without the rails environment being present. This causes mailroom to fail starting with 16.6 with errors like the following

==> /var/log/gitlab/mailroom/current <==
2023-11-17_15:58:51.82444 /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/redis/wrapper.rb:113:in `secret_file': uninitialized constant Gitlab::Redis::Wrapper::Settings (NameError)
2023-11-17_15:58:51.82455 Did you mean?  String
2023-11-17_15:58:51.82458 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/redis/wrapper.rb:135:in `encrypted_secrets'
2023-11-17_15:58:51.82461 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/redis/wrapper.rb:159:in `parse_encrypted_config'
2023-11-17_15:58:51.82462 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/redis/wrapper.rb:144:in `redis_store_options'
2023-11-17_15:58:51.82464 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/redis/wrapper.rb:104:in `db'
2023-11-17_15:58:51.82465 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/mail_room.rb:114:in `redis_config'
2023-11-17_15:58:51.82466 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/mail_room.rb:90:in `fetch_config'
2023-11-17_15:58:51.82467 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/mail_room.rb:82:in `block in configs'
2023-11-17_15:58:51.82469 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/mail_room.rb:82:in `to_h'
2023-11-17_15:58:51.82470 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/mail_room.rb:82:in `configs'
2023-11-17_15:58:51.82472 	from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/mail_room.rb:64:in `enabled_configs'
2023-11-17_15:58:51.82473 	from /opt/gitlab/embedded/service/gitlab-rails/config/mail_room.yml:4:in `<main>'
2023-11-17_15:58:51.82474 	from /opt/gitlab/embedded/lib/ruby/3.0.0/erb.rb:905:in `eval'
2023-11-17_15:58:51.82475 	from /opt/gitlab/embedded/lib/ruby/3.0.0/erb.rb:905:in `result'
2023-11-17_15:58:51.82477 	from /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/gitlab-mail_room-0.0.23/lib/mail_room/configuration.rb:18:in `initialize'
2023-11-17_15:58:51.82478 	from /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/gitlab-mail_room-0.0.23/lib/mail_room/cli.rb:44:in `new'
2023-11-17_15:58:51.82479 	from /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/gitlab-mail_room-0.0.23/lib/mail_room/cli.rb:44:in `initialize'
2023-11-17_15:58:51.82481 	from /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/gitlab-mail_room-0.0.23/bin/mail_room:5:in `new'
2023-11-17_15:58:51.82482 	from /opt/gitlab/embedded/lib/ruby/gems/3.0.0/gems/gitlab-mail_room-0.0.23/bin/mail_room:5:in `<top (required)>'
2023-11-17_15:58:51.82483 	from /opt/gitlab/embedded/bin/mail_room:25:in `load'
2023-11-17_15:58:51.82484 	from /opt/gitlab/embedded/bin/mail_room:25:in `<main>'

Steps to reproduce

Deploy a GitLab 16.6 instance with incoming email enabled.

Example Project

What is the current bug behavior?

Mailroom fails to start

What is the expected correct behavior?

Mailroom should work fine

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes