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