Merge branch '7003-fail-to-start-server-without-ar-connection' into 'master'

Prevent web-server to start without a working db connection"

Closes #7003

See merge request gitlab-org/gitlab!25160

changelogs/unreleased/7003-fail-to-start-server-without-ar-connection.yml

+---
+title: Refuse to start web server without a working ActiveRecord connection
+merge_request: 25160
+author:
+type: other

config/initializers/active_record_lifecycle.rb

@@ -14,6 +14,8 @@ end
 
 if defined?(ActiveRecord::Base)
   Gitlab::Cluster::LifecycleEvents.on_before_fork do
+    raise 'ActiveRecord connection not established. Unable to start.' unless Gitlab::Database.exists?
+
     # the following is highly recommended for Rails + "preload_app true"
     # as there's no need for the master process to hold a connection
     ActiveRecord::Base.connection.disconnect!