NameError in ActiveSupport::LoggerThreadSafeLevel after concurrent-ruby update

What actually broke

The NameError is not in your code. Older Rails 6.0/7.0-era activesupport code references the Logger constant inside ActiveSupport::LoggerThreadSafeLevel but never required Ruby's logger library itself — it got loaded transitively because concurrent-ruby required it. When bundle update pulled a concurrent-ruby release that dropped that require, the constant vanished and boot (or the first log call) exploded. Confirm with:

bundle exec rails -v
bundle list | grep -E 'concurrent-ruby|logger'

1. Which Rails line is immune?

Based on available knowledge, the 6.1 series was the first to declare a direct dependency on the logger gem, so 6.1.x and later should not depend on the transitive require. Treat this as likely, not gospel — verify against your installed gemspec before relying on it:

grep -n 'logger' $(bundle show activesupport)/activesupport.gemspec

If you see add_dependency "logger", that line is safe. If you're on 6.0.x or 7.0.x and don't see it, you need a workaround below.

2. Is pinning concurrent-ruby safe?

Yes, as a temporary measure, with a caveat. Pinning to the last version that still required logger restores the old behavior immediately:

# Gemfile
gem "concurrent-ruby", "< 1.2"  # adjust boundary after checking changelogs

The risk is real but bounded: other gems (e.g., anything using newer concurrent-ruby primitives) may demand newer versions, and you'd be frozen out of its bug fixes. Run bundle update concurrent-ruby after pinning and check Bundler resolves cleanly; if it conflicts, fall back to the require workaround. Remove the pin once you upgrade Rails.

3. Does an initializer fix it?

Not reliably. Rails initializers run after frameworks are loaded, and LoggerThreadSafeLevel can be evaluated during ActiveSupport's own load — before config/initializers/*.rb ever executes. Put the require earlier, at the top of config/application.rb before require "rails/all":

require_relative "boot"
require "logger"   # compat: concurrent-ruby no longer requires it
require "rails/all"

Even earlier, in config/boot.rb, is safest if the error surfaces during Bundler setup. Verify by booting in a clean environment (RAILS_ENV=production bundle exec rails runner 'puts Logger.name') and by running your full boot, not just a console — consoles can mask load-order bugs.

Recommendation

1. Short term: add the early require "logger" (works regardless of concurrent-ruby version) or pin concurrent-ruby — not both, to keep the fix auditable.
2. Long term: upgrade to a Rails line whose gemspec declares logger, then delete the workaround.

The one detail that would change this advice: your exact Rails version and the concurrent-ruby version Bundler resolved to — share those if the early require still fails, since it would indicate something else references Logger even earlier in boot.