What I tried to do

I observed that I18n.locale is unexpectedly reset when entering a new Fiber context, because I18n stores its config using Thread.current[:i18n_config].
In Ruby, Thread.current[:key] is fiber-local, not strictly thread-local, so each Fiber gets its own isolated config.

This behavior may affect Fiber-based environments, including:

• CSV library v3.2.6+ (uses Fibers internally for enumeration)
• Falcon web server (uses Fibers for request handling)
• Code that relies on Enumerator (which internally creates Fibers)

What I expected to happen

I18n.locale should be consistent across Fibers in the same Thread.

[before] I18n.locale=en
[fiber] I18n.locale=en
[after] I18n.locale=en

What actually happened

When a new Fiber is created, I18n.locale falls back to I18n.default_locale because the config is missing in the new Fiber context.

[before] I18n.locale=en
[fiber] I18n.locale=ja
[after] I18n.locale=en

Versions of i18n, rails, and anything else you think is necessary

Ruby version: 3.4.6
I18n version: 1.14.7

Steps to reproduce

require "i18n"

I18n.available_locales = [:en, :ja]
I18n.default_locale = :ja
I18n.locale = :en

puts "[before] I18n.locale=#{I18n.locale}"  # => en

Fiber.new do
  puts "[fiber] I18n.locale=#{I18n.locale}"  # => ja (unexpected!)
end.resume

puts "[after] I18n.locale=#{I18n.locale}"   # => en

Root cause analysis

In lib/i18n.rb, the config method uses fiber-local storage:

def config
  Thread.current[:i18n_config] ||= I18n::Config.new
end

However, Thread.current[:key] is fiber-local in Ruby.
Each Fiber has its own Thread.current hash, so :i18n_config can be missing/reset when code runs inside a Fiber, causing I18n.locale to fall back to default_locale.

Real-world example

This breaks behavior in the CSV library (v3.2.6+):

require "csv"
require "i18n"

I18n.locale = :en

CSV.parse("name\nTaro\n", headers: true, header_converters: [
  ->(f, info) { puts I18n.locale }  # => :ja (wrong!)
])

This happens because CSV internally uses Enumerator#next, which creates Fibers.

References

• Related CSV issue: ruby/csv#acc05116