fixes default locale for new threads (WIP)

sets a fallback locale and a default locale
to be copied over to new threads

Co-authored-by: Stefanni Brasil <stefannibrasil@gmail.com>

lib/faker.rb

@@ -14,12 +14,17 @@ module Faker
   module Config
     class << self
       def locale=(new_locale)
+        if new_locale == @fallback_locale
+          @inital_locale = nil
+        else
+          @inital_locale ||= new_locale
+        end
+
         Thread.current[:faker_config_locale] = new_locale
       end
 
       def locale
-        # Because I18n.locale defaults to :en, if we don't have :en in our available_locales, errors will happen
-        Thread.current[:faker_config_locale] || (I18n.available_locales.include?(I18n.locale) ? I18n.locale : I18n.available_locales.first)
+        Thread.current[:faker_config_locale] ||= @inital_locale
       end
 
       def own_locale
@@ -33,6 +38,16 @@ def random=(new_random)
       def random
         Thread.current[:faker_config_random] || Random
       end
+
+      def setup!
+        @fallback_locale = if I18n.available_locales.include?(I18n.locale)
+                             I18n.locale
+                           else
+                             I18n.available_locales.first
+                           end
+
+        self.locale = @fallback_locale
+      end
     end
   end
 
@@ -265,3 +280,5 @@ def disable_enforce_available_locales
 
 # require faker objects
 Dir.glob(File.join(mydir, 'faker', '/**/*.rb')).sort.each { |file| require file }
+
+Faker::Config.setup!

test/test_determinism.rb

@@ -42,6 +42,47 @@ def test_thread_safety
     threads.each(&:join)
   end
 
+  def test_locale_setting
+    # if locale is not set, fallback to :en
+    assert_equal :en, Faker::Config.locale
+
+    # locale can be updated initially
+    # and it becomes the default value
+    # for new threads
+    Faker::Config.locale = :pt
+
+    assert_equal :pt, Faker::Config.locale
+
+    t1 = Thread.new do
+      # child thread has initial locale equal to
+      # latest locale set on main thread
+      # instead of the fallback value
+      assert_equal :pt, Faker::Config.locale
+      refute_equal :en, Faker::Config.locale
+
+      # child thread can set its own locale
+      Faker::Config.locale = :es
+      assert_equal :es, Faker::Config.locale
+    end
+
+    t1.join
+
+    # child thread won't change locale of other threads
+    assert_equal :pt, Faker::Config.locale
+
+    t2 = Thread.new do
+      # initial default locale is copied over to new thread
+      assert_equal :pt, Faker::Config.locale
+
+      Faker::Config.locale = :it
+      assert_equal :it, Faker::Config.locale
+    end
+
+    t2.join
+
+    assert_equal :pt, Faker::Config.locale
+  end
+
   private
 
   def deterministic_random?(first, method_name)