Fixes #20799

When `#perform_later` is called the locale isn't stored on the
queue, which results in a locale reset when the job is performed.

An example of the problem:

    I18n.locale = 'de'
    HelloJob.perform_now # german message, correct

but

    I18n.locale = 'de'
    HelloJob.perform_later # english message, incorrect

This PR attaches the current I18n.locale to every job during the
serialization process. It is then restored during deserialization
and used to perform the job with the correct locale.

It falls back to the default locale if no serialized locale is
found in order to provide backward compatibility with previously
stored jobs. It is not necessary to clear the queue for the update.

11 files changed, 111 insertions, 2 deletions

diff --git a/activejob/CHANGELOG.md b/activejob/CHANGELOG.md
index c523723fc4..1bd97eed38 100644
--- a/activejob/CHANGELOG.md
+++ b/activejob/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Include I18n.locale into job serialization/deserialization and use it around
+    `perform`.
+
+    Fixes #20799.
+
+    *Johannes Opper*
+
 *   Allow `DelayedJob`, `Sidekiq`, `qu`, and `que` to report the job id back to
     `ActiveJob::Base` as `provider_job_id`.
 
diff --git a/activejob/lib/active_job/base.rb b/activejob/lib/active_job/base.rb
index fd49b3fda5..5d7c4cfb91 100644
--- a/activejob/lib/active_job/base.rb
+++ b/activejob/lib/active_job/base.rb
@@ -5,6 +5,7 @@ require 'active_job/enqueuing'
 require 'active_job/execution'
 require 'active_job/callbacks'
 require 'active_job/logging'
+require 'active_job/translation'
 
 module ActiveJob #:nodoc:
   # = Active Job
@@ -60,6 +61,7 @@ module ActiveJob #:nodoc:
     include Execution
     include Callbacks
     include Logging
+    include Translation
 
     ActiveSupport.run_load_hooks(:active_job, self)
   end
diff --git a/activejob/lib/active_job/core.rb b/activejob/lib/active_job/core.rb
index 0528572cd0..eac7279309 100644
--- a/activejob/lib/active_job/core.rb
+++ b/activejob/lib/active_job/core.rb
@@ -20,6 +20,9 @@ module ActiveJob
 
       # ID optionally provided by adapter
       attr_accessor :provider_job_id
+
+      # I18n.locale to be used during the job.
+      attr_accessor :locale
     end
 
     # These methods will be included into any Active Job object, adding
@@ -68,7 +71,8 @@ module ActiveJob
         'job_class'  => self.class.name,
         'job_id'     => job_id,
         'queue_name' => queue_name,
-        'arguments'  => serialize_arguments(arguments)
+        'arguments'  => serialize_arguments(arguments),
+        'locale'     => I18n.locale
       }
     end
 
@@ -96,6 +100,7 @@ module ActiveJob
       self.job_id               = job_data['job_id']
       self.queue_name           = job_data['queue_name']
       self.serialized_arguments = job_data['arguments']
+      self.locale               = job_data['locale'] || I18n.locale
     end
 
     private
diff --git a/activejob/lib/active_job/translation.rb b/activejob/lib/active_job/translation.rb
new file mode 100644
index 0000000000..67e4cf4ab9
--- /dev/null
+++ b/activejob/lib/active_job/translation.rb
@@ -0,0 +1,11 @@
+module ActiveJob
+  module Translation #:nodoc:
+    extend ActiveSupport::Concern
+
+    included do
+      around_perform do |job, block, _|
+        I18n.with_locale(job.locale, &block)
+      end
+    end
+  end
+end
diff --git a/activejob/test/cases/job_serialization_test.rb b/activejob/test/cases/job_serialization_test.rb
index db22783030..e23380a7bf 100644
--- a/activejob/test/cases/job_serialization_test.rb
+++ b/activejob/test/cases/job_serialization_test.rb
@@ -12,4 +12,20 @@ class JobSerializationTest < ActiveSupport::TestCase
     GidJob.perform_later @person
     assert_equal "Person with ID: 5", JobBuffer.last_value
   end
+
+  test 'serialize includes current locale' do
+    assert_equal :en, HelloJob.new.serialize['locale']
+  end
+
+  test 'deserialize sets locale' do
+    job = HelloJob.new
+    job.deserialize 'locale' => :es
+    assert_equal :es, job.locale
+  end
+
+  test 'deserialize sets default locale' do
+    job = HelloJob.new
+    job.deserialize({})
+    assert_equal :en, job.locale
+  end
 end
diff --git a/activejob/test/cases/translation_test.rb b/activejob/test/cases/translation_test.rb
new file mode 100644
index 0000000000..d5e3aaf9e3
--- /dev/null
+++ b/activejob/test/cases/translation_test.rb
@@ -0,0 +1,20 @@
+require 'helper'
+require 'jobs/translated_hello_job'
+
+class TranslationTest < ActiveSupport::TestCase
+  setup do
+    JobBuffer.clear
+    I18n.available_locales = [:en, :de]
+    @job = TranslatedHelloJob.new('Johannes')
+  end
+
+  teardown do
+    I18n.available_locales = [:en]
+  end
+
+  test 'it performs the job in the given locale' do
+    @job.locale = :de
+    @job.perform_now
+    assert_equal "Johannes says Guten Tag", JobBuffer.last_value
+  end
+end
diff --git a/activejob/test/integration/queuing_test.rb b/activejob/test/integration/queuing_test.rb
index d345092dee..e1634972d1 100644
--- a/activejob/test/integration/queuing_test.rb
+++ b/activejob/test/integration/queuing_test.rb
@@ -68,4 +68,21 @@ class QueuingTest < ActiveSupport::TestCase
     refute delayed_test_job.provider_job_id.nil?,
       'Provider job id should by set for delayed jobs by provider'
   end
+
+  test 'current locale is kept while running perform_later' do
+    skip if adapter_is?(:inline)
+
+    begin
+      I18n.available_locales = [:en, :de]
+      I18n.locale = :de
+
+      TestJob.perform_later @id
+      wait_for_jobs_to_finish_for(5.seconds)
+      assert job_executed
+      assert_equal 'de', job_output
+    ensure
+      I18n.available_locales = [:en]
+      I18n.locale = :en
+    end
+  end
 end
diff --git a/activejob/test/jobs/translated_hello_job.rb b/activejob/test/jobs/translated_hello_job.rb
new file mode 100644
index 0000000000..9657cd3f54
--- /dev/null
+++ b/activejob/test/jobs/translated_hello_job.rb
@@ -0,0 +1,10 @@
+require_relative '../support/job_buffer'
+
+class TranslatedHelloJob < ActiveJob::Base
+  def perform(greeter = "David")
+    translations = { en: 'Hello', de: 'Guten Tag' }
+    hello        = translations[I18n.locale]
+
+    JobBuffer.add("#{greeter} says #{hello}")
+  end
+end
diff --git a/activejob/test/support/integration/dummy_app_template.rb b/activejob/test/support/integration/dummy_app_template.rb
index 09a68738ad..4ffdb8cffa 100644
--- a/activejob/test/support/integration/dummy_app_template.rb
+++ b/activejob/test/support/integration/dummy_app_template.rb
@@ -8,13 +8,17 @@ require "#{File.expand_path("../jobs_manager.rb",  __FILE__)}"
 JobsManager.current_manager.setup
 CODE
 
+initializer 'i18n.rb', <<-CODE
+I18n.available_locales = [:en, :de]
+CODE
+
 file 'app/jobs/test_job.rb', <<-CODE
 class TestJob < ActiveJob::Base
   queue_as :integration_tests
 
   def perform(x)
     File.open(Rails.root.join("tmp/\#{x}"), "w+") do |f|
-      f.write x
+      f.write I18n.locale
     end
   end
 end
diff --git a/activejob/test/support/integration/test_case_helpers.rb b/activejob/test/support/integration/test_case_helpers.rb
index 7e87ede275..39aee6d407 100644
--- a/activejob/test/support/integration/test_case_helpers.rb
+++ b/activejob/test/support/integration/test_case_helpers.rb
@@ -45,4 +45,8 @@ module TestCaseHelpers
     def job_executed
       Dummy::Application.root.join("tmp/#{@id}").exist?
     end
+
+    def job_output
+      File.read Dummy::Application.root.join("tmp/#{@id}")
+    end
 end
diff --git a/guides/source/active_job_basics.md b/guides/source/active_job_basics.md
index 22f3c0146a..1715fc327a 100644
--- a/guides/source/active_job_basics.md
+++ b/guides/source/active_job_basics.md
@@ -280,6 +280,19 @@ UserMailer.welcome(@user).deliver_later
 ```
 
 
+Internationalization
+--------------------
+
+Each job uses the `I18n.locale` set when the job was created. Useful if you send
+emails asynchronously:
+
+```ruby
+I18n.locale = :eo
+
+UserMailer.welcome(@user).deliver_later # Email will be localized to Esparanto.
+```
+
+
 GlobalID
 --------