ActiveJob::Base#queue_adapter is now a class_attribute

This allows different `queue_adapters` to be used in each `ActiveJob`
class heirarchy. Previously, all subclasses used a single global queue
adapter.

activejob/lib/active_job/queue_adapter.rb

@@ -1,4 +1,5 @@
 require 'active_job/queue_adapters/inline_adapter'
+require 'active_support/core_ext/class/attribute'
 require 'active_support/core_ext/string/inflections'
 
 module ActiveJob
@@ -7,34 +8,45 @@ module ActiveJob
   module QueueAdapter #:nodoc:
     extend ActiveSupport::Concern
 
+    included do
+      class_attribute :_queue_adapter, instance_accessor: false, instance_predicate: false
+      self.queue_adapter = :inline
+    end
+
     # Includes the setter method for changing the active queue adapter.
     module ClassMethods
-      mattr_reader(:queue_adapter) { ActiveJob::QueueAdapters::InlineAdapter }
+      def queue_adapter
+        _queue_adapter
+      end
 
       # Specify the backend queue provider. The default queue adapter
       # is the :inline queue. See QueueAdapters for more
       # information.
       def queue_adapter=(name_or_adapter_or_class)
+        self._queue_adapter = interpret_adapter(name_or_adapter_or_class)
+      end
+
+      private
+
+      def interpret_adapter(name_or_adapter_or_class)
         case name_or_adapter_or_class
         when Symbol, String
-          self.queue_adapter = load_adapter(name_or_adapter_or_class)
+          load_adapter(name_or_adapter_or_class)
         else
-          @@queue_adapter = if queue_adapter?(name_or_adapter_or_class)
+          if queue_adapter?(name_or_adapter_or_class)
             name_or_adapter_or_class
           elsif queue_adapter_class?(name_or_adapter_or_class)
             ActiveSupport::Deprecation.warn "Passing an adapter class is deprecated " \
-            "and will be removed in Rails 5.1. Please pass an adapter name " \
-            "(.queue_adapter = :#{name_or_adapter_or_class.name.demodulize.remove('Adapter').underscore}) " \
-            "or an instance (.queue_adapter = #{name_or_adapter_or_class.name}.new) instead."
-            name_or_adapter_or_class.new
+              "and will be removed in Rails 5.1. Please pass an adapter name " \
+              "(.queue_adapter = :#{name_or_adapter_or_class.name.demodulize.remove('Adapter').underscore}) " \
+              "or an instance (.queue_adapter = #{name_or_adapter_or_class.name}.new) instead."
+              name_or_adapter_or_class.new
           else
             raise ArgumentError
           end
         end
       end
 
-      private
-
       QUEUE_ADAPTER_METHODS = [:enqueue, :enqueue_at].freeze
 
       def queue_adapter?(object)

activejob/lib/active_job/test_helper.rb

@@ -1,3 +1,4 @@
+require 'active_support/core_ext/class/subclasses'
 require 'active_support/core_ext/hash/keys'
 
 module ActiveJob
@@ -7,8 +8,17 @@ module ActiveJob
 
     included do
       def before_setup
-        @old_queue_adapter = queue_adapter
-        ActiveJob::Base.queue_adapter = :test
+        test_adapter = ActiveJob::QueueAdapters::TestAdapter.new
+
+        @old_queue_adapters = (ActiveJob::Base.subclasses << ActiveJob::Base).select do |klass|
+          # only override explicitly set adapters, a quirk of `class_attribute`
+          klass.singleton_class.public_instance_methods(false).include?(:_queue_adapter)
+        end.map do |klass|
+          [klass, klass.queue_adapter].tap do
+            klass.queue_adapter = test_adapter
+          end
+        end
+
         clear_enqueued_jobs
         clear_performed_jobs
         super
@@ -16,7 +26,9 @@ module ActiveJob
 
       def after_teardown
         super
-        ActiveJob::Base.queue_adapter = @old_queue_adapter
+        @old_queue_adapters.each do |(klass, adapter)|
+          klass.queue_adapter = adapter
+        end
       end
 
       # Asserts that the number of enqueued jobs matches the given number.

activejob/test/cases/queue_adapter_test.rb

@@ -1,5 +1,19 @@
 require 'helper'
 
+module ActiveJob
+  module QueueAdapters
+    class StubOneAdapter
+      def enqueue(*); end
+      def enqueue_at(*); end
+    end
+
+    class StubTwoAdapter
+      def enqueue(*); end
+      def enqueue_at(*); end
+    end
+  end
+end
+
 class QueueAdapterTest < ActiveJob::TestCase
   test 'should forbid nonsense arguments' do
     assert_raises(ArgumentError) { ActiveJob::Base.queue_adapter = Mutex }
@@ -12,13 +26,31 @@ class QueueAdapterTest < ActiveJob::TestCase
         'fake'
       end
 
-      def enqueue(*)
-      end
-
-      def enqueue_at(*)
-      end
+      def enqueue(*); end
+      def enqueue_at(*); end
     end
 
     assert_deprecated { ActiveJob::Base.queue_adapter = klass }
   end
+
+  test 'should allow overriding the queue_adapter at the child class level without affecting the parent or its sibling' do
+    base_queue_adapter = ActiveJob::Base.queue_adapter
+
+    child_job_one = Class.new(ActiveJob::Base)
+    child_job_one.queue_adapter = :stub_one
+
+    assert_not_equal ActiveJob::Base.queue_adapter, child_job_one.queue_adapter
+    assert_kind_of ActiveJob::QueueAdapters::StubOneAdapter, child_job_one.queue_adapter
+
+    child_job_two = Class.new(ActiveJob::Base)
+    child_job_two.queue_adapter = :stub_two
+
+    assert_kind_of ActiveJob::QueueAdapters::StubTwoAdapter, child_job_two.queue_adapter
+    assert_kind_of ActiveJob::QueueAdapters::StubOneAdapter, child_job_one.queue_adapter, "child_job_one's queue adapter should remain unchanged"
+    assert_equal base_queue_adapter, ActiveJob::Base.queue_adapter, "ActiveJob::Base's queue adapter should remain unchanged"
+
+    child_job_three = Class.new(ActiveJob::Base)
+
+    assert_not_nil child_job_three.queue_adapter
+  end
 end

activejob/test/cases/test_case_test.rb

@@ -4,6 +4,15 @@ require 'jobs/logging_job'
 require 'jobs/nested_job'
 
 class ActiveJobTestCaseTest < ActiveJob::TestCase
+  # this tests that this job class doesn't get its adapter set.
+  # that's the correct behaviour since we don't want to break
+  # the `class_attribute` inheritence
+  class TestClassAttributeInheritenceJob < ActiveJob::Base
+    def self.queue_adapter=(*)
+      raise 'Attemping to break `class_attribute` inheritence, bad!'
+    end
+  end
+
   def test_include_helper
     assert_includes self.class.ancestors, ActiveJob::TestHelper
   end