Restrict Task.named to only return Task classes (Shopify#349)

app/tasks/maintenance_tasks/task.rb

@@ -16,9 +16,12 @@ class << self
       #
       # @raise [NotFoundError] if a Task with the given name does not exist.
       def named(name)
-        name.constantize
-      rescue NameError
-        raise NotFoundError.new("Task #{name} not found.", name)
+        task = name.safe_constantize
+        raise NotFoundError.new("Task #{name} not found.", name) unless task
+        unless task.is_a?(Class) && task < Task
+          raise NotFoundError.new("#{name} is not a Task.", name)
+        end
+        task
       end
 
       # Returns a list of concrete classes that inherit from the Task

test/tasks/maintenance_tasks/task_test.rb

@@ -29,6 +29,14 @@ class TaskTest < ActiveSupport::TestCase
       assert_equal 'Maintenance::DoesNotExist', error.name
     end
 
+    test ".named raises Not Found Error if the name doesn't refer to a Task" do
+      error = assert_raises(Task::NotFoundError) do
+        Task.named('Array')
+      end
+      assert_equal 'Array is not a Task.', error.message
+      assert_equal 'Array', error.name
+    end
+
     test '.process calls #process' do
       item = mock
       Maintenance::TestTask.any_instance.expects(:process).with(item)