Make sure that when serialing an just deserialized job arguments are there

When a job was just deserialized `arguments` is `nil` and the serialized
arguments are in the `@serialized_arguments` variable. If we try to
serialize this job again the arguments are going to be `nil` instead of
what was serialized.

The test we had was not checking this case because it was deserializing
the job in the same object that had the arguments.

To fix this, when the `@serialized_arguments` are present we return it
instead of the result of the `arguments` serialized.

activejob/lib/active_job/core.rb

@@ -88,7 +88,7 @@ def serialize
         "provider_job_id" => provider_job_id,
         "queue_name" => queue_name,
         "priority"   => priority,
-        "arguments"  => serialize_arguments(arguments),
+        "arguments"  => serialize_arguments_if_needed(arguments),
         "executions" => executions,
         "locale"     => I18n.locale.to_s,
         "timezone"   => Time.zone.try(:name)
@@ -133,19 +133,31 @@ def deserialize(job_data)
     end
 
     private
+      def serialize_arguments_if_needed(arguments)
+        if arguments_serialized?
+          @serialized_arguments
+        else
+          serialize_arguments(arguments)
+        end
+      end
+
       def deserialize_arguments_if_needed
-        if defined?(@serialized_arguments) && @serialized_arguments.present?
+        if arguments_serialized?
           @arguments = deserialize_arguments(@serialized_arguments)
           @serialized_arguments = nil
         end
       end
 
-      def serialize_arguments(serialized_args)
-        Arguments.serialize(serialized_args)
+      def serialize_arguments(arguments)
+        Arguments.serialize(arguments)
       end
 
       def deserialize_arguments(serialized_args)
         Arguments.deserialize(serialized_args)
       end
+
+      def arguments_serialized?
+        defined?(@serialized_arguments) && @serialized_arguments
+      end
   end
 end

activejob/test/cases/job_serialization_test.rb

@@ -23,16 +23,16 @@ class JobSerializationTest < ActiveSupport::TestCase
 
   test "serialize and deserialize are symmetric" do
     # Round trip a job in memory only
-    h1 = HelloJob.new
-    h1.deserialize(h1.serialize)
+    h1 = HelloJob.new("Rafael")
+    h2 = HelloJob.deserialize(h1.serialize)
+    assert_equal h1.serialize, h2.serialize
 
     # Now verify it's identical to a JSON round trip.
     # We don't want any non-native JSON elements in the job hash,
     # like symbols.
-    payload = JSON.dump(h1.serialize)
-    h2 = HelloJob.new
-    h2.deserialize(JSON.load(payload))
-    assert_equal h1.serialize, h2.serialize
+    payload = JSON.dump(h2.serialize)
+    h3 = HelloJob.deserialize(JSON.load(payload))
+    assert_equal h2.serialize, h3.serialize
   end
 
   test "deserialize sets locale" do