Fix invalid durations in ActiveJob serialization

Durations that were round-tripped through ActiveJob::Arguments.serialize
would appear fine at a first glance, but trying to perform
duration-math on them would fail:

```
irb(main):001:0> d = ActiveJob::Arguments.deserialize(ActiveJob::Arguments.serialize([1.year]))[0]
=> 1 year
irb(main):002:0> d + 1.day
activesupport-6.1.4.4/lib/active_support/duration.rb:242:in `+': undefined method `merge' for [[:years, 1]]:Array (NoMethodError)
```

Author: jdelStrother

activejob/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Fix deserialization of ActiveSupport::Duration
+
+    Previously, a deserialized Duration would return an array from Duration#parts.
+    It will now return a hash just like a regular Duration.
+
+    This also fixes an error when trying to add or subtract from a deserialized Duration
+    (eg `duration + 1.year`).
+
+    *Jonathan del Strother*
+
 *   `perform_enqueued_jobs` is now compatible with all Active Job adapters
 
     This means that methods that depend on it, like Action Mailer's `assert_emails`,

activejob/lib/active_job/serializers/duration_serializer.rb

@@ -4,14 +4,16 @@ module ActiveJob
   module Serializers
     class DurationSerializer < ObjectSerializer # :nodoc:
       def serialize(duration)
+        # Ideally duration.parts would be wrapped in an array before passing to Arguments.serialize,
+        # but we continue passing the bare hash for backwards compatibility:
         super("value" => duration.value, "parts" => Arguments.serialize(duration.parts))
       end
 
       def deserialize(hash)
         value = hash["value"]
         parts = Arguments.deserialize(hash["parts"])
-
-        klass.new(value, parts)
+        # `parts` is originally a hash, but will have been flattened to an array by Arguments.serialize
+        klass.new(value, parts.to_h)
       end
 
       private

activejob/test/cases/argument_serialization_test.rb

@@ -5,6 +5,8 @@
 require "active_job/arguments"
 require "models/person"
 require "active_support/core_ext/hash/indifferent_access"
+require "active_support/core_ext/integer/time"
+require "active_support/duration"
 require "jobs/kwargs_job"
 require "jobs/arguments_round_trip_job"
 require "support/stubs/strong_parameters"
@@ -188,6 +190,12 @@ def self.permitted?
     end
   end
 
+  test "should maintain a functional duration" do
+    duration = perform_round_trip([1.year]).first
+    assert_kind_of Hash, duration.parts
+    assert_equal 2.years, duration + 1.year
+  end
+
   test "should disallow non-string/symbol hash keys" do
     assert_raises ActiveJob::SerializationError do
       ActiveJob::Arguments.serialize [ { 1 => 2 } ]