Lookup global max_job_runtime at runtime

The existing approach set `JobIteration.max_job_runtime` as the default
value of the `job_iteration_max_job_runtime` `class_attribute`. However,
this approach would close around the value at the time the `Iteration`
module was included in the host `class`.

This means that if a job class is defined before the host application
sets `max_job_runtime`, the setting will not be honoured. For example,
if using the `maintenance_tasks` gem, which defines a `TaskJob`, the
default would be read when the gem is loaded, prior to initializers
running, which is where the host application typically sets the global
`max_job_runtime`.

This commit removes the static default passed the `class_attribute`, and
instead overrides the reader it provides with an implementation that
delegates to `JobIteration.max_job_runtime`, ensuring we use the current
global value at runtime.

It should be noted that this does mean it's possible for the restriction
on _increasing_ the `max_job_runtime` to fail to hold:

```ruby
class MyJob < ApplicationJob
  include JobIteration::Iteration
  self.job_iteration_max_job_runtime = 1.hour
  # ...
end
JobIteration.max_job_runtime = 1.minute
```

but this is unlikely to occur in practice outside of jobs classes
provided by gems (as the host app's classes would be defined after
setting the global max in an initializer), but gems probably should be
making decisions about max job runtime anyway.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Bug fixes
 
+- [436](https://github.com/Shopify/job-iteration/pull/436) - Defer reading `JobIteration.max_job_runtime` until runtime, instead of closing around the value at the time of job definition.
 - [431](https://github.com/Shopify/job-iteration/pull/431) - Use `#id_value` instead of `send(:id)`
 when generating position for cursor based on `:id` column (Rails 7.1 and above, where composite
 primary models are now supported). This ensures we grab the value of the id column, rather than a

lib/job-iteration/iteration.rb

@@ -40,7 +40,7 @@ def inspected_cursor
       end
     end
 
-    included do |_base|
+    included do |base|
       define_callbacks :start
       define_callbacks :shutdown
       define_callbacks :complete
@@ -49,10 +49,16 @@ def inspected_cursor
         :job_iteration_max_job_runtime,
         instance_writer: false,
         instance_predicate: false,
-        default: JobIteration.max_job_runtime,
       )
 
-      singleton_class.prepend(PrependedClassMethods)
+      class << base
+        prepend(PrependedClassMethods)
+
+        def job_iteration_max_job_runtime
+          # Lookup default every time, instead of closing around its value when the class is defined.
+          JobIteration.max_job_runtime
+        end
+      end
     end
 
     module PrependedClassMethods

test/unit/iteration_test.rb

@@ -311,6 +311,17 @@ def test_global_max_job_runtime
     end
   end
 
+  def test_global_max_job_runtime_with_updated_value
+    freeze_time
+    with_global_max_job_runtime(10.minutes) do
+      klass = build_slow_job_class(iterations: 3, iteration_duration: 30.seconds)
+      with_global_max_job_runtime(1.minute) do
+        klass.perform_now
+        assert_partially_completed_job(cursor_position: 2)
+      end
+    end
+  end
+
   def test_per_class_max_job_runtime_with_default_global
     freeze_time
     parent = build_slow_job_class(iterations: 3, iteration_duration: 30.seconds)