Raise a more specific error when the job class can't be instantiated

During a rolling deploy, jobs maybe be enqueued that the job processor
doesn't know about yet. Currently it's not easily possible for the job
adapter to know if the exception originated from user code or not.

Author: Earlopain

activejob/CHANGELOG.md

@@ -1,2 +1,9 @@
+*   Raise a more specific error during deserialization when a previously serialized job class is now unknown.
+
+    `ActiveJob::UnknownJobClassError` will be raised instead of a more generic
+    `NameError` to make it easily possible for adapters to tell if the `NameError`
+    was raised during job execution or deserialization.
+
+    *Earlopain*
 
 Please check [8-0-stable](https://github.com/rails/rails/blob/8-0-stable/activejob/CHANGELOG.md) for previous changes.

activejob/lib/active_job.rb

@@ -39,6 +39,7 @@ module ActiveJob
   autoload :Arguments
   autoload :DeserializationError, "active_job/arguments"
   autoload :SerializationError, "active_job/arguments"
+  autoload :UnknownJobClassError, "active_job/core"
   autoload :EnqueueAfterTransactionCommit
 
   eager_autoload do

activejob/lib/active_job/core.rb

@@ -1,6 +1,13 @@
 # frozen_string_literal: true
 
 module ActiveJob
+  # Raised during job payload deserialization when it references an uninitialized job class.
+  class UnknownJobClassError < NameError
+    def initialize(job_class_name)
+      super("Failed to instantiate job, class `#{job_class_name}` doesn't exist", job_class_name)
+    end
+  end
+
   # = Active Job \Core
   #
   # Provides general behavior that will be included into every Active Job
@@ -60,7 +67,10 @@ def successfully_enqueued?
     module ClassMethods
       # Creates a new job instance from a hash created with +serialize+
       def deserialize(job_data)
-        job = job_data["job_class"].constantize.new
+        job_class = job_data["job_class"].safe_constantize
+        raise UnknownJobClassError, job_data["job_class"] unless job_class
+
+        job = job_class.new
         job.deserialize(job_data)
         job
       end

activejob/test/cases/job_serialization_test.rb

@@ -38,6 +38,17 @@ class JobSerializationTest < ActiveSupport::TestCase
     assert_equal h2.serialize, h3.serialize
   end
 
+  test "deserialize raises a specific exception on unknown `job_class`" do
+    payload = HelloJob.new.serialize
+
+    # Simulate the job class being missing, for example during rolling deploys when
+    # the server enqueues a new job but the job processor hasn't been restarted yet.
+    payload["job_class"] = "IDontExist"
+    assert_raises(ActiveJob::UnknownJobClassError) do
+      HelloJob.deserialize(payload)
+    end
+  end
+
   test "deserialize sets locale" do
     job = HelloJob.new
     job.deserialize "locale" => "es"