Raise error when serializing an anonymous class.

The ModuleSerializer does not support serializing anonymous classes
because when we try to deserialize the anonymous class, it wouldn't
know which class to use (since class name is nil).

For this reason, ModuleSerialzier now raises an error if the class
name is nil. Previously, ModuleSerializer would raise an `undefined
method `constantize' for nil:NilClass` error during deserialization.
It's not clear why the deserialization failed from the error.

In this commit, we raise an explicit error when trying to serialize
an anonymous class indicating this behaviour is not supported.

activejob/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Raise an `SerializationError` in `Serializer::ModuleSerializer`
+    if the module name is not present. 
+
+    *Veerpal Brar*
+
+
 ## Rails 7.0.0.alpha2 (September 15, 2021) ##
 
 *   No changes.

activejob/lib/active_job/serializers/module_serializer.rb

@@ -4,6 +4,7 @@ module ActiveJob
   module Serializers
     class ModuleSerializer < ObjectSerializer # :nodoc:
       def serialize(constant)
+        raise SerializationError, "Serializing an anonymous class is not supported" unless constant.name
         super("value" => constant.name)
       end
 

activejob/test/cases/argument_serialization_test.rb

@@ -50,7 +50,7 @@ class ClassArgument; end
     end
   end
 
-  [ Object.new, Person.find("5").to_gid ].each do |arg|
+  [ Object.new, Person.find("5").to_gid, Class.new ].each do |arg|
     test "does not serialize #{arg.class}" do
       assert_raises ActiveJob::SerializationError do
         ActiveJob::Arguments.serialize [ arg ]