Prevent `Marshal.load` from looping infinitely

Fix a bug in `Marshal.load` that caused it to loop indefinitely when
trying to autoload a constant that resolved to a different name.

This could occur when marshalling an ActiveRecord 4.0 object (e.g. into
memcached) and then trying to unmarshal it with Rails 4.2. The
marshalled payload contains a reference to
`ActiveRecord::ConnectionAdapters::Mysql2Adapter::Column`, which in
Rails 4.2 resolves to
`ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::Column`.

1 files changed, 11 insertions, 0 deletions

diff --git a/activesupport/test/core_ext/marshal_test.rb b/activesupport/test/core_ext/marshal_test.rb
index 5427837d19..07c0c0d8cb 100644
--- a/activesupport/test/core_ext/marshal_test.rb
+++ b/activesupport/test/core_ext/marshal_test.rb
@@ -64,6 +64,17 @@ class MarshalTest < ActiveSupport::TestCase
     end
   end
 
+  test "when one constant resolves to another" do
+    class Parent; C = Class.new; end
+    class Child < Parent; C = Class.new; end
+
+    dump = Marshal.dump(Child::C.new)
+
+    Child.send(:remove_const, :C)
+
+    assert_raise(ArgumentError) { Marshal.load(dump) }
+  end
+
   test "that a real missing class is causing an exception" do
     dumped = nil
     with_autoloading_fixtures do