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`.

activesupport/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Prevent `Marshal.load` from looping infinitely when trying to autoload a constant
+    which resolves to a different name.
+
+    *Olek Janiszewski*
+
 *   Deprecate `Module.local_constants`. Please use `Module.constants(false)` instead.
 
     *Yuichiro Kaneko*

activesupport/lib/active_support/core_ext/marshal.rb

@@ -5,7 +5,10 @@ module ActiveSupport
     rescue ArgumentError, NameError => exc
       if exc.message.match(%r|undefined class/module (.+)|)
         # try loading the class/module
-        $1.constantize
+        loaded = $1.constantize
+
+        raise unless $1 == loaded.name
+
         # if it is an IO we need to go back to read the object
         source.rewind if source.respond_to?(:rewind)
         retry

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