Changes to a dupped `ActionController::Parameters` mutate the original

When `ActionController::Parameters` is duplicated with `#dup`, it doesn't create a duplicate of the instance variables (e.g. `@parameters`) but rather maintains the reference (see <http://ruby-doc.org/core-2.3.1/Object.html>). Given that the parameters object is often manipulated as if it were a hash (e.g. with `#delete` and similar methods), this leads to unexpected behaviour, like the following:

```
params = ActionController::Parameters.new(foo: "bar")
duplicated_params = params.dup
duplicated_params.delete(:foo)

params == duplicated_params
```

This fixes the bug by defining a private `#initialize_copy` method, used internally by `#dup`, which makes a copy of `@parameters`.

1 files changed, 5 insertions, 0 deletions

diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index b326695ce2..fea04d98e0 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -783,6 +783,11 @@ module ActionController
           end
         end
       end
+
+      def initialize_copy(source)
+        super
+        @parameters = source.instance_variable_get(:@parameters).dup
+      end
   end
 
   # == Strong \Parameters