Handle subsequent cache reads which are frozen

More quirks of the Rails cache. Initial testing showed that writing a
frozen cache value returns an unfrozen value. Likely due to poor rigor
in test methodology previous thought was that all reads of the cache
would result in unfrozen values - even if that cache'd entry was
refrozen.

This turns out to be false. The actual behavior is:

- Write frozen object to cache
- Read cache receiving an unfrozen version
- All subsequent cache reads will be unfrozen until one of the read
  versions is re-frozen
- Subsequent cache reads will be frozen

This was causing "500 Internal Server" errors in what appeared to be
ActiveRecord calls. Digging into the stack trace showed it was our
caching.

Author: cupakromer

lib/kracken/controllers/token_authenticatable.rb

@@ -45,7 +45,13 @@ def cache_valid_auth(token, &generate_cache)
         cache_key = "auth/token/#{token}"
         val = Rails.cache.read(cache_key)
         val ||= store_valid_auth(cache_key, &generate_cache)
-        val.transform_values!(&:freeze).freeze if val
+        shallow_freeze(val)
+      end
+
+      def shallow_freeze(val)
+        # nil is always frozen
+        return val if val.frozen?
+        val.transform_values!(&:freeze).freeze
       end
 
       def current_auth_info

spec/kracken/controllers/token_authenticatable_spec.rb

@@ -1,3 +1,4 @@
+require "rails_helper"
 require "support/base_controller_double"
 require "support/using_cache"
 
@@ -89,6 +90,25 @@ def authenticate_or_request_with_http_token(realm = nil)
           ).and be_frozen
         end
 
+        it "handles already frozen auth hashes" do
+          # This seems to happen on the 2nd cache read - despite the hash being
+          # frozen when put into the cache the first cache read will not be
+          # frozen. However, subsequent reads seem to be froze - this may be
+          # Rails version dependent. Regardless, we need to be aware of it.
+
+          # Initial cache request
+          expect(a_controller.authenticate_user_with_token!).to eq(
+            id: :any_id,
+            team_ids: [:some, :team, :ids],
+          ).and be_frozen
+
+          # Secondary cache request
+          expect(a_controller.authenticate_user_with_token!).to eq(
+            id: :any_id,
+            team_ids: [:some, :team, :ids],
+          ).and be_frozen
+        end
+
         it "exposes the auth info via the `current_` helpers", :aggregate_failures do
           expect {
             a_controller.authenticate_user_with_token!