Allow deprecated non-symbol access to nested `config_for` hashes

A change to `Rails::Application.config_for` in
https://github.com/rails/rails/pull/33815 and
https://github.com/rails/rails/pull/33882 has altered the behaviour of
the returned object in a breaking manner. Before that change, nested
hashes returned from `config_for` could be accessed using non-symbol keys.
After the change, all keys are recursively symbolized so non-symbol access
fails to read the expected values.

This is a breaking change for any app that might be relying on the
nested hashes returned from `config_for` calls, and thus should be
deprecated before being removed from the codebase.

This commit introduces a temporary `NonSymbolAccessDeprecatedHash` class
that recursively wraps any nested hashes inside the `OrderedOptions`
object returned from `config_for` and issues a deprecation notice when a
non-symbol based access is performed.

This way, apps that are still relying on the ability to access these
nested hashes using non-symbol keys will be able to observe the
deprecation notices and have time to implement changes before non-symbol
access is removed for good.

A CHANGELOG entry is also added to note that non-symbol access to nested
`config_for` hashes is deprecated.

1 files changed, 36 insertions, 2 deletions

diff --git a/railties/lib/rails/application.rb b/railties/lib/rails/application.rb
index 5a924ab8e6..d0417f8a49 100644
--- a/railties/lib/rails/application.rb
+++ b/railties/lib/rails/application.rb
@@ -7,6 +7,7 @@ require "active_support/key_generator"
 require "active_support/message_verifier"
 require "active_support/encrypted_configuration"
 require "active_support/deprecation"
+require "active_support/hash_with_indifferent_access"
 require "rails/engine"
 require "rails/secrets"
 
@@ -230,8 +231,8 @@ module Rails
         config = YAML.load(ERB.new(yaml.read).result) || {}
         config = (config["shared"] || {}).merge(config[env] || {})
 
-        ActiveSupport::OrderedOptions.new.tap do |config_as_ordered_options|
-          config_as_ordered_options.update(config.deep_symbolize_keys)
+        ActiveSupport::OrderedOptions.new.tap do |options|
+          options.update(NonSymbolAccessDeprecatedHash.new(config))
         end
       else
         raise "Could not load configuration. No such file - #{yaml}"
@@ -590,5 +591,38 @@ module Rails
       def build_middleware
         config.app_middleware + super
       end
+
+      class NonSymbolAccessDeprecatedHash < HashWithIndifferentAccess # :nodoc:
+        def initialize(value = nil)
+          if value.is_a?(Hash)
+            value.each_pair { |k, v| self[k] = v }
+          else
+            super
+          end
+        end
+
+        def []=(key, value)
+          if value.is_a?(Hash)
+            value = self.class.new(value)
+          end
+          super(key.to_sym, value)
+        end
+
+        private
+
+          def convert_key(key)
+            unless key.kind_of?(Symbol)
+              ActiveSupport::Deprecation.warn(<<~MESSAGE.squish)
+                Accessing hashes returned from config_for by non-symbol keys
+                is deprecated and will be removed in Rails 6.1.
+                Use symbols for access instead.
+              MESSAGE
+
+              key = key.to_sym
+            end
+
+            key
+          end
+      end
   end
 end