Decorating the ActiveRecord @config object in a thread-safe manner

Not sure if this is needed as @config[:database] doesn't seem to affect any tests.

lib/active_record_host_pool/connection_adapter_mixin.rb

@@ -3,17 +3,54 @@
 require "active_record/connection_adapters/mysql2_adapter"
 
 module ActiveRecordHostPool
+  module PerThreadHashAccess
+    # PerThreadHashAccess.module_for(...) returns a module which scopes hash accessor methods
+    # to the given connection_id with thread-local values. It uses `super` as a fallback. This is to
+    # make access to the config object on a ActiveRecord::ConnectionAdapters::Mysql2Adapter
+    # thread-safe.
+    #
+    # Because AHRP shares the connection across members of its connection pool it is possible for
+    # multiple threads be simultaneously trying to talk to the database.
+    def self.module_for(connection_id)
+      Module.new do
+        define_method(:[]=) do |key, value|
+          _ahrp_storage[key] = value
+        end
+
+        define_method(:[]) do |key|
+          if _ahrp_storage.key?(key)
+            _ahrp_storage[key]
+          else
+            _ahrp_storage[key] = super(key)
+          end
+        end
+
+        define_method(:fetch) do |*args, &blk|
+          key, = args
+          _ahrp_storage.fetch(key) do
+            _ahrp_storage[key] = super(*args, &blk)
+          end
+        end
+
+        private
+
+        define_method(:_ahrp_storage) do
+          DatabaseSwitch.thread_local_storage[connection_id]
+        end
+      end
+    end
+  end
+
   module DatabaseSwitch
     def self.included(base)
       base.class_eval do
-        attr_reader :_host_pool_current_database
         def _host_pool_current_database
-          _ahrp_per_thread_per_connection_storage[:_host_pool_current_database]
+          _ahrp_storage[:_host_pool_current_database]
         end
 
         def _host_pool_current_database=(database)
-          _ahrp_per_thread_per_connection_storage[:_host_pool_current_database] = database
-          # @config[:database] = _host_pool_current_database if ActiveRecord::VERSION::MAJOR >= 5
+          _ahrp_storage[:_host_pool_current_database] = database
+          @config[:database] = _host_pool_current_database if ActiveRecord::VERSION::MAJOR >= 5
         end
 
         alias_method :execute_without_switching, :execute
@@ -31,12 +68,13 @@ def _host_pool_current_database=(database)
     end
 
     def self.thread_local_storage
-      @thread_local_storage = Thread.current[:active_record_host_pool] ||= Hash.new { |h,k| h[k] = {} }
+      Thread.current[:active_record_host_pool] ||= Hash.new { |h, k| h[k] = {} }
     end
 
     def initialize(*)
-      self._cached_current_database = nil
       super
+
+      @config.singleton_class.prepend PerThreadHashAccess.module_for(@connection.object_id)
     end
 
     def execute_with_switching(*args)
@@ -71,11 +109,11 @@ def disconnect_with_host_pooling!
     attr_accessor :_no_switch
 
     def _cached_current_database
-      _ahrp_per_thread_per_connection_storage[:_cached_current_database]
+      _ahrp_storage[:_cached_current_database]
     end
 
     def _cached_current_database=(database)
-      _ahrp_per_thread_per_connection_storage[:_cached_current_database] = database
+      _ahrp_storage[:_cached_current_database] = database
     end
 
     def _cached_connection_object_id
@@ -101,7 +139,7 @@ def _switch_connection
       end
     end
 
-    def _ahrp_per_thread_per_connection_storage
+    def _ahrp_storage
       DatabaseSwitch.thread_local_storage[_cached_connection_object_id]
     end