Make we always type cast TimeWithZone objects before passing to mysql2

mysql2 knows how to handle Time and Date objects but doesn't know
about TimeWithZone. This was causing failures when prepared statements
were enabled since we were passing TimeWithZone objects that mysql2 didn't
know how to deal with.

An new environment variable to enable prepared statements were added to
config.example.yml, so we can test in our CI and prevent regressions.

Fixes #41368.

activerecord/lib/active_record/connection_adapters/mysql/quoting.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/time_with_zone"
+
 module ActiveRecord
   module ConnectionAdapters
     module MySQL
@@ -69,10 +71,23 @@ def column_name_with_order_matcher
         private_constant :COLUMN_NAME, :COLUMN_NAME_WITH_ORDER
 
         private
+          # Override +_type_cast+ we pass to mysql2 Date and Time objects instead
+          # of Strings since mysql2 is able to handle those classes more efficiently.
           def _type_cast(value)
             case value
-            when Date, Time then value
-            else super
+            when ActiveSupport::TimeWithZone
+              # We need to check explicitly for ActiveSupport::TimeWithZone because
+              # we need to transform it to Time objects but we don't want to
+              # transform Time objects to themselves.
+              if ActiveRecord::Base.default_timezone == :utc
+                value.getutc
+              else
+                value.getlocal
+              end
+            when Date, Time
+              value
+            else
+              super
             end
           end
       end

activerecord/test/cases/base_test.rb

@@ -26,6 +26,7 @@
 require "models/bird"
 require "models/car"
 require "models/bulb"
+require "models/pet"
 require "concurrent/atomic/count_down_latch"
 require "active_support/core_ext/enumerable"
 
@@ -312,6 +313,18 @@ def test_preserving_time_objects_with_time_with_zone_conversion_to_default_timez
     end
   end
 
+  def test_time_zone_aware_attribute_with_default_timezone_utc_on_utc_can_be_created
+    with_env_tz eastern_time_zone do
+      with_timezone_config aware_attributes: true, default: :utc, zone: "UTC" do
+        pet = Pet.create(name: 'Bidu')
+        assert_predicate pet, :persisted?
+        saved_pet = Pet.find(pet.id)
+        assert_not_nil saved_pet.created_at
+        assert_not_nil saved_pet.updated_at
+      end
+    end
+  end
+
   def eastern_time_zone
     if Gem.win_platform?
       "EST5EDT"

activerecord/test/config.example.yml

@@ -40,13 +40,19 @@ connections:
       username: rails
       encoding: utf8mb4
       collation: utf8mb4_unicode_ci
+<% if ENV['MYSQL_PREPARED_STATEMENTS'] %>
+      prepared_statements: true
+<% end %>
 <% if ENV['MYSQL_HOST'] %>
       host: <%= ENV['MYSQL_HOST'] %>
 <% end %>
     arunit2:
       username: rails
       encoding: utf8mb4
       collation: utf8mb4_general_ci
+<% if ENV['MYSQL_PREPARED_STATEMENTS'] %>
+      prepared_statements: true
+<% end %>
 <% if ENV['MYSQL_HOST'] %>
       host: <%= ENV['MYSQL_HOST'] %>
 <% end %>