Invalidate query cache for all connections in the current thread

This change ensures that all query cahces are cleared across all
connections per handler for the current thread so if you write on one
connection the read will have the query cache cleared.

activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb

@@ -17,7 +17,7 @@ module ActiveRecord
           method_names.each do |method_name|
             base.class_eval <<-end_code, __FILE__, __LINE__ + 1
               def #{method_name}(*)
-                clear_query_cache if @query_cache_enabled
+                ActiveRecord::Base.clear_query_caches_for_current_thread if @query_cache_enabled
                 super
               end
             end_code

activerecord/lib/active_record/connection_handling.rb

@@ -176,6 +176,15 @@ module ActiveRecord
       config_hash
     end
 
+    # Clears the query cache for all connections associated with the current thread.
+    def clear_query_caches_for_current_thread
+      ActiveRecord::Base.connection_handlers.each_value do |handler|
+        handler.connection_pool_list.each do |pool|
+          pool.connection.clear_query_cache if pool.active_connection?
+        end
+      end
+    end
+
     # Returns the connection currently associated with the class. This can
     # also be used to "borrow" the connection to do database work unrelated
     # to any of the specific Active Records.

activerecord/test/cases/query_cache_test.rb

@@ -502,6 +502,44 @@ class QueryCacheTest < ActiveRecord::TestCase
     }.call({})
   end
 
+  def test_clear_query_cache_is_called_on_all_connections
+    skip "with in memory db, reading role won't be able to see database on writing role" if in_memory_db?
+    with_temporary_connection_pool do
+      ActiveRecord::Base.connection_handlers = {
+        writing: ActiveRecord::Base.default_connection_handler,
+        reading: ActiveRecord::ConnectionAdapters::ConnectionHandler.new
+      }
+
+      ActiveRecord::Base.connected_to(role: :reading) do
+        ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations["arunit"])
+      end
+
+      mw = middleware { |env|
+        ActiveRecord::Base.connected_to(role: :reading) do
+          @topic = Topic.first
+        end
+
+        assert @topic
+
+        ActiveRecord::Base.connected_to(role: :writing) do
+          @topic.title = "It doesn't have to be crazy at work"
+          @topic.save!
+        end
+
+        assert_equal "It doesn't have to be crazy at work", @topic.title
+
+        ActiveRecord::Base.connected_to(role: :reading) do
+          @topic = Topic.first
+          assert_equal "It doesn't have to be crazy at work", @topic.title
+        end
+      }
+
+      mw.call({})
+    end
+  ensure
+    ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
+  end
+
   private
 
     def with_temporary_connection_pool