Fix query cache for multiple connections

Currently the query cache is only aware of one handler so once we added
multiple databases switching on the handler we broke query cache for
those reading connections.

While #34054 is the proper fix, that fix is not straight forward and I
want to make sure that the query cache isn't just broken for all other
connections not in the main handler.

activerecord/lib/active_record/query_cache.rb

@@ -26,15 +26,22 @@ module ActiveRecord
     end
 
     def self.run
-      ActiveRecord::Base.connection_handler.connection_pool_list.
+      pools = []
-        reject { |p| p.query_cache_enabled }.each { |p| p.enable_query_cache! }
+
+      ActiveRecord::Base.connection_handlers.each do |key, handler|
+        pools << handler.connection_pool_list.reject { |p| p.query_cache_enabled }.each { |p| p.enable_query_cache! }
+      end
+
+      pools.flatten
     end
 
     def self.complete(pools)
       pools.each { |pool| pool.disable_query_cache! }
 
-      ActiveRecord::Base.connection_handler.connection_pool_list.each do |pool|
+      ActiveRecord::Base.connection_handlers.each do |_, handler|
-        pool.release_connection if pool.active_connection? && !pool.connection.transaction_open?
+        handler.connection_pool_list.each do |pool|
+          pool.release_connection if pool.active_connection? && !pool.connection.transaction_open?
+        end
       end
     end
 

activerecord/test/cases/query_cache_test.rb

@@ -55,6 +55,22 @@ class QueryCacheTest < ActiveRecord::TestCase
     assert_cache :off
   end
 
+  def test_query_cache_is_applied_to_connections_in_all_handlers
+    ActiveRecord::Base.connected_to(role: :reading) do
+      ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations["arunit"])
+    end
+
+    mw = middleware { |env|
+      ro_conn = ActiveRecord::Base.connection_handlers[:reading].connection_pool_list.first.connection
+      assert_predicate ActiveRecord::Base.connection, :query_cache_enabled
+      assert_predicate ro_conn, :query_cache_enabled
+    }
+
+    mw.call({})
+  ensure
+    ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
+  end
+
   def test_query_cache_across_threads
     with_temporary_connection_pool do
       begin