Fix query cache when using shared connections

Enables the query cache on the correct connection when
shared connections across threads are enabled

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

@@ -426,7 +426,7 @@ def lock_thread=(lock_thread)
       # #connection can be called any number of times; the connection is
       # held in a cache keyed by a thread.
       def connection
-        @thread_cached_conns[connection_cache_key(@lock_thread || Thread.current)] ||= checkout
+        @thread_cached_conns[connection_cache_key(current_thread)] ||= checkout
       end
 
       # Returns true if there is an open connection being used for the current thread.
@@ -435,7 +435,7 @@ def connection
       # #connection or #with_connection methods. Connections obtained through
       # #checkout will not be detected by #active_connection?
       def active_connection?
-        @thread_cached_conns[connection_cache_key(Thread.current)]
+        @thread_cached_conns[connection_cache_key(current_thread)]
       end
 
       # Signal that the thread is finished with the current connection.
@@ -730,6 +730,10 @@ def connection_cache_key(thread)
           thread
         end
 
+        def current_thread
+          @lock_thread || Thread.current
+        end
+
         # Take control of all existing connections so a "group" action such as
         # reload/disconnect can be performed safely. It is no longer enough to
         # wrap it in +synchronize+ because some pool's actions are allowed

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

@@ -33,17 +33,17 @@ def initialize(*)
         end
 
         def enable_query_cache!
-          @query_cache_enabled[connection_cache_key(Thread.current)] = true
+          @query_cache_enabled[connection_cache_key(current_thread)] = true
           connection.enable_query_cache! if active_connection?
         end
 
         def disable_query_cache!
-          @query_cache_enabled.delete connection_cache_key(Thread.current)
+          @query_cache_enabled.delete connection_cache_key(current_thread)
           connection.disable_query_cache! if active_connection?
         end
 
         def query_cache_enabled
-          @query_cache_enabled[connection_cache_key(Thread.current)]
+          @query_cache_enabled[connection_cache_key(current_thread)]
         end
       end
 

activerecord/test/cases/query_cache_test.rb

@@ -536,6 +536,23 @@ def test_clear_query_cache_is_called_on_all_connections
     ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
   end
 
+  test "query cache is enabled in threads with shared connection" do
+    ActiveRecord::Base.connection_pool.lock_thread = true
+
+    assert_cache :off
+
+    thread_a = Thread.new do
+      middleware { |env|
+        assert_cache :clean
+        [200, {}, nil]
+      }.call({})
+    end
+
+    thread_a.join
+
+    ActiveRecord::Base.connection_pool.lock_thread = false
+  end
+
   private
     def with_temporary_connection_pool
       old_pool = ActiveRecord::Base.connection_handler.retrieve_connection_pool(ActiveRecord::Base.connection_specification_name)