UpdateProcThreadAttribute() documentation says that "... handles must be created as inheritable handles ..." and a comment says that "If using PROC_THREAD_ATTRIBUTE_HANDLE_LIST, pass TRUE to bInherit in CreateProcess. Otherwise, you will get an ERROR_INVALID_PARAMETER."

http://msdn.microsoft.com/en-us/library/windows/desktop/ms686880%28v=vs.85%29.aspx

Seriously? What is the purpose of PROC_THREAD_ATTRIBUTE_HANDLE_LIST if it does not avoid the race condition? It's "just" to not inherit some inheritable handles? In Python 3.4, files and sockets are created non-inheritable by default, so PROC_THREAD_ATTRIBUTE_HANDLE_LIST may not improve anything :-/