fork() only leaves the one thread running in the child, and at that point the fd tables are no longer shared, so trying to detect and close unwanted descriptors

fork() only leaves the one thread running in the child, and at that point the fd tables are no longer shared, so trying to detect and close unwanted descriptors in the child after fork is not racy by itself as a way of mitigating the possibility of uncontrollable non-CLOEXEC opens elsewhere in the process (though this doesn't preclude it being a bad idea for other reasons).