IMO closing an OS-level file descriptor in one thread while it is in use by another thread is a bad idea, full of race conditions and undefined behaviour. An AttributeError sounds like a best-case scenario. It is like freeing a memory allocation in one thread while another thread is accessing the memory. What if the other thread was slow, or a syscall was interrupted, and hasn’t (re-)started the recv() call in time? What if a third thread opens a file and reuses the file descriptor you just closed?

I suggest to close this.