Hey again, I've just realised that there's a bug in my patch. I initially
wrote it so that the thread was the one to close the handles, but then I
decided that the main thread should close them as soon as they're not
required any more. That, and also the fact that they needed to be cleaned
up in destroy_thread_data as well.

Long story made short, I forgot to change the thread's local handle
storage into a handle duplication. I've attached the patch. (full.diff as
to 7.12.1, partial.diff as to my previous patch)