> I'm imagining (for POSIX platforms) adding some kind of check for
> signals when the system call returns EINTR.  If the signal handler
> raises an exception, like an interrupt should raise a
> KeyboardInterrupt, we can just give a different return code and
> propagate the exception.

Yes, this is what I'm proposing too.

> It also seems like this behavior can be extended gradually to
> different platforms, since I don't have the resources to change and
> test every threading implementation.

There is only one active POSIX threading implementation in 3.2, in
Python/thread_pthread.h.
(and the only non-POSIX one is for Windows)