> I'm pretty sure you mean WNOWAIT, right?

Right.

> revalidate pid licenses

What does this mean?

> I think we can patch up this last race condition by adding yet one more lock

Wouldn't it be sufficient to add

if self.returncode is not None:
    return self.returncode

at the top of poll()?

(And in fact, you don't even need to add it, since an equivalent of this check is already there.)

I think this makes calling poll() from wait() unnecessary too.