Summary of my review for Georg's benefit: I had one minor formatting nit with the patch (which Stefan can easily fix before committing), but it otherwise looked fine to me.

I also agree that the old implicit truncation was unusable in practice, as the *actual* length is not returned from the function, and in fact cannot be returned because the return type is int rather than Py_ssize_t. A length mismatch therefore needs to be an error.