> MvL> If you are absolutely certain that a cast cannot possibly truncate, 
> MvL> add a comment explaining why that is.
> 
> Ah yes, sorry, I forgot to add a comment: done in r87746.

But the comment is actually wrong: It says

len <= buf_size <= INT_MAX (see above)

however, len > buf_size may happen, after this code:

    if (len > buf_size) {
        PyErr_Format(PyExc_ValueError,
                     "read() returned too much data: "
                     "%i bytes requested, %zd returned",
                     buf_size, len);
        goto finally;
    }