The patch looks fine (as far as it goes).

I do think the same should be done to the reader: IO libraries typically
provide a promise that concurrent threads can read, and will get the
complete stream in an overlapped manner (i.e. each input byte goes to
exactly one thread - no input byte gets lost, and no input byte is
delivered to multiple threads). I don't think this is currently the
case: two threads reading simultaneously may very well read the same
bytes twice, and then, subsequently, skip bytes (i.e. when both
increment _read_pos, but both see the original value of pos)