Copy length bytes from fileobj src to fileobj dst.
       If length is None, copy the entire content.

def copyfileobj(src, dst, length=None):
    """Copy length bytes from fileobj src to fileobj dst.
       If length is None, copy the entire content.
    """
    if length == 0:
        return
    if length is None:
        while True:
            buf = src.read(16*1024)
            if not buf:
                break
            dst.write(buf)
        return

    BUFSIZE = 16 * 1024
    blocks, remainder = divmod(length, BUFSIZE)
    for b in range(blocks):
        buf = src.read(BUFSIZE)
        if len(buf) < BUFSIZE:
            raise IOError("end of file reached")
        dst.write(buf)

    if remainder != 0:
        buf = src.read(remainder)
        if len(buf) < remainder:
            raise IOError("end of file reached")
        dst.write(buf)
    return