Get the hex-encoded hash of the fd's data

def get_file_hash( fd, hashfunc, fd_len=None ):
    """
    Get the hex-encoded hash of the fd's data
    """

    h = hashfunc()
    fd.seek(0, os.SEEK_SET)

    count = 0
    while True:
        buf = fd.read(65536)
        if len(buf) == 0:
            break

        if fd_len is not None:
            if count + len(buf) > fd_len:
                buf = buf[:fd_len - count]

        h.update(buf)
        count += len(buf)

    hashed = h.hexdigest()
    return hashed