Return a position in a file which is known to be read & handled.
    It assumes a buffered file and streaming processing.

def get_known_read_position(fp, buffered=True):
    """ 
    Return a position in a file which is known to be read & handled.
    It assumes a buffered file and streaming processing. 
    """
    buffer_size = io.DEFAULT_BUFFER_SIZE if buffered else 0
    return max(fp.tell() - buffer_size, 0)