def _copy_stream(src, dest, length=0):
    """Similar to shutil.copyfileobj, but supports limiting data size.

    As for why this is required, refer to
    https://www.python.org/dev/peps/pep-0333/#input-and-error-streams

    Yes, there are WSGI implementations which do not support EOFs, and
    believe me, you don't want to debug this.

    Args:
        src: source file-like object
        dest: destination file-like object
        length: optional file size hint
            If not 0, exactly length bytes will be written.
            If 0, write will continue until EOF is encountered.
    """
    if length == 0:
        shutil.copyfileobj(src, dest)
        return

    bytes_left = length
    while bytes_left > 0:
        buf_size = min(_BUFFER_SIZE, bytes_left)
        buf = src.read(buf_size)
        dest.write(buf)
        bytes_left -= buf_size