update-recovery-worker.py

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def main(request, response):

    # Set mode to 'init' for initial fetch.

    mode = b'init'

    if b'update-recovery-mode' in request.cookies:

        mode = request.cookies[b'update-recovery-mode'].value

    # no-cache itself to ensure the user agent finds a new version for each update.

    headers = [(b'Cache-Control', b'no-cache, must-revalidate'),

               (b'Pragma', b'no-cache')]

    extra_body = b''

    if mode == b'init':

        # Install a bad service worker that will break the controlled

        # document navigation.

        response.set_cookie(b'update-recovery-mode', b'bad')

        extra_body = b"addEventListener('fetch', function(e) { e.respondWith(Promise.reject()); });"

    elif mode == b'bad':

        # When the update tries to pull the script again, update to

        # a worker service worker that does not break document

        # navigation.  Serve the same script from then on.

        response.delete_cookie(b'update-recovery-mode')

    headers.append((b'Content-Type', b'application/javascript'))

    return headers, b'%s' % (extra_body)