Conflict with Super Cache plugin

[iandunn]

I haven't had a chance to figure out exactly what's causing this yet, so this may be invalid, but I've noticed that sites with the pwa plugin active seem to disable WP Super Cache by making it think that the current visitor is logged in.

To reproduce:

1. Open an Incognito window
2. Open the network dev tool
3. Visit https://2019.europe.wordcamp.org/contact/
4. View the response for the /contact/ request, and you'll see <!-- Cached page generated by WP-Super-Cache on 2019-07-04 00:51:12 --> at the bottom, which is expected. At this point the service worker hasn't been installed yet.
5. Refresh the page now that the service worker is installed, and you'll see <!-- WP Super Cache: Caching disabled for known user. User logged in or cookie found. --> at the bottom of the response.

If you curl https://2019.europe.wordcamp.org/contact/ you'll always get the correct cached response. If you repeat the steps above with a site that doesn't have pwa activated -- e.g., 2019.us -- then you'll also always see the correct cached response.

[westonruter]

Ah, interesting. The PWA plugin does set a cookie when the service worker is installed:

https://github.com/xwp/pwa-wp/blob/b0dcc2d668cd3dbb410af89b9af45e548436e73e/wp-includes/service-workers.php#L161-L165

And then WP-Super-Cache is doing:

	} elseif ( $wp_cache_not_logged_in == 1 && ! empty( $_COOKIE ) ) {
		wp_cache_debug( 'wpsc_is_caching_user_disabled: true because cookie found' );
		return true;

The cookie is being used to prevent script concatenation:

https://github.com/xwp/pwa-wp/blob/b0dcc2d668cd3dbb410af89b9af45e548436e73e/wp-includes/service-workers.php#L261-L268

The intention here is that the service worker will precache the scripts being used in the admin so that they will be served immediately form the browser cache and thus there is no need to concatenate. This intention may be misguided.

Nevertheless, what is for sure is that this only applies to the admin. Script concatenation via load-scripts.php is not a frontend thing. Therefore, there the PWA plugin should avoid setting this cookie for the front service worker. It should be restricted to the admin service worker only, at least.

This will also improve frontend performance a bit by not having this extra cookie being sent in all requests.

[westonruter]

That being said, should WP-Super-Cache really be preventing any page caching when there is any cookie present? That seems like it would cause caching to be disabled very often.

[westonruter]

For example, doesn't the ubiquitous wordpress_test_cookie cookie cause caching to be disabled in WP-Super-Cache?

[westonruter]

Here's a fix: #189.

[iandunn]

Ah, you're right: https://github.com/Automattic/wp-super-cache/issues/620

I'll try to test #189 tomorrow, thanks!