Remove or replace resourceRegistries etag

[mauritsvanrees]

This was initially removed in Plone 5.2 because it was for the old portal_css/javascripts tools. I restored it in proper form for the "new" resource registries of 5.2 in PR #65, basically reading the timestamp.txt. But with Plone 6 the timestamp.txt in portal_resources is not there and not needed.

So what do we do with this etag? Remove it? Or can it be replaced with an etag that works on Plone 6?

The point of this etag is to prevent this scenario:

• You have an html page in the Varnish cache, and this points to a resource with a unique hash.
• You change the resource, probably by a change on the file system, and restart Plone.
• A user requests the page, Varnish returns the cached version, and then the browser requests the resource with the old unique hash, so it gets the old version.

If the page is cached using etags, then the job of the resourceRegistries etag, is to return a different value, because it sees the resource has changed.

Can we come up with a sane replacement of this etag?

[mauritsvanrees]

Starting with Plone 6.0.4, the portal_registry will have a modification time, and it is updated whenever the Resource Registries change or an add-on is installed. See plone/Products.CMFPlone#3771. We could use this time in the etag.

@jensens Does this make sense to you?

[jensens]

I think this is an excellent idea!