Preview showing cached version with Varnish in Acquia Cloud

Had the same issue with Platform.sh

To fix this, I had to add the cookie wse_preview to the .platform/routes.yaml settings file as per https://docs.platform.sh/define-routes/cache.html

I'd imagine the solution will be similar for Acquia as well.

This should be mentioned in the readme somewhere for the future folks.

I have confirmed this buggy behavior on Acquia Cloud with Varnish. If anyone needs an immediate fix, we can always cache bust by adding a random query string to the end of the url. That's a lot to ask of previewers, though, especially if they're going to be clicking around to multiple pages during the preview. But in a pinch!

I like the idea of starting a session in order to bypass Varnish (or other reverse proxy caches). I made a patch that I tested in an Acquia Cloud dev environment. It seems to have done the trick.

I'm going to put in an MR. I'm going to start lazy and eschew DI. If the maintainer approves the approach, we could then polish it.

After some testing in the wild, we determined that the caching of the redirect was problematic. We don't want it anywhere in the Drupal cache or the browser cache, otherwise the session might not get started when the user tries the preview link.