Varnish sends 304 header for ESI pages although some of the included objects might have changed

Problem/Motivation

When checking whether it is possible to send a 304 instead of a full 200 response, Varnish does not respect the cache-characteristics of embedded ESI fragments. In order to make this work it would be necessary to merge all Last-Modified and ETag response headers generated by all ESI fragments making up a response and synthesize a new value which is then delivered to the browser. Better 304 support has been on the ESI wishlist for some time but it did not happen until now.

Proposed resolution

Disable HTTP revalidation by removing ETag and Last-Modified headers from the response upon vcl_deliver on requests where ESI processing was turned on.

Remaining tasks

User interface changes

API changes