nginx not returning updated headers from origin server on conditional GET

[jchannon]

I have nginx and its cache working as expected apart from one minor issue.
When a request is made for the first time it hits the origin server, returns
a 200 and nginx caches that response. If I make another request I can see
from the X-Cache-Status header that the cache has been hit. When I wait a
while knowing the cache will have expired I can see nginx hit my origin
server doing a conditional GET because I have proxy_cache_revalidate on;
defined.

When I check if the resource has changed in my app on the origin server I
see it hasn't and return a 304 with a new Expires header. Some may argue why
are you returning a new Expires header if the origin server says nothing has
changed and you are returning 304. The answer is, the HTTP RFC says that
this can be done https://tools.ietf.org/html/rfc7234#section-4.3.4

One thing I have noticed, no matter what headers are I add or modify, when
the origin server returns 304 nginx will give a response with the first set
of response headers it saw for that resource.

Also if I change the Cache-Control:max-age header value from the first
request when I return the 304 response it appears nginx obeys the new value
as my resource is cached for that time however the response header value is
that of what was given on the first request not the value that I modified on
the 304 response.   This applies to all subsequent requests if the origin
server issues a 304.

I am running nginx version: nginx/1.10.1

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,269457,269457#msg-269457