If-Modified-Since and If-None-Match conditionals with X-Accel-Redirect responses ?

[Tom Lanyon]

We've got an upstream returning X-Accel-Redirect responses to an internal 
location (which itself proxies to another upstream). Something like:

        upstream fileserver {
                server foo;
        }

        upstream backend {
                server bar;
        }

        server {
                location /filestore {
                        internal;
                        proxy_cache filestore_cache;
                        proxy_pass http://fileserver;
                }
                
                # responses from backend include header:
                # X-Accel-Redirect: /filestore/<pathname>
                location / {
                        proxy_set_header Host $host;
                        proxy_pass http://backend;
                }
        }

Both upstreams return Last-Modified and ETag headers. We want to keep the ETag 
and Last-Modified from the original backend (not the fileserver), so we changed 
the X-Accel-Redirect upstream to:

        location /filestore {
                internal;
                
                set $first_etag $upstream_http_etag;
                set $first_last_modified $upstream_http_last_modified;
                add_header ETag $first_etag;
                add_header Last-Modified $first_last_modified;
                
                proxy_cache filestore_cache;
                proxy_pass http://fileserver;
        }

This appears to work OK, because a response returns the ETag and Last-Modified 
we'd expect:
        ETag: "ce79a35523521ae64290db620e1073b1ea145497"
        Expires: Fri, 09 Oct 2015 05:46:35 GMT
        Last-Modified: Fri, 17 Jan 2014 05:20:40 GMT

But if we send a matching:
        If-None-Match: "ce79a35523521ae64290db620e1073b1ea145497"
or:
        If-Modified-Since: Fri, 17 Jan 2014 05:20:40 GMT
request header, then we get a HTTP 200 response, not a 304 Not Modified as we'd 
hope for.

I assume that this is because we're relying on the ETag and Last-Modified being 
set from temporary variables rather than the ones from the fileserver upstream. 
Is there any way we can work around this to pass through the ETag and 
Last-Modified from the backend sending X-Accel-Redirect responses whilst still 
supporting conditional requests and 304 responses?

Thanks,
Tom

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