proxy_cache and variable

[trungaczne]

Using "proxy_cache" directive with a variable seems to change that
variable's value.

Tested against a fresh install of nginx 1.9.6 (default configure parameters)
on 2 different Ubuntu:14.04 hosts. DigitalOcean droplets.

The following configuration works as expected: It returns 0/1/2 depending on
the mimetype returned by the upstream. The returned header is always
correct.

worker_processes  1;

events {
    worker_connections  1024;
}


http {
    include       mime.types;
    default_type  application/octet-stream;

    sendfile        on;

    keepalive_timeout  65;

    map $upstream_http_content_type $should_not_cache {
        default        1;
        "~image/"      2;
        "~css"         0;
    }
    add_header X-Should-Not-Cache $should_not_cache;

    server {
        listen       80;
        server_name  myupstream.com;

        location / {
            proxy_pass   http://<upstreamip>;
            proxy_set_header Host $host;

        }
    }

}

This on the other hand always return the default value of the "map"
directive (X-Should-Not-Cache always return 1 regardless of whether it's
css/image/etc):

worker_processes  1;

events {
    worker_connections  1024;
}


http {
    include       mime.types;
    default_type  application/octet-stream;

    sendfile        on;

    keepalive_timeout  65;

    map $upstream_http_content_type $should_not_cache {
        default        1;
        "~image/"      2;
        "~css"         0;
    }

    proxy_cache_path  /tmp/low levels=1:2  keys_zone=0:1m  inactive=1m 
max_size=1g;
    proxy_cache_path  /tmp/medium levels=1:2  keys_zone=1:1m  inactive=1m 
max_size=1g;
    proxy_cache_path  /tmp/high levels=1:2  keys_zone=2:10m  inactive=10m 
max_size=1g;
    proxy_cache_key "$scheme$request_method$host$request_uri$is_args$args";
    proxy_cache_methods GET HEAD;
    proxy_cache_valid 200 302 10m;
    proxy_cache_valid 404      1m;
    proxy_cache $should_not_cache;

    add_header X-Should-Not-Cache $should_not_cache;

    server {
        listen       80;
        server_name  myupstream.com;

        location / {
            proxy_pass   http://<upstreamip>;
            proxy_set_header Host $host;

        }
    }

}

(basically I'm trying to have different cache zones for different mime types
with different time parameters)

Also, in the second config, commenting out the "proxy_cache" directive makes
the header correct again.

Can anybody explain this behavior?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,262527,262527#msg-262527