Serving stale content, when proxy_cache_background_update is on and Cache-Control disappears

[karol-gro@…]

nginx for Windows

I set up nginx proxy with following config

worker_processes  1;

events {
    worker_connections  1024;
}


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

    keepalive_timeout  65;
	
	proxy_cache_path C:\\folder\\cache levels=1:2 keys_zone=my_cache:1m max_size=1g inactive=12h use_temp_path=off;

    server {
        listen       4080;
        listen       [::]:4080;
        server_name  localhost;

        location /api/ {
            proxy_pass https://example.com;
            proxy_http_version 1.1;
            proxy_set_header X-TenantKey SomeKey;
        }

        location / {
            rewrite "^/([^/]*/){3}(.*)$" /$2 last ;

            proxy_pass https://SOMEACCOUNT.blob.core.windows.net/app/;
            proxy_http_version 1.1;

            proxy_cache my_cache;
            proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
            proxy_cache_lock on;
            proxy_cache_background_update on;
        }
    }
}

Reproduction steps:

1. Create index.html file on blob storage with Test-1 content and Cache-Control: max-age=30
2. Open IP:4080/a/b/c/index.html in HTTP client. It should work
3. Change content to Test-2, keeping the Cache-Control. After some 30s you can see new content served by nginx
4. Change content to Test-3 and remove Cache-Control header.

Expected result:
As all responses without cache-control, it will be treated as not-cachable. Background updater removes it from cache and nginx will proxy to source on each request

Actual result:
Nginx serves still an old Test-2 with cache-control, even after 2 minutes.

[Maxim Dounin]

Duplicate of #853.