Nginx ignores proxy_no_cache

[Maxim Dounin]

Hello!

On Sun, Apr 07, 2024 at 04:56:20PM +0300, Maxim Dounin wrote:

> On Sun, Apr 07, 2024 at 01:36:21PM +0200, Kirill A. Korinsky wrote:
> 
> > Greetings,
> > 
> > Let assume that I would like behavior on LB from the backend and force it to
> > cache only resposnes that have a X-No-Cache header with value NO.
> > 
> > Nginx should cache a response with any code, if it has such headers.
> > 
> > This works well until the backend is unavailable and nginx returns a
> > hardcoded 502 that doesn't have a control header, but such a response is
> > cached anyway.
> > 
> > Here is the config that allows to reproduce the issue:
> > 
> >   http {
> >       default_type  application/octet-stream;
> > 
> >       proxy_cache_path        /tmp/nginx_cache keys_zone=the_zone:1m;
> >       proxy_cache             the_zone;
> >       proxy_cache_valid       any 15m;
> >       proxy_cache_methods     GET HEAD POST;
> > 
> >       add_header              X-Cache-Status $upstream_cache_status always;
> > 
> >       map $upstream_http_x_no_cache $no_cache {
> >           default             1;
> >           "NO"                0;
> >       }
> > 
> >       proxy_no_cache          $no_cache;
> > 
> >       upstream echo {
> >           server 127.127.127.127:80;
> >       }
> > 
> >       server {
> >           listen       1234;
> >           server_name  localhost;
> > 
> >           location / {
> >               proxy_pass http://echo;
> >           }
> >       }
> >   }
> > 
> > when I run:
> > 
> >   curl -D - http://127.0.0.1:1234/
> > 
> > it returns MISS on the first request, and HIT on the second one.
> > 
> > Here I expect both requests to return MISS.
> 
> Thanks for the report.
> 
> Indeed, proxy_no_cache is only checked for proper upstream 
> responses, but not when caching errors, including internally 
> generated 502/504 in ngx_http_upstream_finalize_request(), and 
> intercepted errors in ngx_http_upstream_intercept_errors().
> 
> Quick look suggests there will be also issues with caching errors 
> after proxy_cache_bypass (errors won't be cached even if they 
> should), as well as issues with proxy_cache_max_range_offset after 
> proxy_cache_bypass (it will be ignored).
> 
> This needs cleanup / fixes, added to my TODO list.

I've posted a patch to address proxy_no_cache issues along with 
relevant tests here:

https://freenginx.org/pipermail/nginx-devel/2024-June/000382.html
https://freenginx.org/pipermail/nginx-devel/2024-June/000383.html

Please take a look if it works for you.

-- 
Maxim Dounin
http://mdounin.ru/