Hello!

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.

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