id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	uname	nginx_version
2490	the backup upstream response inherits the response value of the previous upstream that failed.	soukichi@…		"When an upstream configuration defining primary and backup servers is set up as follows,
it receiving a response with a status code defined in proxy_next_upstream and with ""Cache-Control: max-age=XX"" header from the primary server, it will be cached the responses from the backup server even that don't have the ""Cache-Control"" header.


{{{
upstream upstream_http {
    server unix:/run/nginx_1.sock max_fails=1 fail_timeout=10s;
    server unix:/run/nginx_2.sock max_fails=1 fail_timeout=10s backup;
}
}}}

primary upstream server's response:

{{{
HTTP/1.1 500 Internal Server Error
Server: -
Date: -
Content-Type: text/html
Content-Length: 174
Connection: keep-alive
Cache-Control: max-age=15
}}}

backup upstream server's response:
{{{
HTTP/1.1 200 OK
Server: -
Date: -
Content-Type: application/octet-stream
Content-Length: 30
Connection: keep-alive
}}}

Based on the debug log, it appears that when receiving the response from the backup server, it is marked as ""http cacheable: 1"", and is cached for the amount of time specified by the ""Cache-Control: max-age=15"" header on the primary server.

{{{
[debug] 8278#0: *1 http write filter: l:0 f:0 s:184
[debug] 8278#0: *1 http file cache set header
[debug] 8278#0: *1 http cacheable: 1
[debug] 8278#0: *1 http proxy filter init s:200 h:0 c:0 l:30
[debug] 8278#0: *1 http upstream process upstream
}}}

It seems that the initialization is insufficient when the upstream transitions because applying this patch prevents the backup response from being cached.


{{{
diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index 9cc202c9..1487e9ca 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -1626,6 +1626,9 @@ ngx_http_proxy_reinit_request(ngx_http_request_t *r)
     r->upstream->pipe->input_filter = ngx_http_proxy_copy_filter;
     r->upstream->input_filter = ngx_http_proxy_non_buffered_copy_filter;
     r->state = 0;
+    if (r->cache != NULL) {
+        r->cache->valid_sec = 0;
+    }
 
     return NGX_OK;
 }
}}}


Is there a better way to initialize to prevent each server in the upstream from affecting the response of the other servers?
※ I understand that it is not common for a status code 500 response to have a ""Cache-Control: max-age=XX"" header. However, I sometimes receive such responses in my nginx reverse proxy and I want to cache them as so-called negative cache.

I am attaching the configuration and debug log.
"	defect	new	minor		nginx-module	1.23.x					" /usr/local/nginx/sbin/nginx -V
nginx version: nginx/1.23.4
built by gcc 11.3.0 (Ubuntu 11.3.0-1ubuntu1~22.04) 
configure arguments: --with-debug"