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.