Excuse me for refer to quite old issue, we found a different behavior regarding their headers order in case of X-Accel-Expires which is not 0 and Cache-Control has any of no-store, no-cache or private.

It is very easy reproducing process.

ngx_http_upstream.c has two method ngx_http_upstream_process_cache_control and ngx_http_upstream_process_accel_expires which operations their cache related headers, Cache-Control is processed in ngx_http_upstream_process_cache_control, X-Accel-Expires is processed in process_accel_expires.

ngx_http_upstream_process_cache_control in ngx_http_upstream.c

4738 if (r->cache->valid_sec != 0 && u->headers_in.x_accel_expires != NULL) {
4739 return NGX_OK;
4740 }
4741
4742 start = h->value.data;
4743 last = start + h->value.len;
4744
4745 if (ngx_strlcasestrn(start, last, (u_char *) "no-cache", 8 - 1) != NULL
4746 || ngx_strlcasestrn(start, last, (u_char *) "no-store", 8 - 1) != NULL
4747 || ngx_strlcasestrn(start, last, (u_char *) "private", 7 - 1) != NULL)
4748 {
4749 u->cacheable = 0;
4750 return NGX_OK;
4751 }


If Cache-Control from upstream has a value of no-store, no-cache or private, u->cacheable = 0; in ngx_http_upstream_process_cache_control

In the case of before processing ngx_http_upstream_process_accel_expires, if it sets u->cacheable = 0 for this procedure, it does not cache according to Cache-Control. X-Accel-Expires is only overwriting valid_sec.

OTOH, In the case of after processing ngx_http_upstream_process_accel_expires, ngx_http_upstream_process_cache_control returns NGX_OK earlier than the procedure of Cache-Control: no-xxx or private.
In this case, it does cache, the cache is following x-accel-expires values. It ignores Cache-Control.

As this result it seems not to override entirely Cache-Control by X-Accel-Expires. And It has differential behavor according to order of upstream header. We could not find this behavior in any nginx documents. Could you tell me what is true?