[PATCH] skip apply "ngx_http_upstream_process_accel_expires" when a request responeded by cache

[Sokichi Takada]

Hello!

In my case, the backend returns x-accel-expires with 304 responses every
time.

nginx calculates valid_sec by "ngx_http_upstream_process_accel_expires" for
any revalidate or HIT responses, thus to get the wrong valid_sec value in
header_filter/log hook handler.
This patch has no impact to every response results, and to be able to get
TRUE valid_sec value in the hook handlers.

---
TAKADA Sokichi


On Mon, Jun 22, 2020 at 7:37 PM Maxim Dounin <mdounin at mdounin.ru> wrote:

> Hello!
>
> On Mon, Jun 22, 2020 at 05:09:16PM +0900, Sokichi Takada wrote:
>
> > > The cache validity time as obtained from cached responses is used
> > > while processing 304 responses during cache revalidation, so the
> > > suggested change looks wrong to me.
> >
> > When processing 304 responses during cache revalidation,
> > ngx_http_upstream_process_accel_expires is called twice.
> >
> > 1st time, in "ngx_http_upstream_process_header" function.
> > src/http/ngx_http_upstream.c: 2387         rc = u->process_header(r);
> > #  process_header = ngx_http_upstream_process_accel_expires
> >
> > This calculates the exact valid_sec value by x-accel-expires of
> upstream's
> > response,  It doesn't use a value of the cache data.
> > Also, the r->cached is 0 at this time.
> >
> > 2nd time, in "ngx_http_upstream_process_header" ->
> > "ngx_http_upstream_test_next" -> "ngx_http_upstream_cache_send" function.
> > src/http/ngx_http_upstream.c: 2423         if
> > (ngx_http_upstream_test_next(r, u) == NGX_OK)    ->
> > src/http/ngx_http_upstream.c: 2535         rc =
> > ngx_http_upstream_cache_send(r, u);                  ->
> > src/http/ngx_http_upstream.c: 1073     rc = u->process_header(r);
> >
> > This calculates unnecessary valid_sec value by x-accel-expires of the
> cache
> > data.
> > Also, this calculation can skip by this patch since the r->cached is 1 at
> > this time.
> >
> > Please look into ngx_http_upstream_test_next.
> > It keeps the valid_sec value of 1st time into "valid" variable at "2523
> >     valid = r->cache->valid_sec;".
> > After that, it "calls ngx_http_upstream_cache_send", then updates cache
> > data with the "valid" variable at "2560             r->cache->valid_sec =
> > valid;".
>
> Backend is not required to return cache control headers along with
> the 304 response.  If it doesn't - nginx uses the headers returned
> in the cached response.
>
> --
> Maxim Dounin
> http://mdounin.ru/
> _______________________________________________
> nginx-devel mailing list
> nginx-devel at nginx.org
> http://mailman.nginx.org/mailman/listinfo/nginx-devel
>


-- 
-------------------------------------
Sokichi TAKADA <takada at jocdn.co.jp>
JOCDN Inc.
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