[PATCH] Mail: nginx_error:cache file has too long header

[Maxim Dounin]

Hello!

On Mon, Nov 26, 2018 at 06:11:33PM -0800, Yongguang Bai wrote:

> # HG changeset patch
> # User Yongguang Bai <yongguang.bai at quantil.com>
> # Date 1543276278 28800
> #      Mon Nov 26 15:51:18 2018 -0800
> # Node ID 4d48e1f3ebf6f0634e8e2cbd057d9fcfbea9d988
> # Parent  a7ff19afbb14795fef14f599a304d0ad21052b70
> Mail: nginx_error:cache file has too long header
> 
> This error is printed when cached file is expired and the
> response is changed in proxied server, and Nginx is under
> heavy load.
> 
> diff -r a7ff19afbb14 -r 4d48e1f3ebf6 src/http/ngx_http_file_cache.c
> --- a/src/http/ngx_http_file_cache.c	Mon Nov 26 18:29:56 2018 +0300
> +++ b/src/http/ngx_http_file_cache.c	Mon Nov 26 15:51:18 2018 -0800
> @@ -376,6 +376,10 @@
>      ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
>                     "http file cache fd: %d", of.fd);
>  
> +    if(c->uniq != of.uniq){
> +        c->body_start = ngx_pagesize;
> +    }
> +
>      c->file.fd = of.fd;
>      c->file.log = r->connection->log;
>      c->uniq = of.uniq;

This change looks wrong.  In particular, ngx_pagesize, which is 
usually 4k, can be smaller than existing c->body_start, and this 
change will make things worse instead of fixing anything.

Also, could you please elaborate what exactly you are trying to 
fix?

There is a known race which may lead to "cache file has too long 
header" error - and, actually the error was added to ensure that 
this race is properly handled if happens, see here:

http://hg.nginx.org/nginx/rev/6f97afc238de
http://mailman.nginx.org/pipermail/nginx-devel/2011-September/001287.html

Note that this race can only result in problems if for some reason 
the backend returns responses with different headers nearly at the 
same time.  This may indicate that there is something wrong with 
the backend.

Also, there is a known bug with multiple variants which also can 
lead to the same error.  The patch can be found here:

http://mailman.nginx.org/pipermail/nginx-devel/2018-January/010774.html

If you are hitting this bug, please test if the patch works for 
you.

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