Client disconnect in ngx_http_image_filter_module

[Maxim Dounin]

Hello!

On Tue, Apr 28, 2015 at 04:56:54PM +0700, Dan Podeanu wrote:

> 
> I have encountered a bug in ngx_http_image_filter_module when used in conjunction with ngx_http_proxy_module ; the configuration is as following:
> 
> location /img/ {
> 	proxy_pass http://mybucket.s3.amazonaws.com <http://mybucket.s3.amazonaws.com/>;
> 	image_filter resize 150 100;
> }
> 
> The steps to reproduce are rather complicated as they depend on how TCP fragments the response coming from the proxy:
> 
> - If http://mybucket.s3.amazonaws.com <http://mybucket.s3.amazonaws.com/> returns, in the first TCP packet, a sizable amount of data (1-2k), the image is resized correctly.
> 
> - If http://mybucket.s3.amazonaws.com <http://mybucket.s3.amazonaws.com/> returns, in the first TCP packet, a small amount of data (HTTP header, or HTTP header + a few bytes), the content is marked as not an image and NGX_HTTP_UNSUPPORTED_MEDIA_TYPE is returned (disconnecting the client), irrespective on whether or not subsequent data would complete the response to a valid image.
> 
> Nginx appears to give up right away on waiting for data if the contents of the first TCP packet received from the proxy does not contain a valid image header- i.e. ngx_http_image_test() will return NGX_HTTP_IMAGE_SIZE, etc.
> 
> In my experience this was triggered by a subtle change in AWS S3 that fragments the TCP header more.
> 
> Versions affected: 1.6.2, 1.6.3, 1.7.2, 1.8.0, etc.
> 
> 
> 
> Attaching a patch which fixes it for my use case (1.6.2) below; the other versions can be fixed similarly:
> 
> ————————————————————————
> diff -ruN nginx-1.6.2.orig/src/http/modules/ngx_http_image_filter_module.c nginx-1.6.2/src/http/modules/ngx_http_image_filter_module.c
> --- nginx-1.6.2.orig/src/http/modules/ngx_http_image_filter_module.c	2014-09-16 19:23:19.000000000 +0700
> +++ nginx-1.6.2/src/http/modules/ngx_http_image_filter_module.c	2015-04-28 16:25:47.000000000 +0700
> @@ -317,9 +317,8 @@
>                  }
>              }
>  
> -            return ngx_http_filter_finalize_request(r,
> -                                              &ngx_http_image_filter_module,
> -                                              NGX_HTTP_UNSUPPORTED_MEDIA_TYPE);
> +            /* wait to consume more data, do not give up on the request right away */
> +            return NGX_OK;
>          }
>  
>          /* override content type */
> ————————————————————————
> 
> 
> Thoughts ? Is this the right fix ? Should I also post on the nginx Trac ?

Thanks, looks like a valid problem - and yes, please file a ticket 
on trac.  The fix doesn't looks correct for me though - with such 
a change nginx will silently skip first bytes of a response.

-- 
Maxim Dounin
http://nginx.org/