Filter insertion and ordering

[Maxim Dounin]

Hello!

On Tue, Jan 08, 2013 at 12:34:43PM +0000, Nick Kew wrote:

> OK, so my module needs to insert filters.
> I put the usual in a post_config function:
> 
>    /* Insert headers_out filter */
>  ngx_http_next_header_filter = ngx_http_top_header_filter;
>  ngx_http_top_header_filter = my_header_filter;
> 
>  /* Insert body_out filter */
>  ngx_http_next_body_filter = ngx_http_top_body_filter;
>  ngx_http_top_body_filter = my_body_filter;
> 
> Only they never get called!
> 
> With the aid of gdb I put a breakpoint in my post_config,
> and I see ngx_http_top_[header|body]_filters are NULL
> at the point where the post_config is being called.
> Must mean my module is getting called first, right?
> Guess that figures if it was linked in last and works
> in Apache 1.x-style order.
> 
> Setting a breakpoint in another module's filter, I get to:
> 
> 613	    return ngx_http_write_filter(r, &out);
> (gdb) bt
> #0  ngx_http_header_filter (r=0x7f7f728b6000) at ngx_http_header_filter_module.c:613
> #1  0x0000000109020834 in ngx_http_chunked_header_filter (r=0x7f7f728b6000) at ngx_http_chunked_filter_module.c:68
> … [more header filters] ...
> #9  0x0000000108fe2945 in ngx_http_send_header (r=0x7f7f728b6000) at ngx_http_core_module.c:1941
> #10 0x0000000109013def in ngx_http_upstream_send_response (r=0x7f7f728b6000, u=0x7f7f72838450) at ngx_http_upstream.c:2063
> 
> I infer ngx_http_header_filter must've inserted itself after my module,
> and is not calling a "next" filter.  Clearly not useful!
> 
> Apart from the obvious WTF, this raises another issue: how does my
> module determine its order in the chain?  Its body filter needs to come
> before anything that might encode it (like compression or chunking),
> but the header filter should ideally come after those operations to
> see exactly what will be sent to the Client!
> 
> How do I take control?

Filter ordering is determinded during configure - config script of 
your module should place it into appropriate list, usually 
HTTP_AUX_FILTER_MODULES.  Your symptoms suggests you've added your 
filter module into HTTP_MODULES list instead.

You may want to refer here for basic instructions:

http://www.evanmiller.org/nginx-modules-guide.html#compiling

And to auto/modules code for all the details.

-- 
Maxim Dounin
http://nginx.com/support.html